how to write essay on volcano

Essays About Volcanoes: Top 5 Examples and 10 Prompts

Do you need to write essays about volcanoes but don’t know where to start? Check out our top essay examples and prompts to help you write a high-quality essay.

Considered the planet’s geologic architects, volcanoes are responsible for more than 80% of the Earth’s surface . The mountains, craters, and fertile soil from these eruptions give way to the very foundation of life itself, making it possible for humans to survive and thrive.

Aside from the numerous ocean floor volcanoes, there are 161 active volcanoes in the US . However, these beautiful and unique landforms can instantly turn into a nightmare, like Mt. Tambora in Indonesia, which killed 92,000 people in 1815 .

Various writings are critical to understanding these openings in the Earth’s crust, especially for students studying volcanoes. It can be tricky to write this topic and will require a lot of research to ensure all the information gathered is accurate.

To help you, read on to see our top essay examples and writing prompts to help you begin writing.

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Top 5 Essay Examples

1. short essay on volcanoes by prasad nanda , 2. types of volcanoes by reena a , 3. shield volcano, one of the volcano types by anonymous on gradesfixer.com, 4. benefits and problems caused by volcanoes by anonymous on newyorkessays.com, 5. volcanoes paper by vanessa strickland, 1. volcanoes and their classifications, 2. a dormant volcano’s eruption, 3. volcanic eruptions in the movies, 4. the supervolcano: what is it, 5. the word’s ring of fire, 6. what is a lahar, 7. why does a volcano erupt, 8. my experience with volcanic eruptions, 9. effects of volcanic eruptions, 10. what to do during volcanic disasters.

“The name, “volcano” originates from the name Vulcan, a god of fire in Roman mythology.”

Nanda briefly defines volcanoes, stating they help release hot pressure that builds up deep within the planet. Then, he discusses each volcano classification, including lava and magma’s roles during a volcanic eruption. Besides interesting facts about volcanoes (like the Ojos del Salado as the world’s tallest volcano), Nanda talks about volcanic eruptions’ havoc. However, he also lays down their benefits, such as cooled magma turning to rich soil for crop cultivation.

“The size, style, and frequency of eruptions can differ greatly but all these elements are correlated to the shape of a volcano.”

In this essay, Reena identifies the three main types of volcanoes and compares them by shape, eruption style, and magma type and temperature. A shield volcano is a broad, flat domelike volcano with basaltic magma and gentle eruptions. The strato or composite volcano is the most violent because its explosive eruption results in a lava flow, pyroclastic flows, and lahar. Reena shares that a caldera volcano is rare and has sticky and cool lava, but it’s the most dangerous type. To make it easier for the readers to understand her essay, she adds figures describing the process of volcanic eruptions.

“All in all, shield volcanoes are the nicest of the three but don’t be fooled, it can still do damage.”

As the essay’s title suggests, the author focuses on the most prominent type of volcano with shallow slopes – the shield volcano. Countries like Iceland, New Zealand, and the US have this type of volcano, but it’s usually in the oceans, like the Mauna Loa in the Hawaiian Islands. Also, apart from its shape and magma type, a shield volcano has regular but calmer eruptions until water enters its vents.

“Volcanic eruptions bring both positive and negative impacts to man.”

The essay delves into the different conditions of volcanic eruptions, including their effects on a country and its people. Besides destroying crops, animals, and lives, they damage the economy and environment. However, these misfortunes also leave behind treasures, such as fertile soil from ash, minerals like copper, gold, and silver from magma, and clean and unlimited geothermal energy. After these incidents, a place’s historic eruptions also boost its tourism.

“Beautiful and powerful, awe-inspiring and deadly, they are spectacular reminders of the dynamic forces that shape our planet.”

Strickland’s essay centers on volcanic formations, types, and studies, specifically Krakatoa’s eruption in 1883. She explains that when two plates hit each other, the Earth melts rocks into magma and gases, forming a volcano. Strickland also mentions the pros and cons of living near a volcanic island. For example, even though a tsunami is possible, these islands are rich in marine life, giving fishermen a good living.

Are you looking for more topics like this? Check out our round-up of essay topics about nature .

10 Writing Prompts For Essays About Volcanoes

Do you need more inspiration for your essay? See our best essay prompts about volcanoes below:

Identify and discuss the three classifications of volcanoes according to how often they erupt: active, dormant or inactive, and extinct. Find the similarities and differences of each variety and give examples. At the end of your essay, tell your readers which volcano is the most dangerous and why.

Volcanoes that have not erupted for a very long time are considered inactive or dormant, but they can erupt anytime in the future. For this essay, look for an inactive volcano that suddenly woke up after years of sleeping. Then, find the cause of its sudden eruption and add the extent of its damage. To make your piece more interesting, include an interview with people living near dormant volcanoes and share their thoughts on the possibility of them exploding anytime.

Essays About Volcanoes: Volcanic eruptions in the movies

Choose an on-screen depiction of how volcanoes work, like the documentary “ Krakatoa: Volcano of Destruction .” Next, briefly summarize the movie, then comment on how realistic the film’s effects, scenes, and dialogues are. Finally, conclude your essay by debating the characters’ decisions to save themselves.

The Volcanic Explosivity Index (VEI) criteria interpret danger based on intensity and magnitude. Explain how this scale recognizes a supervolcano. Talk about the world’s supervolcanoes, which are active, dormant, and extinct. Add the latest report on a supervolcano’s eruption and its destruction.

Identify the 15 countries in the Circum-Pacific belt and explore each territory’s risks to being a part of The Ring of Fire. Explain why it’s called The Ring of Fire and write its importance. You can also discuss the most dangerous volcano within the ring.

If talking about volcanoes as a whole seems too generic, focus on one aspect of it. Lahar is a mixture of water, pyroclastic materials, and rocky debris that rapidly flows down from the slopes of a volcano. First, briefly define a lahar in your essay and focus on how it forms. Then, consider its dangers to living things. You should also add lahar warning signs and the best way to escape it.

Use this prompt to learn and write the entire process of a volcanic eruption. Find out the equipment or operations professionals use to detect magma’s movement inside a volcano to signal that it’s about to blow up. Make your essay informative, and use data from reliable sources and documentaries to ensure you only present correct details.

If you don’t have any personal experience with volcanic eruptions, you can interview someone who does. To ensure you can collect all the critical points you need, create a questionnaire beforehand. Take care to ask about their feelings and thoughts on the situation.

Write about the common effects of volcanic eruptions at the beginning of your essay. Next, focus on discussing its psychological effects on the victims, such as those who have lost loved ones, livelihoods, and properties.

Help your readers prepare for disasters in an informative essay. List what should be done before, during, and after a volcanic eruption. Include relevant tips such as being observant to know where possible emergency shelters are. You can also add any assistance offered by the government to support the victims.Here’s a great tip: Proper grammar is critical for your essays. Grammarly is one of our top grammar checkers. Find out why in this Grammarly review .

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Essay On The Volcano – 10 Lines, Short & Long Essay For Kids

Key Points To Remember When Writing An Essay On The Volcano For Lower Primary Classes

10 lines on the volcano for kids, a paragraph on the volcano for children, short essay on volcano in 200 words for kids, long essay on volcano for children, interesting facts about volcanoes for children, what will your child learn from this essay.

A volcano is a mountain formed through an opening on the Earth’s surface and pushes out lava and rock fragments through that. It is a conical mass that grows large and is found in different sizes. Volcanoes in Hawaiian islands are more than 4000 meters above sea level, and sometimes the total height of a volcano may exceed 9000 meters, depending on the region it is found. Here you will know and learn how to write an essay on a volcano for classes 1, 2 & 3 kids. We will cover writing tips for your essay on a volcano in English and some fun facts about volcanoes in general.

Volcanoes are formed as a result of natural phenomena on the Earth’s surface. There are several types of volcanoes, and each may emit multiple gases. Below are some key points to remember when writing an essay on a volcano:

Start with an introduction about how volcanoes are formed. How they impact the Earth, what they produce, and things to watch out for.
Discuss the different types of volcanoes and talk about the differences between them.
Cover the consequences when volcanoes erupt and the extent of the damage on Earth.
Write a conclusion paragraph for your essay and summarise it.

When writing a few lines on a volcano, it’s crucial to state interesting facts that children will remember. Below are 10 lines on volcanoes for an essay for classes 1 & 2 kids.

Some volcanoes erupt in explosions, and then some release magma quietly.
Lava is hot and molten red in colour and cools down to become black in colour.
Hot gases trapped inside the Earth are released when a volcano erupts.
A circle of volcanoes is referred to as the ‘Ring of Fire.’
Volcano formations are known as seismic activities.
Active volcanoes are spread all across the earth.
Volcanoes can remain inactive for thousands of years and suddenly erupt.
Most volcanic eruptions occur underwater and result from plates diverging from the margins.
Volcanic hazards happen in the form of ashes, lava flows, ballistics, etc.
Volcanic regions have turned into tourist attractions such as the ones in Hawaii.

Volcanoes can be spotted at the meeting points of tectonic plates. Like this, there are tons of interesting facts your kids can learn about volcanoes. Here is a short paragraph on a volcano for children:

A volcano can be defined as an opening in a planet through which lava, gases, and molten rock come out. Earthquake activity around a volcano can give plenty of insight into when it will erupt. The liquid inside a volcano is called magma (lava), which can harden. The Roman word for the volcano is ‘vulcan,’ which means God of Fire. Earth is not the only planet in the solar system with volcanoes; there is one on Mars called the Olympus Mons. There are mainly three types of volcanoes: active, dormant, and extinct. Some eruptions are explosive, and some happen as slow-flowing lava.

Small changes occur in volcanoes, determining if the magma is rising or not flowing enough. One of the common ways to forecast eruptions is by analysing the summit and slopes of these formations. Below is a short essay for classes 1, 2, & 3:

As a student, I have always been curious about volcanoes, and I recently studied a lot about them. Do you know? Krakatoa is a volcano that made an enormous sound when it exploded. Maleo birds seek refuge in the soil found near volcanoes, and they also bury their eggs in these lands as it keeps the eggs warm. Lava salt is a popular condiment used for cooking and extracted from volcanic rocks. And it is famous for its health benefits and is considered superior to other forms of rock or sea salts. Changes in natural gas composition in volcanoes can predict how explosive an eruption can be. A volcano is labelled active if it constantly generates seismic activity and releases magma, and it is considered dormant if it has not exploded for a long time. Gas bubbles can form inside volcanoes and blow up to 1000 times their original size!

Volcanic eruptions can happen through small cracks on the Earth’s surface, fissures, and new landforms. Poisonous gases and debris get mixed with the lava released during these explosions. Here is a long essay for class 3 kids on volcanoes:

Lava can come in different forms, and this is what makes volcanoes unique. Volcanic eruptions can be dangerous and may lead to loss of life, damaging the environment. Lava ejected from a volcano can be fluid, viscous, and may take up different shapes.

When pressure builds up below the Earth’s crust due to natural gases accumulating, that’s when a volcanic explosion happens. Lava and rocks are shot out from the surface to make room on the seafloor. Volcanic eruptions can lead to landslides, ash formations, and lava flows, called natural disasters. Active volcanoes frequently erupt, while the dormant ones are unpredictable. Thousands of years can pass until dormant volcanoes erupt, making their eruption unpredictable. Extinct volcanoes are those that have never erupted in history.

The Earth is not the only planet in the solar system with volcanoes. Many volcanoes exist on several other planets, such as Mars, Venus, etc. Venus is the one planet with the most volcanoes in our solar system. Extremely high temperatures and pressure cause rocks in the volcano to melt and become liquid. This is referred to as magma, and when magma reaches the Earth’s surface, it gets called lava. On Earth, seafloors and common mountains were born from volcanic eruptions in the past.

What Is A Volcano And How Is It Formed?

A volcano is an opening on the Earth’s crust from where molten lava, rocks, and natural gases come out. It is formed when tectonic plates shift or when the ocean plate sinks. Volcano shapes are formed when molten rock, ash, and lava are released from the Earth’s surface and solidify.

Types Of Volcanoes

Given below various types of volcanoes –

1. Shield Volcano

It has gentle sliding slopes and ejects basaltic lava. These are created by the low-viscosity lava eruption that can reach a great distance from a vent.

2. Composite Volcano (Strato)

A composite volcano can stand thousands of meters tall and feature mudflow and pyroclastic deposits.

3. Caldera Volcano

When a volcano explodes and collapses, a large depression is formed, which is called the Caldera.

4. Cinder Cone Volcano

It’s a steep conical hill formed from hardened lava, tephra, and ash deposits.

Causes Of Volcano Eruptions

Following are the most common causes of volcano eruptions:

1. Shifting Of Tectonic Plates

When tectonic plates slide below one another, water is trapped, and pressure builds up by squeezing the plates. This produces enough heat, and gases rise in the chambers, leading to an explosion from underwater to the surface.

2. Environmental Conditions

Sometimes drastic changes in natural environments can lead to volcanoes becoming active again.

3. Natural Phenomena

We all understand that the Earth’s mantle is very hot. So, the rock present in it melts due to high temperature. This thin lava travels to the crust as it can float easily. As the area’s density is compromised, the magma gets to the surface and explodes.

How Does Volcano Affect Human Life?

Active volcanoes threaten human life since they often erupt and affect the environment. It forces people to migrate far away as the amount of heat and poisonous gases it emits cannot be tolerated by humans.

Here are some interesting facts:

The lava is extremely hot!
The liquid inside a volcano is known as magma. The liquid outside is called it is lava.
The largest volcano in the solar system is found on Mars.
Mauna Loa in Hawaii is the largest volcano on Earth.
Volcanoes are found where tectonic plates meet and move.

Your child will learn a lot about how Earth works and why volcanoes are classified as natural disasters, what are their types and how they are formed.

Now that you know enough about volcanoes, you can start writing the essay. For more information on volcanoes, be sure to read and explore more.

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Essay on Volcano

Students are often asked to write an essay on Volcano in their schools and colleges. And if you’re also looking for the same, we have created 100-word, 250-word, and 500-word essays on the topic.

Let’s take a look…

100 Words Essay on Volcano

What is a volcano.

A volcano is a crack in the Earth’s surface. Through this crack, melted rock, ash, and gases can escape from deep inside the Earth. Think of it like a soda bottle. If you shake it and then open the top, everything rushes out. That’s similar to what happens during a volcanic eruption.

Types of Volcanoes

There are mainly three types: shield, cone, and composite. Shield volcanoes are broad and flat. Cone volcanoes are steep and pointy. Composite volcanoes are tall and can be very explosive. Each type acts differently when it erupts.

Why Do Volcanoes Erupt?

Deep inside the Earth, it’s so hot that rocks melt into liquid called magma. When magma is lighter than the rock around it, it moves up. If it reaches the Earth’s surface, it erupts. This can happen because of the Earth’s plates moving and creating pressure.

Living with Volcanoes

People live near volcanoes for the fertile soil, which is good for farming. But, living close to a volcano can be dangerous. Scientists help by monitoring volcanoes to predict eruptions and keep people safe.

250 Words Essay on Volcano

A volcano is a crack in the Earth’s surface where molten rock, ash, and gases from deep inside the Earth come out. Think of it like a soda bottle that’s been shaken up. When you open the cap, everything rushes out because of the pressure. In the same way, when a volcano erupts, it releases pressure from beneath the Earth’s crust.

There are different kinds of volcanoes, mainly based on their shape and how often they erupt. Some are called shield volcanoes because they’re broad and low, like a warrior’s shield. Others are called stratovolcanoes, which are tall and steep. They usually have more explosive eruptions. Then there are cinder cone volcanoes, which are smaller and made of bits of rock and ash.

Volcanoes erupt because of the movement of tectonic plates, which are big pieces of the Earth’s surface. When these plates move, they can cause magma from deep inside the Earth to push its way up to the surface. This magma then becomes lava when it comes out of the volcano.

The Impact of Volcanoes

Volcanoes can be dangerous, destroying homes and forests with their lava flows and ash. But they also create new land and bring important nutrients to the soil, which can help plants grow. Plus, the gases they release into the atmosphere can affect the Earth’s climate.

Understanding volcanoes helps us prepare for their eruptions and appreciate the powerful forces that shape our planet.

500 Words Essay on Volcano

Volcanoes: nature’s fiery breath.

Volcanoes are fascinating natural wonders that capture our imaginations. These colossal mountains showcase the immense power of nature, capable of awe-inspiring eruptions and destruction. Let’s explore the world of volcanoes and delve into some of their most intriguing aspects.

A Peek Inside a Volcano

Imagine a giant underground chamber filled with molten rock, known as magma. This magma is incredibly hot, and it’s constantly pushing against the Earth’s crust. When the pressure becomes too intense, it finds a way to escape, and that’s when a volcano erupts.

Types of Volcanic Eruptions

There are various types of volcanic eruptions, each with its own characteristics. Some eruptions are explosive, sending ash and lava soaring high into the air. Others are more gentle, with lava flowing slowly out of the volcano. Some eruptions produce glowing clouds of ash, called pyroclastic flows, which can race down the volcano’s slopes at high speeds.

Volcanic Hazards

While volcanoes can be a sight to behold, they also pose potential hazards. Lava flows can destroy entire villages and forests, and ash clouds can disrupt air travel. Volcanic eruptions can also trigger earthquakes, landslides, and tsunamis.

Predicting Volcanic Eruptions

Volcanoes and the environment.

Volcanic eruptions can have both positive and negative impacts on the environment. On the one hand, they can release harmful gases and ash into the atmosphere, which can affect air quality and climate. On the other hand, volcanic eruptions can create new landforms, provide fertile soil for agriculture, and support unique ecosystems.

Conclusion: The Majestic Force of Nature

Volcanoes are a powerful reminder of the Earth’s dynamic nature. They can be both destructive and awe-inspiring, showcasing the incredible forces that shape our planet. By studying volcanoes, we can better understand the Earth’s processes and prepare for potential hazards, while still appreciating their majestic beauty.

Apart from these, you can look at all the essays by clicking here .

Volcanoes and Their Characteristics Essay

To find inspiration for your paper and overcome writer’s block
As a source of information (ensure proper referencing)
As a template for you assignment

Works Cited

Volcanoes always presented a broad area for researches in terms of their close relationship between their forms, structures, the styles of their eruption, and the mineral composition of their magma and lava. In the following paper, different types of volcanic mountains will be examined and compared in order to make conclusions concerning the relationship between the volcanoes’ structure and their features. Generally, after evaluating data, it appears that depending on the process of volcano formation it may be related to one of the four existing volcano types which are known for their different characteristics and “conduct” while eruptions.

First of all, speaking about different types of volcanoes’ structure, it should be mentioned that depending on their geographic location and, thus, appurtenance to a certain lithosphere platform with varied features, volcanoes demonstrate different ways of eruption ranging from the so-called “quiet” ones to the very disastrous and dangerous (Furniss 56). In general, volcanoes are subdivided into the following categories: shield volcanoes, composite volcanoes, lava domes or plug domes, and, finally, cinder cones which can be distinguished by their sizes, forms, and functioning. All of these volcano types have their peculiarities in structure and way of eruption.

In addition, the very notion of “volcano” is to be explained. A volcano is a geological landform that is made up of two parts – the upper one called a cone, and the lower one called fissure where volcanic material is accumulated (Hess 391). According to Furniss (57),

The vast majority of volcanoes are found along the boundaries of tectonic plates. Convergent boundaries, where the plates are crashing into each other, host around 90 percent of what we generally think of as volcanoes. Here, as one plate is pushed below another–a process is known as subduction–it melts, the resultant magma rising and causing volcanic eruptions. Volcanoes also form away from plate boundaries, above so-called volcanic hotspots.

Volcanoes are known for their disastrous nature which many times led to serious tragedies for humanity. During volcanoes’ eruptions, numerous developing processes in the crust of the earth are taking place. According to Furniss (58),

Several factors are used to assign eruptions a score, including the volume of erupted material, the height of the eruption column, and the duration. The index ranges between zero and eight, with each increase in score representing a ten-fold increase in the various factors. The highest score on the list, VEI8, is reserved for eruptions that emit more than 1,000 cubic kilometers of material.

Below, all the four types of volcanoes will be addressed along with their features and popularities.

Discussing shield volcanoes, it should be said that they are formed as a result of a huge amount of free lava spilling from a vent and coming up abundantly and widely; gradually congealing lava forms a low and wide mountain of dome shape which is called a shield volcano. These types of volcanoes can be very high. Among them are popular volcanoes in Hawaii.

With regards to composite volcanoes, it should be stated that they are formed as a result of the eruption of both lava and tephra which occurs from a central vent. At the end of the erupting process lava and tephra form a cone in the shape of a tower which is called a composite volcano. These volcanoes tend to develop into mountains of beautiful and symmetrical forms. Among the most famous volcanoes of this type are Mount Fuji in Japan and Mount Rainer in Washington (Hegner 88).

Addressing plug domes, it should be stated that they are formed as a result of congealing of very viscous lava (for example, rhyolite one) which is too thick to flow at a remote distance. As a result of this process, the mountain grows from below and from within. This mountain is called a plug dome. The volcanoes of this type are rather young and can be found in numerous parts of the earth including Mono Lake in California.

And finally, speaking about cinder cones, it should be stated that they are formed as a result of tephra’s building up. A cone-shaped mountain that is the result of this process is called a cinder cone volcano. These volcanic mountains are the smallest ones of all types and are generally less in their size than 500 meters. An example of such a volcanic mountain is SP Mountain situated in Arizona, and belonging to the Colorado Plato.

Concluding on all the information related above, it should be stated that there exists a close connection between the structure of any particular volcano and the style of its functioning and its magma and lava nature. Evaluating a row of facts about the functioning of different types of volcanoes, it appears that depending on the process of volcano formation it may be related to one of the four existing volcano types which are known for their different characteristics and “conduct”. In general, volcanoes are subdivided into the following categories: shield volcanoes, composite volcanoes, lava domes or plug domes, and, finally, cinder cones.

Furniss, Charlie. “Volcanoes: They’re the Most Powerful Expressions of Nature’s Might, Responsible for Mass Extinctions, Global Climate Change, and the Demise of Entire Civilisations, but How Much Do We Really Know About Volcanoes? and Just How Close Are We to the Holy Grail of Accurately Predicting When They’re Going to Explode?.” Geographical Mar. 2007: 52+. Questia . Web.

Hegner, E., et al. “Testing Tectonic Models with Geochemical Provenance Parameters in Greywacke.” Journal of the Geological Society (2005): 87+. Questia . Web.

Hess, Darrel. Physical Geography Laboratory Manual (10th ed.), The United States: Prentice-Hall, 2010. Print.

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125 Volcano Essay Topic Ideas & Examples

Inside This Article

Volcanoes are one of the most fascinating natural phenomena on Earth. From their explosive eruptions to the creation of new land, there is no shortage of topics to explore when it comes to volcanoes. If you are looking for inspiration for your next essay on volcanoes, look no further. Here are 125 volcano essay topic ideas and examples to get you started:

The science behind volcanic eruptions
The different types of volcanoes and how they form
The role of plate tectonics in volcanic activity
The impact of volcanic eruptions on the environment
The history of volcanic eruptions around the world
The cultural significance of volcanoes in different societies
The relationship between volcanoes and climate change
The benefits of living near a volcano
The risks and challenges of living near a volcano
The technology used to monitor and predict volcanic eruptions
The impact of volcanic eruptions on air travel
The economic impact of volcanic eruptions on local communities
The history of volcano research and exploration
The myths and legends surrounding volcanoes
The impact of volcanic eruptions on wildlife and ecosystems
The role of volcanoes in the formation of Earth's landforms
The connection between volcanic activity and earthquakes
The role of volcanic eruptions in shaping human history
The impact of volcanic eruptions on agriculture and food security
The relationship between volcanic eruptions and volcanic gases
The role of volcanoes in the formation of mineral deposits
The impact of volcanic eruptions on water quality
The connection between volcanic eruptions and geothermal energy
The impact of volcanic eruptions on tourism
The history of famous volcanic eruptions, such as Mount Vesuvius and Mount St. Helens
The impact of volcanic eruptions on indigenous communities
The role of volcanoes in the creation of new land
The impact of volcanic eruptions on global climate
The relationship between volcanic eruptions and volcanic lightning
The role of volcanoes in the formation of volcanic islands
The impact of volcanic eruptions on human health
The connection between volcanic activity and volcanic hotspots
The role of volcanic eruptions in the formation of volcanic ash clouds
The impact of volcanic eruptions on marine ecosystems
The relationship between volcanic eruptions and volcanic mudflows
The role of volcanic eruptions in the formation of volcanic craters
The impact of volcanic eruptions on infrastructure and buildings
The connection between volcanic activity and volcanic earthquakes
The role of volcanic eruptions in the formation of volcanic domes
The impact of volcanic eruptions on air quality
The relationship between volcanic eruptions and volcanic gas emissions
The role of volcanoes in the formation of volcanic arcs
The impact of volcanic eruptions on soil fertility
The connection between volcanic activity and volcanic tremors
The role of volcanic eruptions in the formation of volcanic calderas
The impact of volcanic eruptions on water resources
The relationship between volcanic eruptions and volcanic tsunamis
The role of volcanoes in the formation of volcanic rift zones
The impact of volcanic eruptions on wildlife migration patterns
The connection between volcanic activity and volcanic vents
The role of volcanic eruptions in the formation of volcanic cones
The impact of volcanic eruptions on indigenous knowledge and traditions
The relationship between volcanic eruptions and volcanic seamounts
The role of volcanoes in the formation of volcanic craters
The impact of volcanic eruptions on human migration patterns
The connection between volcanic activity and volcanic eruptions
The role of volcanic eruptions in the formation of volcanic plateaus
The impact of volcanic eruptions on geothermal energy production
The relationship between volcanic eruptions and volcanic islands
The role of volcanoes in the formation of volcanic plains
The impact of volcanic eruptions on soil erosion
The connection between volcanic activity and volcanic rocks
The role of volcanic eruptions in the formation of volcanic ridges
The impact of volcanic eruptions on biodiversity
The relationship between volcanic eruptions and volcanic vents
The role of volcanoes in the formation of volcanic ridges
The impact of volcanic eruptions on human settlements
The connection between volcanic activity and volcanic rifts
The role of volcanic eruptions in the formation of volcanic chains
The relationship between volcanic eruptions and volcanic cones
The role of volcanoes in the formation of volcanic fields
The impact of volcanic eruptions on wildlife habitats
The role of volcanic eruptions in the formation of volcanic belts
The impact of volcanic eruptions on cultural heritage sites
The relationship between volcanic eruptions and volcanic ash clouds
The impact of volcanic eruptions on agricultural productivity
The role of volcanic eruptions in the formation of volcanic arches
The relationship between volcanic eruptions and volcanic fissures
The role of volcanoes in the formation of volcanic calderas
The connection between volcanic activity and volcanic domes
The role of volcanic eruptions in the formation of volcanic rift zones
The connection between volcanic activity and volcanic plateaus
The connection between volcanic activity and volcanic islands
The role of volcanic eruptions in the formation of volcanic plains
The relationship between volcanic eruptions and volcanic rocks

With so many interesting topics to choose from, you are sure to find the perfect subject for your volcano essay. Whether you are interested in the science behind volcanic eruptions, the cultural significance of volcanoes, or the impact of volcanic activity on the environment, there is a topic on this list that is sure to inspire you. Happy writing!

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Home — Essay Samples — Environment — Natural Disasters — The Environmental Effects of Volcanoes: A Comprehensive Analysis

The Environmental Effects of Volcanoes: a Comprehensive Analysis

Categories: Natural Disasters

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Published: Mar 6, 2024

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Air pollution and climate change, positive effects of volcanic activity, mitigating the environmental impacts of volcanoes.

The release of gases and particulate matter into the atmosphere
Global cooling caused by volcanic ash blocking out sunlight
Contribution to air pollution and respiratory problems
Impact on climate change through the release of carbon dioxide
Creation of new landmasses supporting new ecosystems
Increased biodiversity on newly formed landmasses
Contribution to the formation of mineral deposits
Use of volcanic ash and rocks for various purposes
Close monitoring of volcanic activity to predict eruptions
Development of effective mitigation strategies
Reducing reliance on fossil fuels and supporting renewable energy sources
Protection of vulnerable ecosystems and biodiversity

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How to Write a Report on Volcanoes

Facts on Volcanology

Geology reports don't have to lull readers to dreamland when you explain how a natural force can explode with more power than an atomic bomb, obliterate most of an island, change the weather and hurl shock waves around the globe. These are some of the incredible effects your report can describe when you discuss volcanoes -- one of Earth's most powerful forces.

Why Volcanoes Exist

Pressure causes a multitude of physical actions to occur. Combine heat and pressure and you may create a volcano. Begin your report by explaining how magma -- hot, liquid rock below the earth -- rises because its density is less than the density of the surrounding rocks. The distance the magma moves vertically depends on factors such as the mass of the rocks it must go through and its density. Under intense pressure, dissolved gas in the magma helps propel it upward where it can make it to the surface and into the air depending on the volcano's type. Geologists call magma "lava" when it leaves a volcano via an eruption or vent.

Define a Volcano's Status

According to the Global Volcanism Program, an extinct volcano is one people don't expect to erupt again, while an active volcano is one that has erupted in the last 10,000 years. Place these important facts into your report along with the definition of dormant: a volcano expected to erupt one day, but which hasn't in the last 10,000 years.

Not All Volcanoes Go "BOOM!"

Talk about various types of volcanoes, such as Mt. St. Helens, a powerful stratovolcano that explodes with fury, hurling gas, rocks and ash high into the air. Shield volcanoes like Hawaii's Kilauea don't erupt as violently -- they create rivers of lava that flow down the mountainside. Because the lava in shield volcanoes has low viscosity, they erupt less violently, creating gentle slopes around the mountain. Stratovolcanoes have high-viscosity lava, causing them to erupt more violently and form steep-sided slopes. Magma can also flow from fractures in a volcano without causing an explosive eruption -- scientists call this a "curtain of fire."

Location, Location, Location

You don't see too many volcanoes around the neighborhood because they only form in certain places -- including under water. Submarine volcanoes sit an average of 2,600 meters (8,500 feet) below the oceans. According to some theories, over a million submarine volcanoes dot the ocean floor. The continents rest on tectonic plates in motion below the planet's surface. Explain how you find most volcanoes in places where these plates move away from one another at divergent plate boundaries, or towards one another at convergent plate boundaries. Hot spots, such as the one beneath Iceland, also create volcanoes. A hot spot is a location where magma has made its way through the Earth's crust.

How Volcanoes Affect the World

Krakatoa erupted with fury in 1883, flinging ash up to 80 kilometers (49.7 miles) into the air, which lowered Earth's temperatures until 1888. The eruption also created a shock wave that circled the Earth seven times and triggered a massive tsunami that killed over 36,000 people. Lava flows are always a concern when volcanoes cause them near populated areas. Explain how lava usually moves too slowly to engulf people, but pyroclastic flows can travel down volcano slopes at up to 200 kilometers (124.3 feet) per hour. Composed of ash and hot gas, these flows kill anything in their path. On the positive side, tell your readers how volcanoes can create new islands, produce fertile soil, and produce pumice and other useful products.

What happens after volcanoes erupt, facts on & causes of volcanoes, facts on volcano eruptions for kids, how do volcanoes cause erosion, characteristics of composite volcanoes, how does a stratovolcano erupt, what are the types of volcanic explosions, background information for a volcano science project, stages of a volcano eruption, what are the results of a volcano eruption, effects of mauna loa's eruptions, three types of stress on the earth's crust, what forms when two continental plates collide, forces that cause landforms, composite volcano facts for kids, mauna loa facts for kids, types of volcanoes and their characteristics, volcanoes & their types of eruptions, how do earthquake activities influence the formation....

Cornell University Earth and Atmospheric Sciences: Discover Volcanoes
Geology.com: Plate Tectonics and the Hawaiian Hot Spot
Volcano - When a Mountain Explodes; Linda Barr
Tulane University: Volcanoes and Volcanic Eruptions
Oregon State University Volcano World: Historical Eruption Sounds
Natural Disasters; Lee Davis

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Essay on volcanoes | geology.

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After reading this article you will learn about:- 1. Introduction to Volcanoes 2. Volcano Formation 3. Volcanic Landforms 4. Major Gases Emitted by Volcanoes 5. Lightning and Whirlwinds 6. Features Produced by the Escape of Gases from Volcanic Lavas 7. Volcanic Products 8. Source of the Explosive Energy 9. Classification of Pyroclastics 10. Lahars-Mudflows on Active and Inactive Cones and Other Details.

Essay Contents:

Essay on the Volcanoes and Atmospheric Pollution

Essay # 1. Introduction to Volcanoes :

A volcano is a cone shaped hill or mountain which is built-up around an opening in the earth’s surface through which hot gases, rock fragments and lavas are ejected.

Due to the accumulation of the solid fragments around the conduit a conical mass is built which increases in size to become a large volcanic mountain. The conical mass so built-up is called a volcano. However the term volcano is taken to include not only the central vent in the earth but also the mountain or hill built around it.

Volcanoes are in varying sizes, varying from small conical hills to loftiest mountains on the earth’s surface. The volcanoes of the Hawaiian Islands are nearly 4300 metres above sea level since they are built over the floor of the Pacific ocean which at the site is 4300 to 5500 metres deep, the total height of the volcano may be about 9000 m or more.

The very high peaks in the Andes, in the Cascade Range of the Western United States, Mt. Baker, Mt. Adams, Mt. Hood etc. are all volcanoes which have now become extinct. Over 8000 independent eruptions have been identified from earth’s volcanoes. There are many inaccessible regions and ocean floors where volcanoes have occurred undocumented or unnoticed.

The eruption of a volcano is generally preceded by earthquakes and by loud rumblings like thunder which may continue on a very high scale during the eruption. The loud rumblings are due to explosive movement of gases and molten rock which are held under very high pressure. Before eruption of a volcano fissures are likely to be opened, nearby lakes likely to be drained and hot springs may appear at places.

The eruptive activity of volcanoes is mostly named after the well-known volcanoes, which are known for particular type of behaviour, like Strambolian, Vulcanian, Vesuvian, Hawaiian types of eruption. Volcanoes may erupt in one distinct way or may erupt in many ways, but, the reality is, these eruptions provide a magical view inside the earth’s molten interior.

The nature of a volcanic eruption is determined largely by the type of materials ejected from the vent of the volcano. Volcanic eruptions may be effusive (fluid lavas) or dangerous and explosive with blasts of rock, gas, ash and other pyroclasts.

Some volcanoes erupt for just a few minutes while some volcanoes spew their products for a decade or more. Between these two main types viz. effusive and explosive eruptions, there are many subdivisions like, eruption of gases mixed with gritty pulverised rock forming tall dark ash clouds seen for many kilometres, flank fissure eruptions with lava oozing from long horizontal cracks on the side of a volcano.

There is also the ground hugging lethally hot avalanches of volcanic debris called pyroclastic flows. When magma rises, it may encounter groundwater causing enormous phreatic, i.e., steam eruptions. Eruptions may also release suffocating gases into the atmosphere. Eruptions may produce tsunamis and floods and may trigger earthquakes. They may unleash ravaging rockslides and mudflows.

Volcanoes which have had no eruptions during historic times, but may still show fairly fresh signs of activity and have been active in geologically recent times are said to be dormant. There are also volcanoes which were formerly active but are of declining activity a few of which may be emitting only steam and other gases.

Geysers are hot springs from which water is expelled vigorously at intervals and are characteristics of regions of declining volcanic activity. Geysers are situated in Iceland, the Yellowstone park in USA and in New Zealand.

In contrast to the explosive type of volcanoes, there exist eruptions of great lava flows quietly pouring out of fissures developed on the earth’s surface. These eruptions are not accompanied by explosive outbursts. These are fissure eruptions.

Ex: Deccan Trap formations in India. The lavas in these cases are mostly readily mobile and flow over low slopes. The individual flows are seldom over a few meters in thickness; the average thickness may be less than 15 meters. If the fissure eruptions have taken place in valleys however, the thickness may be much greater.

A noteworthy type of volcano is part of the world encircling mid-ocean ridge (MOR) visible in Iceland. The MOR is really a single, extremely long, active, linear volcano, connecting all spreading plate boundaries through all oceans. Along its length small, separate volcanoes occur. The MOR exudes low-silica, highly fluid basalt producing the entire ocean floor and constituting the largest single structure on the face of the earth.

Essay # 2. Location of Volcanoes:

Volcanoes are widely distributed over the earth, but they are more abundant in certain belts. One such belt encircles the Pacific ocean and includes many of the islands in it. Other volcanic areas are the island of West Indies, those of the West coast of Africa, the Mediterranean region and Iceland.

Most volcanoes occur around or near the margins of the continents and so these areas re regarded as weak zones of the earth’s crust where lavas can readily work their way upward. There are over 400 active volcanoes and many more inactive ones. Numerous submarine volcanoes also exist.

Since it is not possible to examine the magma reservoir which fees a volcano our information must be obtained by studying the material ejected by the volcano. This material consists of three kinds of products, viz. liquid lava, fragmented pyroclasts and gases. There may exist a special problem in studying the gases, both in collecting them under hazardous conditions or impossible conditions.

It may also be difficult to ascertain that the gases collected are true volcanic gases and are not contaminated with atmospheric gases. Investigation of the composition of extruded rock leads to a general, although not very detailed, correlation between composition and intensity of volcanic eruption.

In general, the quite eruptions are characteristic of those volcanoes which emit basic or basaltic lavas, whereas the violent eruptions are characteristic of volcanoes emitting more silicic rocks.

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Essay # 3 . formation of volcanoes :.

The term volcano is used to mean both the opening in the earth’s crust, i.e. the vent through which the eruption of magma occurs as well as the hill built- up by the erupted material. Volcanoes occur where the cracks in the earth’s crust lead to the magma chamber.

The liquid magma which is lighter than the surrounding rocks is under high pressure is pushed up towards the surface through these cracks. In this process the gases dissolved in the magma which expand are released providing an upward push to the magma.

As the magma gets closer to the surface, due to the reducing confining pressure to overcome, the magma and the gases flow faster. The magma, depending on its viscosity may quietly pour to the surface in the form of a flood of molten rock or it may explosively spurt out the molten rock to considerable heights as showers on the surrounding region with solid rock fragments and globs of molten rock. The liquid magma discharged to the surface is called lava.

Essay # 4 . Volcanic Landforms :

Many surface features of volcanic origin are created. These features range from towering peaks and huge lava sheets to small and low craters. The features created by a volcano vary depending on the type of eruption, the material erupted and the effects of erosion.

Four types of volcanic landforms are formed:

i. Ash and Cinder Cones or Explosion Cones:

These appear where explosive eruptions take place. When very hot solid fragments from a central crater (or a subsidiary crater) are ejected. A concave cone of height not exceeding 300 m is formed.

ii. Lava Cones:

These are formed from slowly upwelling lava.

These are of two types:

(a) Steep Sided Volcanoes:

These are formed from sticky acid lava which gets hardened quickly. The highly viscous lava which is squeezed out makes spines like tower.

(b) Shield Volcanoes:

These show gently sloping dome features. These are formed from runny lava which flows long distances, before getting hardened.

iii. Composite Cones or Strato-Volcanoes or Strato Cones:

These volcanoes have concave cone shaped sides of alternating ash and lava layers. These are common in most very high volcanoes. In some cases solid lava may plug the main pipe to the crater. Then pent up gases may blast the top off.

When the magma chamber empties, the summit of the volcano collapses. As a consequence, the feature produced is a vast shallow cavity called a Caldera. Strato volcanoes are the accumulated products of many volcanoes. Chemically most of these products are andesite. Some are dacite and a few are basalt and rhyolite. Due to this chemical mix and characteristic interlayering of lava flows, this volcano is called strato volcano.

iv. Shield Volcanoes:

When a volcano vent produces many successive basaltic lava flows stacked one on top of another in eruptive order, the resulting landform is called a shield volcano. A cinder cone and its associated lava flow can be thought of as the initial building blocks of a shield volcano.

A cinder cone is monogenetic because it forms from a single short-lived eruption (of a few years to a decade or two in duration). In contrast, a shield volcano that is an accumulation of the products of many eruptions over a period of say thousands to hundreds of thousands of years is polygenic.

On land these volcanoes have low angle cones. When they form under water they start with a steeper shape because the lava freezes much faster and does not travel far. The shape fattens to the shield form as the cone builds above the sea level.

v. Plateau Basalts or Lava Plains:

These form the bulk of many volcanic fields. These are features which occur where successive flows of basic lava leaks through fissures, over land surface and then cools and hardens forming a blanket-like feature.

The surface appearance of a flow provides information on the composition and temperature of the magma before it solidified. Very hot low viscosity basalt flows far and fast and produces smooth ropy surfaces. Cooler and less-fluid basalt flows form irregular, jagged surfaces littered with blocks.

The lava flows have blanketed to about 2000 m thickness covering 6,50,000 sq.km. in the Indian Deccan Plateau. Such lava flows have also created the U.S. Columbia River Plateau, the Abyssinian Plateau, the Panama Plateau of South America and the Antrim Plateau of Northern Ireland.

Magmas like dacite and rhyolite that have high silica contents are cooler and more viscous than basalt and hence they do not flow far resulting in the features, lobes, pancakes and domes. Domes often plug up the vent from which they issued, sometimes creating catastrophic explosions and may create a crater.

Eroded volcanoes have their importance. They give us a glimpse of the interior plumbing along which the magma rose to the surface. At the end of an eruption, magma solidifies in the conduits along which it had been rising. The rock so formed is more resistant than the shattered rock forming the walls and hence these lava filled conduits are often left behind when the rest of the volcano has been eroded away.

The filling of the central vertical vent is somewhat circular in section and forms a spire called a neck. The filling of cracks along which lava rose forms nearly vertical tabular bodies called dikes. Sometimes magma works its way along cracks that are nearly horizontal, often along bedding planes of sedimentary rocks. This results in the formation of table-like bodies called sills.

Essay # 5 . Major Gases Emitted by Volcanoes :

Volcanic gases present within the magma are released as they reach the earth’s surface, escaping at the major volcanic opening or from fissures and vents along the side of the volcano. The most prevalent gases emitted are steam, carbon dioxide and hydrogen sulphide. Carbon dioxide is an invisible, odourless poisonous gas. The table below shows the gases emitted from volcanoes.

Essay # 6 . Lightning and Whirlwinds :

Lightning flashes accompany most volcanic eruptions, especially those involving dust. The cause of this lightning is believed to be either contact of sea water with magma or generation of static electricity by friction between colliding particles carried in the erupting gases. Lightning is characteristic of vulcanian eruptions and is common during glowing avalanches.

Whirlwinds are seen during many volcanic eruptions. They are seen above hot lavas. Sometimes they form inverted cones extending a little below the eruption cloud. Energy for the whirlwinds might be from the hot gases and lava, high velocity gas jets in the eruption, heat released into the atmosphere during falls of hot tephra or where lava flows into the sea creating steam.

Essay # 7 . Features Produced by the Escape of Gases from Volcanic Lavas :

The gases of volcanic lavas produce several interesting features while they escape. They expand in the lava of the flow and thus cause the formation of Scoriaceous and Pumiceous rocks. By their explosion, they blow the hardened lava above them in the conduit, into bits and thus produce pyroclastic material.

They form clouds above volcanoes, the rain from which assists in the production of mud flows. When the volcano becomes inactive, they escape aiding in the formation of jumaroles, geysers and hot springs. Scoriaceous rocks are extremely porous. They are formed by the expansion of the steam and other gases beneath the hardened crust of a lava. The final escape of the gases from the hardening lava leaves large rounded holes in the rock.

Pumice is a rock also formed by the expansion and escape of gases. In pumice, many of the holes are in the form of long, minute, closed tubes which make the rock so light that it will float on water.

These tubes are formed by the expansive force of large amounts of gases in an extremely viscous lava that cools very rapidly, forming a glassy rock. Pumice is the rock that is usually formed from the lava ejected from explosive volcanoes. It can be blown to kilometres by explosions.

Essay # 8 . Volcanic Products :

Volcanoes give out products in all the states of matter – gases, liquids and solids.

Steam, hydrogen, sulphur and carbon dioxide are discharged as gases by a volcano. The steam let out by a volcano condenses in the air forming clouds which shed heavy rains. Various gases interact and intensify the heat of the erupting lavas. Explosive eruptions cause burning clouds of gas with scraps of glowing lava called nuees ardentes.

The main volcanic product is liquid lava. Sticky acid lava on cooling, solidifies and hardens before flowing long distances. Such lava can also block a vent resulting in pressure build-up which was relieved by an explosion. Basic fluid lava of lesser viscosity flows to great distances before hardening.

Some lava forms are produced by varying conditions as follows. Clinkery block shaped features are produced when gas spurted from sluggish molten rock capped by cooling crust. These are called Aa.

Pahoehoe is a feature which has a wrinkled skin appearance caused by molten lava flowing below it.

Pillow lava is a feature resembling pillows. This feature piles up when fast cooling lava erupts under water.

Products in explosive outbursts are called Pyroclasts. These consist of either fresh material or ejected scraps of old hard lava and other rock. Volcanic bombs include pancake-flat scoria shaped on impacting the ground and spindle bombs which are twisted at ends as they whizzle through the air. Acid lava full of gas formed cavities produces a light volcanic rock.

Pumice which is so light it can float on water. The product Ignimbrite shows welded glassy fragments. Lapilli are hurled out cinder fragments. Vast clouds of dust or very tiny lava particles are called volcanic ash. Volcanic ash mixed with heavy rain creates mudflows.

Sometimes mudflows can bury large areas of land. Powerful explosions can smoother land for many kilometres around with ash and can hurl huge amount of dust into the higher atmosphere. Violent explosions destroy farms and towns, but volcanic ash provides rich soil for crops.

i. Hot springs:

The underground hot rocks heat the spring waters creating hot springs. The hot springs shed minerals dissolved in them resulting in crusts of calcium carbonate and quartz (geyserite).

ii. Smoker:

This is a submarine hot spring at an oceanic spreading ridge. This submarine spring emits sulphides and builds smoky clouds.

iii. Geyser:

Periodically steam and hot water are forced up from a vent by super-heated water in pipe like passage deep down. Famous geysers are present in Iceland and Yellowstone National Park.

iv. Mud volcano:

This is a low mud cone deposited by mud-rich water gushing out of a vent.

v. Solfatara:

This is a volcanic vent which emits steam and sulphurous gas.

vi. Fumarole:

This is a vent which emits steam jets as at Mt. Etna, Sicily and Valley of Ten Thousand smokes in Alaska.

vii. Mofette:

This is a small vent which emits gases including carbon dioxide. These occur in France, Italy and Java.

Various terms used while describing volcanic features are given below:

i. Magma Chamber:

Magma is created below the surface of the earth (at depth of about 60 km) and is held in the magma chamber until sufficient pressure is built-up to push the magma towards the surface.

This is a pipe like passage through which the magma is pushed up from the magma chamber.

This is the outlet end of the pipe. Magma exits out of the vent. If a vent erupts only gases, it is called fumarole.

iv. Crater:

Generally the vent opens out to a depression called crater at the top of the volcano. This is caused due to the collapse of the surface materials.

v. Caldera:

This is a very big crater formed when the top of an entire volcanic hill collapses inward.

When the erupted materials cover the vent, a volcanic dome is created covering the vent. Later as the pressure of gas and magma rises, another eruption occurs shattering the dome.

A mountain-like structure created over thousands of years as the volcanic lava, ash, rock fragments are poured out onto the surface. This feature is called volcanic cone.

viii. Pyroclastic Flow :

A pyroclastic flow (also known as nuee ardentes (French word) is a ground hugging, turbulent avalanche of hot ash. pumice, rock fragments, crystals, glass shards and volcanic gas. These flows can rush down the steep slopes of a volcano at 80 to 160 km/li, burning everything in their path.

Temperatures of these flows can reach over 500°C. A deposit of this mixture is also often referred to as pyroclastic flow. An even more energetic and dilute mixture of searing volcanic gases and rock-fragments is called a pyroclastic surge which can easily ride up and over ridges.

ix. Seamounts :

A spectacular underwater volcanic feature is a huge localized volcano called a seamount. These isolated underwater volcanic mountains rise from 900 m to 3000 m above the ocean floor, but typically are not high enough to poke above the water surface.

Seamounts are present in all the oceans of the world, with the Pacific ocean having the highest concentration. More than 2000 seamounts have been identified in this ocean. The Gulf of Alaska also has many seamounts. The Axial Seamount is an active volcano off the north coast of Oregon (currently rises about 1400 m above the ocean floor, but its peak is still about 1200 m below the water surface.

Essay # 9 . Source of the Explosive Energy :

The energy for the explosive violence comes from the expansion of the volatile constituents present in the magma, the gas content of which determines the degree of commination of the materials and the explosive violence of the eruption.

This energy is expanded in two ways, firstly in the expulsion of the materials into the atmosphere and secondly, due to expansion within the magma leading to the development of vesicles. The most important gas is steam, which may form between 60 to 90 per cent of the total gas content in a lava. Carbon dioxide, nitrogen and sulphur dioxide occur commonly and hydrogen, carbon monoxide, sulphur and chlorine are also present.

Essay # 10 . Classification of Pyroclastics :

Pyroclastics refer to fragmental material erupted by a volcano. The larger fragments consisting of pieces of crystal layers beneath the volcano or of older lavas broken from the walls of the conduit or from the surface of the crater are called blocks.

Volcanic bombs are masses of new lava blown from the crater and solidified during flight, becoming round or spindle shaped as they are hurled through the air. They may range in size from small pellets up to huge masses weighing many kilonewtons.

Sometimes they are still plastic when they strike the surface and are flattened or distorted as they roll down the side of the cone. Another type called bread crust bomb resembles a loaf of bread with large gaping cracks in the crust.

This cracking of the crust results from the continued expansion of the internal gases. Many fragments of lava and scoria solidified in flight drop back into the crater and are intermixed with the fluid lava and are again erupted.

In contrast to bombs, smaller broken fragments are lapilli (from Italian meaning, little stones) about the size of walnuts; then in decreasing size, cinders, ash and dust. The cinders and ash are pulverized lava, broken up by the force of rapidly expanding gases in them or by the grinding together of the fragments in the crater, as they are repeatedly blown out and dropped back into the crater after each explosion.

Pumice is a type of pyroclastic produced by acidic lavas if the gas content is so great as to cause the magma to froth as it rises in the chimney of the volcano. When the expansion occurs the rock from the froth is expelled as pumice. Pumice is of size ranging from the size of a marble to 30 cm or more in diameter. Pumice will float in water due to many air spaces formed by the expanding gases.

Lava fountains in which steam jets blow the lava into the air produce a material known as Pele’s hair which is identical with rock wool which is manufactured by blowing a jet of steam into a stream of molten rock (Rock wool is used for many types of insulation).

Coarse angular fragments become cemented to form a rock called volcanic breccia. The finer material like cinders and ash forms thick deposits which get consolidated through the percolation of ground water and is called tuff. Tuff is a building stone used in the volcanic regions. It is soft and easily quarried and can be shaped and has enough strength to be set into walls with mortar.

i. Agglomerate:

The debris in and around the vent contains the largest ejected masses of lava bombs which are embedded in dust and ash. A deposit of this kind is known as agglomerate. The layers of ash and dust which are formed for some distance around the volcano and which builds its cone, become hardened into rocks which are called tuffs.

Ash includes all materials with size less than 4 mm. It is pulverized lava, in which the fragments are often sharply angular and formed of volcanic glass; these angular and often curved fragments are called shards.

Since the gas content of ash on expulsion is high it has considerable mobility on reaching the surface; it is also hot and plastic, the result of these conditions being that the fragments often become welded together. The finest of ash is so light that wind can transport it for great distances.

The table below sets out a general classification of pyroclastic rocks based on the particle size of the fragments forming the rocks.

Essay # 10. Lahars-Mudflows on Active and Inactive Cones :

In addition to violent eruptions, large composite cones may generate a type of mudflow called Lahar (Indonesian name). These destructive mudflows occur when volcanic debris becomes saturated with water and rapidly moves down steep volcanic slopes, generally following gullies and stream valleys.

Some lahars are triggered when large volumes of ice and snow melt during an eruption. Others are generated when heavy rainfall saturates weathered volcanic deposits. Thus can occur even when a volcano is not erupting.

Essay # 11. Cooling of Lava :

Although lava is mostly liquid, it often contains gas, fragments of rock and crystals that formed the magma before eruption. When the flow erupts, the liquid portion of the lava congeals (i.e., becomes thick and sticky) rapidly and traps gas bubbles and solid material in a mass woven of microscopic crystals and glass. Parts of the flow may freeze so rapidly that the liquid quenches to a glass (obsidian).

Volatile constituents, mainly water, carbon dioxide, sulphur dioxide and chlorine form gas bubbles in the congealed lava, leaving spherical, elongate and irregular cavities (vesicles) in the solidified rocks. A high concentration of vesicles makes the rock very light and frothy.

Essay # 12. Features of Lava Flows :

After the lava is poured on to the surface, it spreads out as tongues or sheets which flow over the country side. Often the lava finds its way into stream valleys along which it may extend for many kilometres. Some sheets of lava form great lava plateaus covering thousands of square kilometres.

The movement of the molten lava depends on its composition and its temperature. Stiff viscous acidic lavas solidify before travelling far, while the more fluid basic lavas freely flow for long distances before coming to rest. The speed of a lava flow depends on its viscosity (which depends on temperature and composition) as well as the slope of the surface on which it is flowing.

The upper part of a lava flow, is usually made up of a porous sponge like mass known as scoria. The porous feature is due to the escape of the contained gases or due to the expansion of the gases to form bubbles prior to the freezing of the flow. These bubbles or voids which were filled by gas may get elongated to tube like forms during the forward movement of the viscous lava.

The lava flow surfaces develop into one of two contrasting types viz. pahoehoe and Aa. In the pahoehoe type the surface is smooth and billowy and often molded into forms resembling huge coils of rope. This feature is common in basic lava. In the aa type the flow surface presents a mass of angular, jagged scoriaceous blocks with sharp edges and spiny projections.

i. Lava Tubes:

Once the lava surface has started to harden, the interior of the lava may remain in liquid state and mobile for considerable period of flow. The lava movement is by laminar flow giving rise to layered lava which is common in basaltic flows. A lava tube begins to form when a channel carrying lava becomes crusted over with solid rock, while the still molten lava beneath the crust continues to flow.

As these inner molten lava mass drains a void called a lava tube is formed. Lava tubes which do not completely drain have relatively flat floors made of frozen residual molten lava. Pre-existing lava tubes may be re-occupied by lava from later eruptions.

ii. Lava Tree Moulds:

When trees are buried by lava, their shape is often preserved, though the wood may be completely burned away. The hollows left over are called tree moulds.

iii. Pillow Lava:

When lava flows into water or when lava is erupted under water a special feature known as a pillow lava is commonly formed. The lava chills fast to form a glassy but plastic skin around still liquid lava and rolls along like plastic bags filled with liquid.

The round or sausage shaped bags are known as pillows and are heaped one on another. They have rounded tops but their hases fit into the shape of the underlying surfaces. In most cases, pillow lava is formed in the sea but some pillow lava is also formed in fresh water.

iv. Jointing :

As a lava cools, it shrinks and this results in the formation of joints. These may be irregular in originally pasty masses but are likely to attain geometric regularity in originally wide spread very fluid basalts. Due to these joints very thick (tens of metres) columns may be formed depending on the original thickness of the flow.

v. Vesicles and Amygdales :

Vesicles are small cavities in lava, frozen bubbles of gas. Amygdales are vesicles with secondary minerals such as zeolite, calcite or agate. The diameters of the vesicles range from 1 cm to tens of centimetres. Pipe amygdales are cylindrical and perpendicular to the direction of lava flow, due to the movement over wet ground.

Essay # 13. Fissure Eruptions :

Fissure eruptions represent the simplest form of extrusion in which lavas issue quietly from linear cracks in the ground. These lavas are generally basic and mobile. They have a low viscosity and spread rapidly over large areas. In the past geological times vast floods of basalt (a basic rock) have been poured out over different regions and are attributed to eruptions from fissures.

Among the extensive remains of these basalts at the present day are the Deccan Traps which cover an area of about 1024000 sq.km. in peninsular India and reach a thickness which in places exceeds 1800 m which is built up of lava flow. In general, rapid extrusion of very fluid lava and little explosive activity are characteristic of fissure eruptions.

Essay # 14. Quiet and Violent Volcanoes :

Volcanoes are in various sizes and shapes and their behaviour ranges from a quiet state to violently destructive state. Such diversity in the activity of volcanoes is related to the chemistry of the erupting magma. Chemical compositions that produce thin, easy flowing magmas are associated with non-violent eruptions, whereas compositions that produce thick, sluggish (highly viscous) magmas are associated with explosive eruptions.

Magma is a silicate liquid (with rare exception). The most abundant chemical building block is a pyramid-shaped assembly of the element silicon (Si) surrounded by four oxygen (O) atoms. Other elements that are common rock-forming elements including aluminium, iron, magnesium, potassium and calcium occupy the spaces between and around the silicate building blocks.

The overall mixture forms the hot and sticky stuff called magma. Magma takes on a spectrum of chemical compositions, with different relative amounts of these chemical compositions. Magma which forms in the mantle carries the name basalt. Though fairly uniform in composition, even this mother magma is somewhat variable.

Much of this variation in composition depends upon what happens to the basalt magma once it leaves the mantle and begins its upward journey through the crust. For instance, rocks encountered in the crust sometimes are partly melted by and dissolved into the rising mantle-derived magma, and sometimes crystals grow and then separate from magma as it begins to cool.

Both of these processes actually result in an increased silicon content for the modified daughter magma, and thus the natural tendency is for basaltic magma to change to become a more silicic composition. The extent to which such change advances depends on the duration of the magma journey and the chemical character of the crust traversed.

Magma composition is classified and named mainly on the basis of the amount of silicon in the form of silica or silicon dioxide SiO 2 .

The chart in Fig. 15.3 summarizes the names of the common magmas and their associated ranges in silica. A very important property of magma that determines the eruption style and the eventual shape of the volcano it builds, is its resistance to flow, namely its viscosity.

Magma viscosity increases as its silica content increases. Eruptions of highly viscous magmas are violent. The highly viscous rhyolite magma piles up its ticky masses right over its eruptive vent to farm tall steep sided volcanoes.

On the contrary the basaltic magma flows great distances from its eruptive vent to from low, broad volcanic features. Magma in the intermediate viscosity spectrum say the andesite magma tends to form volcanoes of profile shapes between these two extremes.

An additional important ingredient of magma is water. Magmas also contain carbon dioxide and various sulphur-containing gases in solution. These substances are considered volatile since they tend to occur as gases at temperatures and pressures at the surface of the earth.

As basaltic magma changes composition toward rhyolite the volatiles become concentrated in the silica-rich magma. Presence of these volatiles (mainly water) in high concentration produces highly explosive volcanoes. It should be noted that these volatiles are held in magma by confining pressure. Within the earth, the confining pressure is provided by the load of the overlying rocks.

As the magma rises from the mantle to depths about 1.5 km or somewhat less, the rock load is reduced to that extent that the volatiles (mainly water) start to boil. Bubbles rising through highly viscous rhyolitic magma have such difficulty to escape their way, that many carry blobs of magma and fine bits of rock with them and they finally break free and jet violently upward resulting in a violent buoyant eruption column that can rise to kilometres above the earth.

The fine volcanic debris in such a powerful eruption gets dispersed within the upper atmosphere, hide the sunlight affecting the weather. The greater the original gas concentration in a magma and the greater the volume rate of magma leaving the vent, the taller is the eruption column produced.

The gases escaping from magma during eruption mix with the atmosphere and become part of the air humans, animals and plants breath and assimilate. However as magma cools and solidifies to rock during eruption, some of the gas remains trapped in bubbles creating vesicles. Generally all volcanic rocks contain some gas bubbles. A variety of vesicular rhyolite is pumice. Pumice is vesicular to such an extent, it floats in water.

Essay # 15. Classification of Volcanic Activity:

A classification of volcanic activity based on the type of product is shown in Fig. 15.4. The basic subdivision is based on the proportions of the gas, liquid and solid components, which can be represented on a triangular diagram. The four basic triangles represent the domain of four basic kinds of volcanic activity.

Essay # 16. Cone Topped and Flat Topped Volcanoes:

Generally rhyolite volcanoes are flat-topped because rhyolite magma which is extremely viscous, oozes out of the ground, piles up around the vent and then oozes away a bit to form a pancake shape. In contrast basalt volcanoes generally feed lava flows that flow far from the vent, building a cone.

Basaltic tephra (large particles of different size) is a spongy-looking black, rough material of pebble or cobble. Commercially this tephra is known as cinder and is used for gardening and rail-road beds. In some situations basaltic volcanoes develop flat top profile.

Flat topped volcanoes of basalt can form when there is an eruption under a glacier. Instead of getting ejected as tephra to form a cone, it forms a cauldron of lava surrounded by ice and water and eventually solidifying. When the ice melts, a steep-sided, table-shaped mountain known as a tuya remains. Volcanoes of this type are common in Iceland and British Columbia, where volcanoes have repeatedly erupted under glaciers.

Surprisingly, the Pacific ocean is a home to many flat-topped undersea basaltic mountains. These are called seamounts. How these seamounts were formed was a mystery for a long time. Surveying and dredging operations revealed that most seamounts were formerly conical volcanoes projecting above the water.

Geologists found that the conical volcanoes got lowered due to subsidence and the tops of the volcanoes came near the sea water level and the powerful waves mowed them flat. Continued subsidence caused them to drop below the water surface.

Essay # 17. Types of Volcanoes :

There are many types of volcanoes depending on the composition of magma especially on the relative proportion of water and silica contents. If the magma contains little of either of these, it is more liquid and it flows freely forming a shallow rounded hill.

Large water content with little silica permits the vapour to rapidly rise through the molten rock, throwing fountains of fire high into the air. More silica and less water in the magma make the magma more viscous. Such magma flows slowly and builds-up a high dome.

High content of both water and silica create another condition. In such a case the dense silica prevents the water from vaporizing until it is close to the surface and results in a highly explosive way. Such an eruption is called a Vulcan eruption.

Other types of eruption are named after people or regions associated with them. Vesuvian eruption named after Vesuvius is a highly explosive type occurring after a long period of dormancy. This type ejects a huge column of ash and rock to great heights upto 50 km.

A peleean eruption named after the eruption of Mt. Pelee in Martin que in 1902 is a highly violent eruption ejecting a hot cloud of ash mixed with considerable quantity of gas which flows down the sides of the volcano like a liquid. The cloud is termed nuee ardente meaning glowing cloud. Pyroclastic or ash flow refers to a flow of ash, solid rock pieces and gas. Hawaiian eruptions eject fire fountains.

Essay # 18. Violence of Volcanic Eruptions :

Volcanic activity may be classified by its violence, which in turn is generally related to rock type, the course of eruptive activity and the resulting landforms. We may in general distinguish between lava eruptions associated with basic and intermediate magmas and pumice eruptions associated with acid magmas.

The percentage of the fragmentary material in the total volcanic material produced can be used as a measure of explosiveness and if calculated for a volcanic region can be adopted as an Explosion Index (E), useful for comparing one volcanic region with others. Explosion Index for selected volcanic regions by Rittmann (1962) are shown in the table below.

Newhall and Self (1982) proposed a Volcanic Explosivity Index (VEI) which helps to summarize many aspects of eruption and is shown in the table below.

Essay # 19. Famous Volcanoes around the World :

Many volcanoes are present around the world. Some of the largest and well known volcanoes are listed in the table below.

Essay # 20. Volcanic Hazards :

Volcanic eruptions have caused destruction to life and property. In most cases volcanic hazards cannot be controlled, but their impacts can be mitigated by effective prediction methods.

Flows of lava, pyroclastic activity, emissions of gas and volcanic seismicity are major hazards. These are accompanied with movement of magma and eruptive products of the volcano. There are also other secondary effects of the eruptions which may have long term effects.

In most cases volcanoes let out lava which causes property damage rather than injuries or deaths. For instance, in Hawaii lava flows erupted from Kilauea for over a decade and as a consequence, homes, roads, forests, cars and other vehicles were buried in lavas and in some cases were burned by the resulting fires but no lives were lost. Sometimes it has become possible to control or divert the lava flow by constructing retaining walls or by some provision to chill the front of the lava flow with water.

Lava flows move slowly. But the pyroclastic flows move rapidly and these with lateral blasts may kill lives before they can run away. In 1902, on the island of Martinique the most destructive pyroclastic flow of the century occurred resulting in very large number of deaths.

A glowing avalanche rushed out of the flanks of Mount Pelee, running at a speed of over 160 km/h and killed about 29000 people. In A.D. 79 a large number of people of Pompeii and Herculaneum were buried under the hot pyroclastic material erupted by Mount Vesuvius.

The poisonous gas killed many of the victims and their bodies got later buried by pyroclastic material. In 1986, the eruption of the volcano at Lake Nyos, Cameroon killed over 1700 people and over 3000 cattle.

When magma moves towards the surface of the earth rocks may get fractured and this may result in swarms of earthquakes. The turbulent bubbling and boiling of magma below the earth can produce high frequency seismicity called volcanic tremor.

There are also secondary and tertiary hazards connected with volcanic eruptions. A powerful eruption in a coastal setting can cause a displacement of the seafloor leading to a tsunami. Hazardous effects are caused by pyroclastic material after a volcanic eruption has ceased.

Either melt water from snow or rain at the summit of the volcano can mix with the volcanic ash and start a deadly mud flow (called as lahar). Sometimes a volcanic debris avalanche in which various materials like pyroclastic matter, mud, shattered trees etc. is set out causing damage.

Volcanic eruptions produce other effects too. They can permanently change a landscape. They can block river channels causing flooding and diversion of water flow. Mountain terrains can be severely changed.

Volcanic eruptions can change the chemistry of the atmosphere. The effects of eruption on the atmosphere are precipitation of salty toxic or acidic matter. Spectacular sun set, extended period of darkness and stratospheric ozone depletion are all other effects of eruptions. Blockage of solar radiation by fine pyroclastic material can cause global cooling.

Apart from the above negative effects of volcanisms there are a few positive effects too. Periodic volcanic eruptions replenish the mineral contents of soils making it fertile. Geothermal energy is provided by volcanism. Volcanism is also linked with some type of mineral deposits. Magnificent scenery is provided by some volcanoes.

The study of volcanoes has great scientific as well as social interest. Widespread tephra layers inter-bedded with natural and artificial deposits have been used for deciphering and dating glacial and volcanic sequences, geomorphic features and archeological sites.

For example, ash from Mt. St. Helens Volcano in Washington travelled at least 900 km into Alberta. North American Indians fashioned tools and weapons out of volcanic glass, the origin of which is used to trace migratory and trading routes.

Volcanoes are windows through which the scientists look into the interiors of the earth. From volcanoes we learn the composition of the earth at great depths below the surface. We learn about the history of shifting layers of the earth’s crust. We learn about the processes which transform molten material into solid rock.

From the geological historical view point, volcanic activity was crucial in providing to the earth a unique habitat for life. The degassing of molten materials provided water for the oceans and gases for the atmosphere – indeed, the very ingredients for life and its sustenance.

Essay # 21. Volcanoes and Atmospheric Pollution :

During eruptions volcanoes inject solid particles and gases into the atmosphere. Particles may remain in the atmosphere for months to years and rain back on to the earth. Volcanoes also release chlorine and carbon dioxide.

The main products injected into the atmosphere from volcanic eruptions however are volcanic ash particles and small drops of sulphuric acid in the form of a fine spray known as aerosol. Most chlorine released from volcanoes is in the form of hydrochloric acid which is washed out in the troposphere. Volcanoes also emit carbon dioxide.

During the times of giant volcanic eruptions in the past the amount of carbon dioxide released may have been enough to affect the climate. In general global temperatures are cooler for a year or two after a major eruption.

A large magnitude pyroclastic eruption such as a caldera-forming event can be expected to eject huge volumes of fine ash high into the atmosphere where it may remain for several years, carried around the globe by strong air currents in the upper atmosphere.

The presence of this ash will increase the opacity of the atmosphere, that is, it will reduce the amount of sunlight reaching the earth’s surface. Accordingly, the earth’s surface and climate will become cooler. Various other atmospheric effects may be observed. Particularly noticeable is an increase in the intensity of sunsets.

i. Global Warming :

Besides blocking the rays of the sun, the vast clouds of dust and ash that result from a volcanic eruption can also trap ultraviolet radiation within the atmosphere causing global warming.

Volcanic eruptions usually include emissions of gases such as carbon dioxide which can further enhance this warming. Even if it lasted only for a relatively short time, a sudden increase in temperature could in turn have contributed to extinctions by creating an environment unsuitable for many animals.

ii. Geothermal Energy :

Geothermal energy is the heat energy trapped below the surface of the earth. In all volcanic regions, even thousands of years after activity has ceased the magma continues to cool at a slow rate. The temperature increases with depth below the surface of the earth. The average temperature gradient in the outer crust is about 0.56° C per 30 m of depth.

There are regions however, where the temperature gradient may be as much as 100 times the normal. This high heat flow is often sufficient to affect shallow strata containing water. When the water is so heated such surface manifestations like hot springs, fumaroles, geysers and related phenomena often occur.

It may be noted that over 10 per cent of the earth’s surface manifests very high heat flow and the hot springs and related features which are present in such areas have been used throughout the ages, for bathing, laundry and cooking.

In some places elaborate health spas and recreation areas have been developed around the hot-spring areas. The cooling of magma, even though it is relatively close to the surface is such a slow process that probably in terms of human history, it may be considered to supply a source of heat indefinitely.

Temperatures in the earth rise with increasing depth at about 0.56°C per 30 m depth. Thus if a well is drilled at a place where the average surface temperature is say 15.6°C a temperature of 100°C would be expected at about 4500 m depth. Many wells are drilled in excess of 6000 m and temperatures far above the boiling point of water are encountered.

Thermal energy is stored both in the solid rocks and in water and steam filling the pore spaces and fractures. The water and steam serve to transmit the heat from the rocks to a well and then to the surface.

In a geothermal system water also serves as the medium by which heat is transmitted from a deep igneous source to a geothermal reservoir at a depth shallow enough to be tapped by drilling. Geothermal reservoirs are located in the upward flowing part of a water – convective system. Rainwater percolates underground and reaches a depth where it is heated as it comes into contact with the hot rocks.

On getting heated, the water expands and moves upward in a convective system. If this upward movement is unrestricted the water will be dissipated at the surface as hot springs; but if such upward movement is prevented, trapped by an impervious layer the geothermal energy accumulates, and becomes a geothermal reservoir.

Until recently it was believed that the water in a geothermal system was derived mainly from water given off by the cooling of magma below the surface. Later studies have revealed that most of the water is from surface precipitation, with not more than 5 per cent from the cooling magma.

Production of electric power is the most important application of geothermal energy. A geothermal plant can provide a cheap and reliable supply of electrical energy. Geothermal power is nearly pollution free and there is little resource depletion.

Geothermal power is a significant source of electricity in New Zealand and has been furnishing electricity to parts of Italy. Geothermal installations at the Geysers in northern California have a capacity of 550 megawatts, enough to supply the power needs of the city of San Francisco.

Geothermal energy is versatile. It is being used for domestic heating in Italy, New Zealand and Iceland. Over 70 per cent of Iceland’s population live in houses heated by geothermal energy. Geothermal energy is being used for forced raising of vegetables and flowers in green houses in Iceland where the climate is too harsh to support normal growth. It is used for animal husbandry in Hungary and feeding in Iceland.

Geothermal energy can be used for simple heating processes, drying or distillation in every conceivable fashion, refrigeration, tempering in various mining and metal handling operations, sugar processing, production of boric acid, recovery of salts from seawater, pulp and paper production and wood processing.

Geothermal desalinization of sea water holds promise for abundant supply of fresh water. In some areas it is a real alternative to fossil fuels and hydroelectricity and in future may help meet the crisis of our insatiable appetite for energy.

iii. Phenomena Associated with Volcanism :

In some regions of current or past volcanic activity some phenomena related to volcanism are found. Fumaroles, hot springs and geysers are the widely known belonging to this group. During the process of consolidation of molten magma either at the surface or at some depths beneath the surface gaseous emanations may be given off.

These gas vents constitute the fumaroles. The Valley of Ten Thousand Smokes in Alaska is a well-known fumarole and is maintained as a national monument. This group of fumaroles was formed by the eruption of Mount Katmai in 1912. This valley of area of about 130 square kilometres contains thousands of vents discharging steam and gases.

These gases are of varied temperatures and the temperatures vary from that of ordinary steam to superheated steam coming out as dry gas. Many of the gases escaping from the vents may be poisonous, such as hydrogen sulphide and carbon monoxide which are suffocating and may settle at low places in the topography. For example, the fumaroles at the Poison Valley, Java discharge deadly poisonous gases.

Solfataras are fumaroles emitting sulphur gases. At some places, the hydrogen sulphide gases undergo oxidation on exposure to air to form sulphur. The sulphur accumulates in large amount so that the rocks close to the solfataras may contain commercial quantities of sulphur.

Hot springs are also phenomena associated with volcanic activity. Waters from the surface which penetrate into the ground can get heated either by contact with the rocks which are still hot or by gaseous emanations from the volcanic rocks. The water so heated may re-emerge at the surface giving rise to hot springs. In some situations the hot springs may be intermittently eruptive. Such intermittently hot springs are called geysers.

Lava: Types and Eruptions | Volcanoes
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Volcanoes: compilation of essays on volcanoes | disasters | geology.

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Essay on Volcanoes

Essay Contents:

Essay on the Effects of Volcanic Activity

Essay # 1. Meaning of Volcanoes:

Sometimes the molten rock, ash, steam and other gases find their way to the surface of the earth through some vents or openings. These ejected materials accumulate around the vent and give rise to a volcanic cone or a hill. The conical hill along with the vent is known as volcano.

The mouth of a volcano is funnel shaped and hollow. This funnel-shaped hole is called the crater and the opening through which molten rock materials come out from inside is called the pipe or vent.

Fuji Yama in Japan, Visuvius in Italy, Cotopaxi in Equador and Barren Island are good examples of volcanic mountains.

The ejection of materials through the crater is called volcanic eruption. Volcanoes are generally classified into three types. These are active, dormant and extinct.

The active volcanoes generally erupt fairly frequently and are of two types, namely, permanent and intermittent:

(a) Permanent volcanoes:

The volcanoes that eject molten materials continuously are called permanent volcanoes.

(b) Intermittent volcanoes:

These are those volcanoes in which an eruption occurs at the end of a certain period of time. Stromboli of Italy and Etna of Sicily are the examples of active volcanoes.

A volcano which erupted sometime in the past, and is now passive, but may erupt again, is called a dormant volcano. Fuji Yama of Japan and Vesuvius of Italy are the dormant volcanoes.

Extinct volcanoes are those whose activity has not been known to us during the historical time. Kilimanjaro of Africa and Chimborazo of Equador are the examples of extinct volcanoes.

Sometimes it is difficult to say whether a volcano is extinct or dormant. For example, the Vesuvius, the Krakatoa, Mt. Unzen of Japan and Mt. Pintubo of Philippines once thought to be extinct became active recently.

Essay # 2. Formation of Volcanoes :

The actual causes of the formation of volcanoes are not yet definitely known.

The probable causes of their origin are given below:

(a) Difference of Pressure on the Surface:

Pressure below the earth’s surface increases with depth. The interior of the earth is under high pressure. It is at the same time very hot. Because of high pressure and high temperature the materials of the earth’s interior are in the viscous state. Owing to the release of pressure from the top portion for some reason, the viscous materials get expanded. There is then a natural tendency for molten materials to make an upward drive through the fissures or weak zones of the earth. By this way a volcano may be formed.

(b) Chemical Reaction:

There are some radioactive minerals in the interior of the earth. They emit heat continuously and as a result of this, the other materials get expanded and create enormous pressure. Then the underlying expands and molten materials may come out through some fissures forming volcanoes.

(c) Percolation of W ater to Earth’s Interior:

Besides those two causes, a volcano may be caused when water percolating through the surface meets the molten materials. In this case water turns into steam of great volume. Steam, lava, and other materials may then come out to the surface forming a volcano.

Essay # 3. Arisen of Volcanoes:

It is a well-known fact that in all the northern and central part of Europe at the present time there are no active volcanoes. But, the geologist professes to be perfectly certain that there were, during many of the past geological periods, many volcanoes in our island. Before we can understand how he has come to this conclusion we must, according to Hutton’s method, find out how any known volcano has arisen and what traces it has left after it has become extinct.

The nearest European volcano to our islands is Vesuvius, and it is fortunate that we know a great deal about Vesuvius during the last two thousand years. The Neapolitan volcanoes extend from Vesuvius on the east to the Islands of Procida and Ischia on the west.

Before the Christian era, as far back as any tradition is known, there seems to have been no activity at Vesuvius. But a Greek colony, which settled on Ischia about 380 B.C., had to abandon the island owing to an eruption, and there were traditions of volcanic eruptions driving earlier colonists away as well.

Since the date above mentioned there has been complete cessation of vulcanicity in that island except at one time, when there was an emission of lava. On the mainland to the west of Naples there was Lake Avernus, occupying the inside of an ancient crater, where probably poisonous gases were emitted, if what Lucretius tells us is true, that birds could not fly over it without being stifled.

Thus we see that just before and during the beginning of the Christian epoch there was some evidence that the volcanic forces, which had in far-off ages built up high mountains near the Bay of Naples, were not quite extinct.

Vesuvius, which we now recognise as a modern cone within an ancient one, the remains of the latter being called Monte Somma, was, up to the first century, a single regular truncated cone. Plutarch writes of its old crater having steep cliffs and being covered with wild vines. The flanks of the mountain were of a rich soil and were covered with fertile fields stretching down towards the busy towns of Pompeii and Herculaneum.

An especially violent shock on the 5th of February A.D. 63 gave warning of a recrudescence of vulcanicity. Pompeii was badly damaged, many buildings being thrown down. But the inhabitants were rich and at once set about restoring what was damaged. After sixteen years most of the houses were in good repair, and at least two damaged temples had been rebuilt. In August A.D. 79 the great catastrophe occurred, and when the Forum was dug out a few years ago, it was seen that the west side of the colonnade, which had been almost completed when the earthquake of A.D. 63 threw it down, had been only partly rebuilt, and the area was strewn with blocks which were being made ready for the rebuilding by the stone masons.

On August 24 the elder Pliny, who was commander of the Roman fleet, went with his ships to rescue the people at the foot of Vesuvius from danger. Arriving too late he went to Stabiae where, on the next morning, he was suffocated by fumes..

His nephew, the younger Pliny, tells the story of what he saw in a letter to Tacitus. It is easy to follow the course of events. A column of vapour ascended from Vesuvius and then spread out on all sides. This caused intense darkness, which was lit up at times by flashes of lightning.

Volcanic ashes, as the fragments blown out from a volcano are called, fell on ships many miles away, earthquake shocks were frequent, and the sea went back, leaving marine animals high and dry.

Pompeii and Herculaneum:

But the ashes did not only fall on ships and the earthquakes were only, as it were, premonitory warnings of what was to follow. For at last a series of gigantic explosions blew one side of Monte Somma into the air and the enormous mass of material on falling buried up the prosperous cities of Pompeii and Herculaneum.

On the west side the descending ashes mingled with torrents of rain and so gave rise to a huge stream of mud, which poured down upon Herculaneum and sealed it up, on the south side, driven by the wind, the ashes spread out like a cloud and covered Pompeii. From this cloud there descended first broken fragments of pumice about the size of a walnut, which rained down to a depth of about 9 feet, then came fine ash wet with rain and settled for a depth of some 6 or 7 feet over the doomed town.

When the sun set the storm of ashes had ceased, but by that time Herculaneum had vanished and only the roofs of the houses of Pompeii projected above the surface. As far as we can tell, no lava flowed from the mountain during this period. But after this year other periods of activity occurred, and in 1036 we learn that a lava stream did make its way down the mountain-side.

Other times of activity were in the years 1049, 1138, 1306, 1500, and 1631, though the activity in 1500 was very slight. During the long pause before 1631 there occurred in the Phlegraean Fields to the west an event of some importance. In 1538 a new mountain was suddenly formed; it was named Monte Nuovo, and good accounts of its formation have come down to us.

Monte Nuovo:

After several shocks during September 28, we are told that very early on the 29th flames of fire were seen, and shortly after the earth burst open and ashes and pumice were blown out. Later on the inhabitants of the neighbouring town fled in terror to Naples, and it is stated that the sea had retired and left multitudes of fish to die on the shore.

The eruption continued for two days and nights, and on the third day, signs of activity having ceased, several people went up the new hill which was over 400 feet high and 8000 feet round its base. When they looked down into the crater they saw a boiling cauldron. Some days afterwards fresh explosions took place, but with decreasing activity.

No lava was extruded during the eruption, and the whole mountain was built up of large and small fragments which were blown up into the air by explosions and fell close to the orifice. No further volcanic action has occurred here since the time of the formation of the hill and it is now completely covered with grass.

For nearly a century after the birth of Monte Nuovo Vesuvius continued tranquil, and there was no violent eruption for nearly five hundred years. At the bottom of its crater cattle grazed, and its sides were covered with bushes. But in 1631 these grassy flats and woods were blown into the air and seven streams of lava poured down from the mountain and overwhelmed several villages. From the seventeenth century to the present day there has been a constant series of eruptions and no great convulsion.

In fact, the behaviour of the district is very much like that of a safety valve; when there is an opening the forces below cause more or less quiet extrusion of molten rock, but if the opening gets plugged up then the activity ceases, it may be for many years, but there comes a time when the imprisoned forces get sufficiently great to blow away the overlying tract which had sealed up any opening that had before existed.

Probably the most violent volcanic eruption of recent times occurred in the Islands of Krakatoa in August 1883. The Royal Society appointed a committee to investigate the eruption, and the report of this committee is a very remarkable production and stands for all time as an example of what such a report should be.

Krakatoa is an island lying between Java and Sumatra. Although prior to the catastrophe this island had attracted but little attention, it lies in the heart of the great focus of volcanic activity in the world at the present time. Java itself contains forty-nine great volcanic mountains, and from it a chain of volcanoes is continued to the east and to the west.

Krakatoa did not show the regular conical shape of a volcano, but was only a part of a huge crater of which other smaller islands close by were also fragments.

The volcano of which these islands are the remnant must have been formerly one of gigantic dimensions, probably some twenty-five miles in circumference at the sea-level and over 10,000 feet in height. It was built up almost entirely of lava, and rested on beds of post-Tertiary age.

At some unknown date the central mass of the volcano was blown away, and only irregular portions of its crater- ring were left. Then occurred quiet eruptions at the bottom of the crater, causing the formation of small cones which filled up the older crater in parts, and raised these above sea-level. At one edge of the old crater eruptions built up the cone of Krakatoa from lava extrusions and outbursts of ashes.

In historic times the lateral cone of Krakatoa and the other land which was part of the original volcano were seen to be covered with luxuriant vegetation, and the products of its forests attracted the inhabitants of neighbouring islands.

The first eruption recorded as having taken place at Krakatoa is in 1680, though tradition points to former ones having occurred while man lived in the neighbourhood. The eruption was of a moderate type and the island soon recovered its former state, and the vegetation which had been destroyed once more spread and for two hundred years the volcanic forces lay dormant.

In 1880 earthquakes were frequent, and in May 1883 the inhabitants of the town of Batavia, 100 miles away from Krakatoa, heard booming sounds, and for many hours their doors and windows were rattled. On the next day ashes fell on Java and Sumatra at places opposite to Krakatoa, and later a column of steam from Krakatoa showed the position of the disturbance.

The height of the cloud of dust above the island was estimated to be seven miles, and falls of dust were noticed three hundred miles away. This eruption soon decreased in intensity.

An excursion party from Batavia landed on the island and found some depth of fine ash scattered over it, and many” trees had been stripped of their leaves and branches by the fall of this deposit.

On August 11 another visit to Krakatoa showed that the forests had been completely destroyed, and that the fine ash covered the surface of the ground near the shore to a depth of 20 inches. Some fourteen foci of eruption were seen, from which steam and dust were being sent out.

This recrudescence of activity finally culminated on August 26 and 27 in a series of tremendous explosions. During these days three European ships were actually in the Straits in which Krakatoa lies, and as they escaped destruction those on them have been able to give a record of their observations.

The course of the events which happened seem to have been as follows- During the night of the 26th white-hot fragments of lava were blown out and rolled down the mountain-sides, smaller cinders were blown out to sea, and finer ashes, remaining in the air, caused intense darkness in the early morning.

From sunset till midnight there was a continuous roar which moderated in the early morning. Each explosive outburst of steam blew off the crust which formed on the lava and left the white-hot molten material to throw its light on to the overhanging pall of ash, and so lit up the scene for miles around.

But now the somewhat peculiar situation of the volcano made itself felt. It lies close to the sea-level, and so the removal of so much material by the constant explosions allowed the waters of the sea to make their way into the heated mass of lava. This must have chilled and frozen much of the molten rock.

Some flashing into steam of the water no doubt occurred, and this gave rise to waves in the sea which travelled to the shores of Java and Sumatra, and were noted on the evening of the 26th and through the night.

The general result of this action of the sea-water was the closing of the safety-valve while the forces below it were still active, and finally there resulted four tremendous explosions between 5.30 a.m. and 10.52 a.m. on the morning of the 27th.

What happened in this space of time to the island itself was that the whole of the northern and lower portion of the island was blown into the air, while a large part of the cone which existed there also vanished. Where the island had risen to heights of between 300 and 400 feet was now in some places more than 1000 feet below sea-level.

These terrible explosions caused huge waves to travel away from the centre of destruction and, rushing up on to the coasts of Java and Sumatra, stranded all vessels near the shore, devastated towns and villages, destroyed two lighthouses, and caused the deaths of over 36,000 people.

But not only were sea-waves produced, but there were also air-waves. Some were so rapid that they produced sounds even at a place 3080 miles away, others were larger and broke in windows and cracked walls a hundred miles away at Batavia, where also lamps were thrown down, gas- jets extinguished, and a gasometer leaped out of its well.

Other air-waves were traced in their course by barometers at various places scattered widely over the surface of the world, and so we know that they travelled several times round the globe.

At Greenwich the depressions of the barometer due to these air-waves were recorded on every day between noon on August 27 to early in the morning of September 1. During the whole of August 27 eruptive action seems to have continued. Three vessels remained beating about in the Straits near the island, and their crews were busy for hours shovelling off their decks the volcanic dust which descended.

The finer particles of dust, however, floated still farther away. Between 7 and 10 in the morning the sky began to darken at Batavia, soon after lamps had to be lit, and a descent of fine dust began about 10.30 and lasted for nearly an hour, when complete darkness came on, the heavy dust- rain continuing till 1, and a less heavy precipitation till 3 P.M.

Complete darkness was experienced still farther to the east at a place one hundred and fifty miles from the volcano.

What happened during the days succeeding August 27 is doubtful; there may have been several small eruptions, but we know that on October 10 there was an explosion of some magnitude, and a large sea-wave. The commotion in the sea due to the cataclysmic convulsion of August 27 travelled far and wide.

Its terrible effects on the neighbouring islands of Java and Sumatra have already been noticed, and, of course, as it passed away from the centre of disturbance it gradually lost its intensity, still, tide-gauges and eye-observations tell us that to the west it was noticed up both the coasts of India, along the south of Arabia, round the south coast of Africa, to the east coast of Central America and Terra del Fuego, while it also spread to the north and reached the coast of France and was recorded at Devonport and Portland.

To the east it spread to Australia and New Zealand, to Alaska and San Francisco.

In Australia sudden rises of 5 and 6 feet were recorded on the west coast; on the shores of Java the rise was one of about 50 feet, while in the English Channel it was only barely noticeable. But if the effect on the sea near England was so slight there was another effect so pronounced that it must have forced itself on the attention of almost everyone in our islands.

The coarser particles which were shot into the air fell close to the island, finer ones rained down, as we have noted, a hundred miles away in Java, but still finer ones, shot up into the air to enormous heights by the terrific explosions, were carried in the upper regions of the atmosphere all round the globe. One effect of these fine particles floating in the air was to give most extraordinary sunset effects on clear evenings.

The writer very vividly remembers some of them he saw in Kent. The brilliant, almost lurid glows in the western sky lasted long after the usual sunset effects would have passed away, and were far brighter than the normal sunset colours.

These splendid sunset effects were noted in Australia, in India, and throughout Europe and America.

In Surrey particularly fine sunset colourings were noted on September 8, and on several occasions between that date and early November. On November 9 the effect was most wonderful and magnificent. Afterglows were recorded throughout November, December, and January, but during February and March the coloration decreased, and after March no peculiar illumination was observed.

Essay # 4. General Course of an Volcanic Eruption:

The general course of a volcanic eruption becomes plain when we consider such cases as those of Vesuvius, Monte Nuovo, and Krakatoa.

A district may have been quiescent for centuries, but then from some cause the earth’s internal heat produces stresses at some point in the crust below that district and the crust gives. Such a crack even though it be exceedingly small in amount causes a tremor, and things on the surface are shaken. It is rare that any actual Crack appears on the actual surface of the ground, but this does occur sometimes.

The molten rock which is pressed up in some cases finds its way along these cracks and may eventually solidify in them without ever reaching the surface; these rock masses are spoken of as Intrusions. At other times water makes its way down to the heated area and, flashing into steam, produces an explosion.

This, if powerful enough, may blow off a huge amount of the superincumbent crust and cause vast damage, but if an opening has once been drilled the explosions are less violent and merely cause the ejection of blocks of stone or masses of molten rock. These may either fall back into the crater or be scattered over the mountain-sides round it.

Such mountains as Monte Nuovo and many of the so-called cinder cones of Central France have been formed in this way and show no other signs of volcanic activity. But in other cases molten rock wells up the central crater and pours over or breaches its cone and flows down its sides. In this way we get lava streams, and these, owing to subsequent periods of explosive violence, may get covered by volcanic ashes.

But the story of a volcano is not quite so simple as this, for as the volcano rises higher the pressure needed to force the column of molten rock up to near its summit must grow greater, and so it often happens that the internal forces find it easier to produce an opening on the sides of the mountain and so get formed those lateral cones, which in some cases are of such frequent occurrence. These may be merely cinder cones or they may pour out molten rock.

The wind which prevails during an eruption sends the ashes this way or that, and so the deposit of volcanic detritus round a volcano is not so continuous as an aqueous sediment. Lavas and ashes may be mixed up, and the lava stream may end off with great abruptness both at its lower end and along its sides. Moreover, when the whole mass has become firm, subsequent shocks may crack it, and into those cracks molten rock may be squeezed, giving rise to what are called dykes.

In general, therefore, beds which owe their formation to igneous activity do not show anything like the same regularity that is seen in the case of aqueous sediments. Moreover, as a rule, volcanic detritus encloses no life, for though the living things, such as the inhabitants of Pompeii, may get killed on the surface of the earth when it has ashes poured down on to it, yet the mass of the ash above contains no life of the period. It may, however, contain blocks of rocks which enclose fossils of their own age, and so be useful in giving something of a clue to the age of the ash.

Fossiliferous Ash Beds:

But there is another possibility; some ash may fall into the sea, and then as it settles it will entomb some of the life of the period, and successive eruptions may build up a great thickness of rock in which afterwards fossils in plenty may be found. Such a deposit will often be in part like a terrestrial ash bed, and in part like a marine sediment, and these volcanic deposits are the most hopeful ones in which to search for fossils.

When denudation gets to work on a volcano it is obvious that loose ash beds will be scoured away with rapidity, and so when the denuding agents get either to the solid plug of frozen lava in the vent, or to streams of lava down the sides, or to the dyke intrusions, these hard rocks will be left as prominences above the softer and more easily eroded beds.

The discussion of vulcanicity is, however, incomplete unless one considers the origin of the molten rock; here we cannot appeal to active or modern volcanoes, as in their cases we can only see surface phenomena. But, after ages of denudation, ancient volcanoes have been dissected, and their roots, as it were, disclosed.

We then find that there was a large mass of molten rock below them which no doubt was compressed at different times and squeezed upwards. These huge masses of liquid material ultimately froze, and so have arisen the enormous masses of granite so well known in parts of our islands. Such a rock is spoken of as plutonic to distinguish it from a molten rock which solidified on the earth’s surface and is called volcanic.

If one asks what caused the pressure one must remember that our earth has a hot interior, as proved by every coal-mine and deep boring, and a cooled crust, and that as the earth slowly gives out its heat it shrinks, and the crust settles on to the shrinking nucleus. This settling down must give rise to enormous stresses, and it is no wonder that cracks appear and molten material is squeezed up them.

Fissure Eruptions:

But though the Vesuvian type of eruption is the one nearest to our doors and the one most commonly found at present on the earth, if we go to Iceland we shall find another and a very different type of eruption allied to what is called the Fissure type.

In Iceland, though there are volcanoes of the Vesuvian type, the most common eruptions take place from a chain of volcanoes arranged along a large fissure. Small ash cones may be built up at points on the fissure and the position of actual eruption moves on so that the rings of the cones intersect, but at times flows of lava pour out from the fissure without any cones at all, flood over the surrounding country, fill up the valleys, and instead of an undulating plain produce a more or less level lava desert. Such was the course of events in 1783 when one of the greatest extrusions of lava in modern times occurred in Iceland.

When such an outpouring takes place, the rivers which afterwards arise cut their way into the lavas and gradually remove them from the older rocks which they overlie or rest against. Ultimately the lava plain may become a plateau scored by streams that wind over it.

This type of volcanic action differs from the Vesuvian type in that it possesses few, if any, ash beds, and in its occurring with great regularity over a vast area.

The fissure type of rock being pre-eminently a lava there seems little hope of finding fossil evidence of its age. But when we remember that extrusion is not continuous and that rivers make their way over the plateau and form gravels, and that these gravels may be covered by a subsequent flow, we see that it is possible for fossil evidence to be found even amongst the lavas of the fissure type. Where the centre of volcanic activity lay beneath the sea the beds due to the eruption will get covered by marine deposits and so evidence will accumulate to give the age of the eruption, if in after times the sea-floor is raised above sea-level.

Essay # 5. Volcanic Deposits in the British Islands:

Now when we examine the rocks of our islands we find repeated evidence that in various periods there existed centres of vulcanicity in many places. The Pre-Cambrian rocks of Charnwood in Leicestershire, of Shropshire, of the Malverns and elsewhere contain undoubted beds of volcanic ash, while when we come to the succeeding Primary rocks we find abundant proof that volcanic action took place during their formation.

North Wales is one of the districts which must again and again have included volcanoes throughout Cambrian and Ordovician times, while on the other side of. St. George’s Channel Ordovician lavas and ashes are found to the north of Dublin and in the Waterford area. Nearer the centre of Ireland such rocks are seen close to Kildare, and in the west, in Galway and Mayo, extensive coarse and fine ash deposits and lavas have been proved to be of Ordovician age.

In South Wales Ordovician lavas and ashes are seen forming Cader Idris, and they occur in Shropshire, in the Lake District, and in Ayrshire. The Ordovician period, in fact, was one, as far as our islands are concerned, of almost universal volcanic activity. It will be noticed that much of our mountainous country owes its ruggedness and its height to the Ordovician volcanic rocks, which by their hardness have been able to withstand denuding forces.

After this widespread outburst of volcanic forces came the Silurian period, and for long it was thought that there was then a complete cessation of the ejectment of ashes and the outflow of lavas. Comparatively recently, however, it has been shown that Silurian lavas and ashes are to be seen in Kerry in Ireland, to the north of Bristol, and in the Mendips, and in South-west Wales. But elsewhere, as far as we know, throughout our islands deposition of marine sediment went on quietly throughout Silurian times.

After Silurian times there came a period of uplift from Central England northwards, and the formation of a series of large sea lochs in which the Old Red Sandstone was laid down.

The tract of land stretching north from the Cheviots contained many active volcanoes. In South Devon also there was much lava poured out and ash accumulated near what must have been the northern margin of a great ocean stretching southwards.

In South Scotland many of the plugs of igneous rocks which filled up old vents in Old Red Sandstone times have proved much harder to wear down than the surrounding ash cones, and so have remained forming hills standing up somewhat abruptly amongst softer strata. This volcanic tract of Argyllshire is continued to the west in Ulster.

To the south of this line volcanoes of Old Red Sandstone age occurred in the Pentland Hills, where numerous “necks ” of this Age are found. These form the western end of a long line of active volcanoes which stretches westwards to the Ayrshire coast.

The upper Old Red Sandstone beds but rarely enclose evidence of igneous activity, but in Carboniferous times once more the subterranean forces burst into vigorous action.

Carboniferous volcanoes were most abundant in Scotland, and they persisted from the lowest beds to the basement beds of the coal-measures. They were distributed over the central valley from the south of Kintyre to the Firth of Forth.

In England Carboniferous volcanic rocks are seen in Derbyshire, in the Mendips, and near Weston-super-Mare. In the Isle of Man there are the relics of a group of Carboniferous vents, but in Ireland the only evidence of activity during this period is in King’s County and near Limerick.

In Scotland the predominant type of outflow of lava is of the plateau type, but the cinder-cone type of volcano is seen abundantly in the Firth of Forth region and forms the regular type seen elsewhere in the British Isles.

The plateau type of eruption built up enormous sheets of igneous rock which now form the prominent escarpments of many hills in South Scotland, and the vents up which the molten rock rose were plugged up by the frozen lava, and now remain as numerous prominent hills rising abruptly above the softer sandstone around them. Instances of these hills are North Berwick Law and the Bass Rock.

Eventually, but before the close of the Carboniferous period, the plateau extrusions were submerged and buried under the Carboniferous limestone series, and then a new type of eruption began, the cinder-cone type. The relics of these cones are abundant in the Carboniferous beds of Scotland.

After a very considerable time earth movements ridged up the coal-measures and formed a series of inland seas, and in the Permian period we find feeble and short-lived volcanoes in Ayrshire, in East Fife, and in South Devon. In Ayrshire there are several volcanic necks, which descend vertically through the surrounding rocks and form vertical columns of volcanic material. This material is usually a coarse ash, but in some cases molten rock has risen up the vent.

In East Fife, along the shore near St. Andrews and Elie, there are abundant small necks piercing the Carboniferous strata. That they are older than the beds they pierce is proved by the blocks which fill them being of the same mineral character as the Carboniferous beds, while they also contain the same fossils.

At Largo a coarse volcanic ash lies unconformably on the ridges of the newest Carboniferous beds, and though no Permian sedimentary beds are known in this district, these volcanic beds, being post- Carboniferous, are considered to be of Permian age.

In East Fife some sixty necks can be seen, and the larger ones form conspicuous hills such as Largo Law.

In Devonshire lavas and volcanic ashes are found near Exeter, but, as in Ayrshire, there is no thick accumulation of the ashes. In Devon, however, the volcanic activity was far feebler than in Scotland.

The long story of Igneous activity in our islands now comes to an end for an enormous length of time. It is to be noticed that a region of vulcanicity frequently continues to be one throughout the Primary period. Thus in South Scotland we have volcanoes in Cambrian and Ordovician times, then came a rest during Silurian times, which was followed by a renewed outburst on a gigantic scale in the Old Red Sandstone and Carboniferous periods; this activity then became feebler, and though we do find evidences of volcanic action there in Permian times, the volcanic forces were evidently dying out.

After this for untold ages throughout the whole of the Secondary period came silence. No explosions rent the ground, no faulting caused earthquakes, there was no ejectment of lava, the Jurassic period passed and was succeeded by the Cretaceous, and bed after bed was laid down over the greater part of our island. Time after time parts of the land were submerged or were upraised, but of all the enormous thickness of rock which accumulated not one inch is of volcanic origin.

Then we enter Tertiary times, and then once more the subterranean forces got to work and produced, effects on a grander scale than had ever before been witnessed in our island region.

The Tertiary Fissure-Eruptions:

The type of eruption which was characteristic of this Tertiary period of activity was the fissure type; very occasionally there was a small amount of crater building, as is proved by the occurrence of ashes, but the mass of igneous rock extruded was through almost innumerable fissures.

The tract of ground covered by these lavas was immense, and though the outpouring took place in Tertiary times the effect of marine and surface denudation has been so pronounced that the vast original plateau is now represented by mere scattered fragments above sea-level. The chief places where it is to be seen are in Antrim, in Staffa and Iona, in Mull and in Skye, but the plateau extended northwards to the Faroe Islands, and the Iceland volcanoes of to-day are the sole remaining places where the eruptive forces still effect the extrusion of molten rock as they did in days gone by.

The lava is of the type called a basalt, an almost black rock and one which frequently gives rise to a columnar structure, as is well seen in Staffa. This basalt rests on chalk in the Antrim district, but in Skye is found on much older rocks.

This basalt, which builds up huge thicknesses of rock in this north-western area, must not be imagined as coming out in one continuous flow. There were pauses between extrusions, and during those pauses rivers flowed over the plateaus, cut channels in them and produced beds formed of rolled blocks of the lava and of other rocks brought down from the hills; then more molten rock flowed out and buried these channels and their beds of conglomerate or gravel.

Thus we find in amongst the lavas water-formed beds, and some of them in Antrim and in the islands off the coast of Scotland have yielded plant remains, leaves and wood and a portion of a fresh-water fish.

After the building up of vast thicknesses of rock by repeated outwellings of lava there came a later period when igneous rock of a somewhat similar nature was intruded in sheets amongst the lavas. This rock withstands the action of the weather better than the softer basalts, and so as the basalts are now as a rule more or less horizontal, the two sets of rocks produce a very definite type of scenery.

The hills which have been carved out of them are flat-topped, and the sides are ringed with a series of horizontal scarps with almost vertical edges, these being formed by the hard intrusions. As the basalts on weathering form fertile ground, the slopes below the dark cliffs of intrusive rock are commonly covered by rich green grass.

Such scenery is well seen along the west coast of Skye and any one sailing up the west coast of Scotland will again and again see instances of it.

A consideration of the height to which the basalt plateau now rises enables one to see what an enormous amount of denudation has gone on since the Tertiary time, during which the lavas were poured out.

Originally the plateau extended from the island of Mull to the mainland to the north-east, where the promontories of Morven and Ardnamurchan are now seen. But between Mull and the mainland the Sound of Mull now exists twenty miles long and two miles broad. From the deepest part of the Sound to the top of the plateau in Mull is nearly 4000 feet, and the huge mass of basalt which used to occupy and cover the present Sound has been completely washed away.

Elsewhere the same story is told, and we must imagine that in Tertiary times a huge plateau of Basalt stretched from Antrim up between the present west coast of Scotland and the Hebrides, and beyond to the Faroe Islands.

From this general consideration of the history of British vulcanicity throughout past geological ages certain general facts become obvious.

The regions of vulcanicity in our islands all lie to the west of a line from the most south-easterly point of Scotland to Exeter. In this western region volcanic activity has persisted from the very earliest times of which we have any knowledge down to Tertiary times. Not only is this general persistence to be noted, but also the fact that particular portions of the volcanic region have been the sites of recrudescence of action again and again.

In the south-west of England we find volcanoes in Silurian, in Old Red Sandstone, in Carboniferous, and in Permian times.

In the south of Scotland we find plenty of evidence of vulcanicity all through the Ordovician period, in the Old Red Sandstone, in Carboniferous, and in Permian times, while there are also plenty of dykes there originally full of molten rock which rose up the cracks in the Tertiary period.

But not only are certain well-marked areas again and again the scenes of volcanic action, others are quite as conspicuous for the absence of that action.

The Central Highlands of Scotland, though they abut on to the Old Red Sandstone, Carboniferous, and Permian volcanic areas to the south, themselves contain no traces of vents. The southern uplands, almost surrounded by volcanic rocks of various ages, are themselves perfectly free of any opening through which either lavas or ashes were sent out.

There seem to be, therefore, certain regions which are regions of weakness through which the volcanic forces were able to make openings to the surface, and these regions remained weak even though at one time or another those forces failed to continue ejecting matter through them.

Another point to be noticed is that we have no evidence of any slackening of the forces which produce volcanic action, and a further point is that the types of vulcanicity now known as existing at the present day have been seen again and again during the history of the world.

The volcanoes such as Vesuvius can be matched in the Primary period, during which, for instance, material in our English Lake District was accumulated to a depth of some 8000 feet.

The small ash-cones of the Neapolitan area are strikingly alike to the Tertiary cones of the Puy de Dôme district in France, and to the small cones of the Carboniferous period as seen in Scotland.

Lastly, the modern eruptions of Iceland resemble the Tertiary fissure-eruptions of North-east Ireland and West Scotland, and the Carboniferous fissure-eruptions of South Scotland.

Although the Tertiary outburst proves that as a whole the volcanic forces had by that time suffered no diminution, yet when one traces the history of volcanic action it is seen to have waxed and waned.

The enormous outpouring of lavas and ejection of ashes in Ordovician times was followed by an almost complete cessation of volcanic action in the following Silurian period. The succeeding Old Red Sandstone period gives evidence of a vulcanicity somewhat less pronounced than that of Ordovician times, and is succeeded by smaller outpourings in the Carboniferous period, while the following Permian volcanoes are comparatively few and unimportant.

Then for the whole of the Secondary period there was a complete absence of any vulcanicity at all. Then in the older times of the Tertiary age came a period of great violence, which died down, and from that time to this the sound of a volcanic outburst has been unheard in our islands. But the history of our islands shows that from such a long spell of quiescence we cannot draw the conclusion that those sounds will never be heard again.

The time may come when Scotland, or Wales, or the Midland district of England will be deluged with lava, or their lands desolated by downfalls of volcanic ash, when Glasgow or Birmingham may repeat the story of Pompeii, and future ages, by excavation at those places, may learn of the civilisation of the twentieth century.

Essay # 6. Distribution of Volcanoes :

Almost all the active volcanoes are in the young folded mountain region, and the fault zones of Africa. There are more than 1000 volcanoes in the world and out of these about 500 are active or dormant.

These are distributed in three belts:

(b) Mid-World Mountain Belt:

This belt is extended from the Mediterranean Alps to the Himalayan region. Visuvius, Stromboli, Barren Island, Krakatoa, etc., are some of the best known volcanoes of this belt. There are 83 active volcanoes in this belt.

(c) African Rift Valley Belt:

This belt goes from Palestine in the north to Malagasy Island, through the East African region. The Kilimanjaro in Tanzania is a well-known dormant volcano of this belt.

Over and above these, some scattered volcanoes occur in some islands of the Pacific, Atlantic and Indian Oceans.

Essay # 7. Effects of Volcanic Activity :

Like earthquake, volcanic activity also changes the features of the surface of the earth. These changes are both destructive and constructive.

Great calamities take place due to volcanic activity. Often volcanic eruptions bury many beautiful towns. For example, Harculium and Pompii, the two beautiful Italian towns were completely buried by the erupted materials of Visuvius. Volcanic eruptions may be violent if it originates under sea. It causes strong waves which are highly destructive to life and settlements of coastal regions.

On the other hand, volcanoes may have some good effects. It sometimes creates extensive basaltic plateaus. Generally, the volcanic region is rich in minerals and the volcanic soils are fertile.

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Volcano Writing Prompts

These volcano writing prompts will provide your students with creative writing practice.

You can share these volcano writing prompts with your students.

They are written to get your students to recall what they have learned while creating a short story.

This is another free resource for teachers and home school families from The Curriculum Corner.

Volcano writing prompts

Write a story about a group of explorers who stumble upon an undiscovered volcano. Describe the eruption and the explorers’ reaction to it.
Imagine that you are a volcano that has just woken up after hundreds of years of dormancy. Write a first-person narrative describing your thoughts and feelings as you prepare to erupt.
Write a letter to a friend describing the effects of a volcanic eruption on a nearby town. Use sensory language to vividly describe the sights, sounds, and smells of the aftermath.
Create a dialogue between a group of scientists studying a volcano and a local resident who is skeptical of their findings. Use evidence and reasoning to convince the resident that the volcano poses a threat.
Write a poem about the beauty and danger of volcanoes. Use imagery and figurative language to convey the power and majesty of these natural wonders.
Write a diary entry from the perspective of a person living in a village that is in the path of a lava flow. Describe their feelings and actions as they try to evacuate and save their belongings.
Write a news article about a volcanic eruption that recently occurred in a nearby town. Include quotes from eyewitnesses, local officials, and experts to provide a well-rounded account of the event.
Create a persuasive essay arguing for or against the construction of a new hotel near a volcanic site. Consider the economic benefits, environmental impact, and potential risks involved.
Imagine that you are a scientist studying a dormant volcano. Write a report explaining the signs that indicate that the volcano may become active soon, and what precautions should be taken.
Write a short story from the point of view of a volcano. Describe the process of building up pressure and erupting, and how the volcano affects the landscape and the creatures around it.

You can download a printable version of these prompts by clicking on the green apples below. The first two pages contain the prompts on strips so that you can print and have students choose one. The last page contains a list of the prompts. Students can be given the whole page. Or, you can display the page on your screen.

As with all of our resources, The Curriculum Corner creates these for free classroom use. Our products may not be sold. You may print and copy for your personal classroom use. These are also great for home school families!

You may not modify and resell in any form. Please let us know if you have any questions.

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Volcanoes - Free Essay Examples and Topic Ideas

Volcanoes are geological formations that have openings through which molten lava, ash, and gas erupt from the Earth’s surface. They are typically formed at the boundary of tectonic plates, where magma rises and accumulates, eventually leading to an eruption. Volcanoes can range in size from small hills to massive mountains, such as Mount Everest. They can also have varying levels of activity, from dormant to erupting frequently. Eruptions can cause widespread devastation, including destruction of communities, agriculture, and wildlife, and can also have global consequences, such as altering the climate and causing air pollution.

📘 Free essay examples for your ideas about Volcanoes
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Dynamic Earth
Dare to Dream
Volcanoes: Nature’s Incredible Fireworks
The Science of Volcanoes
The World’s Most Active Volcanoes
The World’s Most Dangerous Volcanoes
Mount Pinatubo Volcano Story
Sky View Restobar
Volcanoes Paper
Volcanoes: A Force of Nature
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Geysers and Their Volcanic Nature
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Differences In The Hazards
Intrusive and minor extrusive features of vulcanicity
The Ring of Fire
Volcanoes: The Power of Nature
Volcanoes: Eruptions and Disasters
Topographical Features at Divergent and Convergent Plate Margins
Geologic Hazards And Disasters
Volcanoes: The Making of a Mountain
Volcanoes: The Living Earth
Volcanoes: Wonders of the World

FAQ about Volcanoes

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How To Make Your 3-Paragraph Essay On Volcanoes Stand Out

Writing an essay on volcanoes can be very hard for you, considering that not so any students have ever found themselves in the unfortunate situation of witnessing one. However with the ease of access to information in the world at the moment, not being here to get firsthand experience is nothing more than an excuse which will not get you anywhere. There are so many students like yourself who have used their imagination in the past to make their work easier and clearly brought up really good papers about a volcano that perhaps they have never even been close to.

At times all it takes is your imagination and you will have all the information that you need to deliver some of the best content for your assignment or the research paper that you are working on. When you come to think about it, the following tips will guide you as you prepare to work on this task, and of special emphasis when you are writing a 3-paragraph paper on volcanoes.

Background information

Research into the region, provide statistical information, have graphical representation if possible.

As you prepare to work on this paper, it is important for you to realize the need for some background information. This will go so far in ensuring that you have all the data necessary to deliver a strong introduction, and provide feasible reasons why you had to choose this particular volcano as your study subject.

In most cases, it is the simple things like this one that make the difference between the students that will pass and the ones who will fail. Surely you do not want to be on the latter category.

The region under which the volcano is found should also feature in your work, and not just the geographical feature. You need to look into things like the terrain and the topographical information regarding that area.

While working on this 3-paragraph essay, try and make sure that you can use statistical information to make your work realistic. Statistics can include anything like the population affected in the area, the number of times the volcano has erupted in the past and so forth.

Providing some graphical material to help your cause is also a good idea. These can be graphs of the region, the volcano or any other information such as the weather.

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Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing (2017)

Chapter: 1 introduction, 1 introduction.

Volcanoes are a key part of the Earth system. Most of Earth’s atmosphere, water, and crust were delivered by volcanoes, and volcanoes continue to recycle earth materials. Volcanic eruptions are common. More than a dozen are usually erupting at any time somewhere on Earth, and close to 100 erupt in any year ( Loughlin et al., 2015 ).

Volcano landforms and eruptive behavior are diverse, reflecting the large number and complexity of interacting processes that govern the generation, storage, ascent, and eruption of magmas. Eruptions are influenced by the tectonic setting, the properties of Earth’s crust, and the history of the volcano. Yet, despite the great variability in the ways volcanoes erupt, eruptions are all governed by a common set of physical and chemical processes. Understanding how volcanoes form, how they erupt, and their consequences requires an understanding of the processes that cause rocks to melt and change composition, how magma is stored in the crust and then rises to the surface, and the interaction of magma with its surroundings. Our understanding of how volcanoes work and their consequences is also shared with the millions of people who visit U.S. volcano national parks each year.

Volcanoes have enormous destructive power. Eruptions can change weather patterns, disrupt climate, and cause widespread human suffering and, in the past, mass extinctions. Globally, volcanic eruptions caused about 80,000 deaths during the 20th century ( Sigurdsson et al., 2015 ). Even modest eruptions, such as the 2010 Eyjafjallajökull eruption in Iceland, have multibillion-dollar global impacts through disruption of air traffic. The 2014 steam explosion at Mount Ontake, Japan, killed 57 people without any magma reaching the surface. Many volcanoes in the United States have the potential for much larger eruptions, such as the 1912 eruption of Katmai, Alaska, the largest volcanic eruption of the 20th century ( Hildreth and Fierstein, 2012 ). The 2008 eruption of the unmonitored Kasatochi volcano, Alaska, distributed volcanic gases over most of the continental United States within a week ( Figure 1.1 ).

Finally, volcanoes are important economically. Volcanic heat provides low-carbon geothermal energy. U.S. generation of geothermal energy accounts for nearly one-quarter of the global capacity ( Bertani, 2015 ). In addition, volcanoes act as magmatic and hydrothermal distilleries that create ore deposits, including gold and copper ores.

Moderate to large volcanic eruptions are infrequent yet high-consequence events. The impact of the largest possible eruption, similar to the super-eruptions at Yellowstone, Wyoming; Long Valley, California; or Valles Caldera, New Mexico, would exceed that of any other terrestrial natural event. Volcanoes pose the greatest natural hazard over time scales of several decades and longer, and at longer time scales they have the potential for global catastrophe ( Figure 1.2 ). While

the continental United States has not suffered a fatal eruption since 1980 at Mount St. Helens, the threat has only increased as more people move into volcanic areas.

Volcanic eruptions evolve over very different temporal and spatial scales than most other natural hazards ( Figure 1.3 ). In particular, many eruptions are preceded by signs of unrest that can serve as warnings, and an eruption itself often persists for an extended period of time. For example, the eruption of Kilauea Volcano in Hawaii has continued since 1983. We also know the locations of many volcanoes and, hence, where most eruptions will occur. For these reasons, the impacts of at least some types of volcanic eruptions should be easier to mitigate than other natural hazards.

Anticipating the largest volcanic eruptions is possible. Magma must rise to Earth’s surface and this movement is usually accompanied by precursors—changes in seismic, deformation, and geochemical signals that can be recorded by ground-based and space-borne instruments. However, depending on the monitoring infrastructure, precursors may present themselves over time scales that range from a few hours (e.g., 2002 Reventador, Ecuador, and 2015 Calbuco, Chile) to decades before eruption (e.g., 1994 Rabaul, Papua New Guinea). Moreover, not all signals of volcanic unrest are immediate precursors to surface eruptions (e.g., currently Long Valley, California, and Campi Flegrei, Italy).

Probabilistic forecasts account for this uncertainty using all potential eruption scenarios and all relevant data. An important consideration is that the historical record is short and biased. The instrumented record is even shorter and, for most volcanoes, spans only the last few decades—a miniscule fraction of their lifetime. Knowledge can be extended qualitatively using field studies of volcanic deposits, historical accounts, and proxy data, such as ice and marine sediment cores and speleothem (cave) records. Yet, these too are biased because they commonly do not record small to moderate eruptions.

Understanding volcanic eruptions requires contributions from a wide range of disciplines and approaches. Geologic studies play a critical role in reconstructing the past eruption history of volcanoes,

especially of the largest events, and in regions with no historical or directly observed eruptions. Geochemical and geophysical techniques are used to study volcano processes at scales ranging from crystals to plumes of volcanic ash. Models reveal essential processes that control volcanic eruptions, and guide data collection. Monitoring provides a wealth of information about the life cycle of volcanoes and vital clues about what kind of eruption is likely and when it may occur.

1.1 OVERVIEW OF THIS REPORT

At the request of managers at the National Aeronautics and Space Administration (NASA), the National Science Foundation, and the U.S. Geological Survey (USGS), the National Academies of Sciences, Engineering, and Medicine established a committee to undertake the following tasks:

Summarize current understanding of how magma is stored, ascends, and erupts.
Discuss new disciplinary and interdisciplinary research on volcanic processes and precursors that could lead to forecasts of the type, size, and timing of volcanic eruptions.
Describe new observations or instrument deployment strategies that could improve quantification of volcanic eruption processes and precursors.
Identify priority research and observations needed to improve understanding of volcanic eruptions and to inform monitoring and early warning efforts.

The roles of the three agencies in advancing volcano science are summarized in Box 1.1 .

The committee held four meetings, including an international workshop, to gather information, deliberate, and prepare its report. The report is not intended to be a comprehensive review, but rather to provide a broad overview of the topics listed above. Chapter 2 addresses the opportunities for better understanding the storage, ascent, and eruption of magmas. Chapter 3 summarizes the challenges and prospects for forecasting eruptions and their consequences. Chapter 4 highlights repercussions of volcanic eruptions on a host of other Earth systems. Although not explicitly called out in the four tasks, the interactions between volcanoes and other Earth systems affect the consequences of eruptions, and offer opportunities to improve forecasting and obtain new insights into volcanic processes. Chapter 5 summarizes opportunities to strengthen

research in volcano science. Chapter 6 provides overarching conclusions. Supporting material appears in appendixes, including a list of volcano databases (see Appendix A ), a list of workshop participants (see Appendix B ), biographical sketches of the committee members (see Appendix C ), and a list of acronyms and abbreviations (see Appendix D ).

Background information on these topics is summarized in the rest of this chapter.

1.2 VOLCANOES IN THE UNITED STATES

The USGS has identified 169 potentially active volcanoes in the United States and its territories (e.g., Marianas), 55 of which pose a high threat or very high threat ( Ewert et al., 2005 ). Of the total, 84 are monitored by at least one seismometer, and only 3 have gas sensors (as of November 2016). 1 Volcanoes are found in the Cascade mountains, Aleutian arc, Hawaii, and the western interior of the continental United States ( Figure 1.4 ). The geographical extent and eruption hazards of these volcanoes are summarized below.

The Cascade volcanoes extend from Lassen Peak in northern California to Mount Meager in British Columbia. The historical record contains only small- to moderate-sized eruptions, but the geologic record reveals much larger eruptions ( Carey et al., 1995 ; Hildreth, 2007 ). Activity tends to be sporadic ( Figure 1.5 ). For example, nine Cascade eruptions occurred in the 1850s, but none occurred between 1915 and 1980, when Mount St. Helens erupted. Consequently, forecasting eruptions in the Cascades is subject to considerable uncertainty. Over the coming decades, there may be multiple eruptions from several volcanoes or no eruptions at all.

The Aleutian arc extends 2,500 km across the North Pacific and comprises more than 130 active and potentially active volcanoes. Although remote, these volcanoes pose a high risk to overflying aircraft that carry more than 30,000 passengers a day, and are monitored by a combination of ground- and space-based sensors. One or two small to moderate explosive eruptions occur in the Aleutians every year, and very large eruptions occur less frequently. For example, the world’s largest eruption of the 20th century occurred approximately 300 miles from Anchorage, in 1912.

In Hawaii, Kilauea has been erupting largely effusively since 1983, but the location and nature of eruptions can vary dramatically, presenting challenges for disaster preparation. The population at risk from large-volume, rapidly moving lava flows on the flanks of the Mauna Loa volcano has grown tremendously in the past few decades ( Dietterich and Cashman, 2014 ), and few island residents are prepared for the even larger magnitude explosive eruptions that are documented in the last 500 years ( Swanson et al., 2014 ).

All western states have potentially active volcanoes, from New Mexico, where lava flows have reached within a few kilometers of the Texas and Oklahoma borders ( Fitton et al., 1991 ), to Montana, which borders the Yellowstone caldera ( Christiansen, 1984 ). These volcanoes range from immense calderas that formed from super-eruptions ( Mastin et al., 2014 ) to small-volume basaltic volcanic fields that erupt lava flows and tephra for a few months to a few decades. Some of these eruptions are monogenic (erupt just once) and pose a special challenge for forecasting. Rates of activity in these distributed volcanic fields are low, with many eruptions during the past few thousand years (e.g., Dunbar, 1999 ; Fenton, 2012 ; Laughlin et al., 1994 ), but none during the past hundred years.

1.3 THE STRUCTURE OF A VOLCANO

Volcanoes often form prominent landforms, with imposing peaks that tower above the surrounding landscape, large depressions (calderas), or volcanic fields with numerous dispersed cinder cones, shield volcanoes, domes, and lava flows. These various landforms reflect the plate tectonic setting, the ways in which those volcanoes erupt, and the number of eruptions. Volcanic landforms change continuously through the interplay between constructive processes such as eruption and intrusion, and modification by tectonics, climate, and erosion. The stratigraphic and structural architecture of volcanoes yields critical information on eruption history and processes that operate within the volcano.

Beneath the volcano lies a magmatic system that in most cases extends through the crust, except during eruption. Depending on the setting, magmas may rise

___________________

1 Personal communication from Charles Mandeville, Program Coordinator, Volcano Hazards Program, U.S. Geological Survey, on November 26, 2016.

directly from the mantle or be staged in one or more storage regions within the crust before erupting. The uppermost part (within 2–3 km of Earth’s surface) often hosts an active hydrothermal system where meteoric groundwater mingles with magmatic volatiles and is heated by deeper magma. Identifying the extent and vigor of hydrothermal activity is important for three reasons: (1) much of the unrest at volcanoes occurs in hydrothermal systems, and understanding the interaction of hydrothermal and magmatic systems is important for forecasting; (2) pressure buildup can cause sudden and potentially deadly phreatic explosions from the hydrothermal system itself (such as on Ontake, Japan, in 2014), which, in turn, can influence the deeper magmatic system; and (3) hydrothermal systems are energy resources and create ore deposits.

Below the hydrothermal system lies a magma reservoir where magma accumulates and evolves prior to eruption. Although traditionally modeled as a fluid-filled cavity, there is growing evidence that magma reservoirs may comprise an interconnected complex of vertical and/or horizontal magma-filled cracks, or a partially molten mush zone, or interleaved lenses of magma and solid material ( Cashman and Giordano, 2014 ). In arc volcanoes, magma chambers are typically located 3–6 km below the surface. The magma chamber is usually connected to the surface via a fluid-filled conduit only during eruptions. In some settings, magma may ascend directly from the mantle without being stored in the crust.

In the broadest sense, long-lived magma reservoirs comprise both eruptible magma (often assumed to contain less than about 50 percent crystals) and an accumulation of crystals that grow along the margins or settle to the bottom of the magma chamber. Physical segregation of dense crystals and metals can cause the floor of the magma chamber to sag, a process balanced by upward migration of more buoyant melt. A long-lived magma chamber can thus become increasingly stratified in composition and density.

The deepest structure beneath volcanoes is less well constrained. Swarms of low-frequency earthquakes at mid- to lower-crustal depths (10–40 km) beneath volcanoes suggest that fluid is periodically transferred into the base of the crust ( Power et al., 2004 ). Tomographic studies reveal that active volcanic systems have deep crustal roots that contain, on average, a small fraction of melt, typically less than 10 percent. The spatial distribution of that melt fraction, particularly how much is concentrated in lenses or in larger magma bodies, is unknown. Erupted samples preserve petrologic and geochemical evidence of deep crystallization, which requires some degree of melt accumulation. Seismic imaging and sparse outcrops suggest that the proportion of unerupted solidified magma relative to the surrounding country rock increases with depth and that the deep roots of volcanoes are much more extensive than their surface expression.

1.4 MONITORING VOLCANOES

Volcano monitoring is critical for hazard forecasts, eruption forecasts, and risk mitigation. However, many volcanoes are not monitored at all, and others are monitored using only a few types of instruments. Some parameters, such as the mass, extent, and trajectory of a volcanic ash cloud, are more effectively measured by satellites. Other parameters, notably low-magnitude earthquakes and volcanic gas emissions that may signal an impending eruption, require ground-based monitoring on or close to the volcanic edifice. This section summarizes existing and emerging technologies for monitoring volcanoes from the ground and from space.

Monitoring Volcanoes on or Near the Ground

Ground-based monitoring provides data on the location and movement of magma. To adequately capture what is happening inside a volcano, it is necessary to obtain a long-term and continuous record, with periods spanning both volcanic quiescence and periods of unrest. High-frequency data sampling and efficient near-real-time relay of information are important, especially when processes within the volcano–magmatic–hydrothermal system are changing rapidly. Many ground-based field campaigns are time intensive and can be hazardous when volcanoes are active. In these situations, telemetry systems permit the safe and continuous collection of data, although the conditions can be harsh and the lifetime of instruments can be limited in these conditions.

Ground-based volcano monitoring falls into four broad categories: seismic, deformation, gas, and thermal monitoring ( Table 1.1 ). Seismic monitoring tools,

TABLE 1.1 Ground-Based Instrumentation for Monitoring Volcanoes

Measurement	Instrument	Purpose
Seismic waves	Geophone	Detect lahars (volcanic mudflows) and pyroclastic density currents
	Short-period seismometer	Locate earthquakes, study earthquake mechanics, and detect unrest
	Broadband seismometer	Study earthquakes, tremor, and long-period earthquakes to quantify rock failure, fluid movement, and eruption progress
	Infrasound detector	Track evolution of near-surface eruptive activity
Geodetic	Classical surveying techniques	Detect deformation over broad areas
	Tiltmeter	Detect subtle pressurization or volumetric sources
	Strainmeter	Detect changing stress distributions
	GNSS/Global Positioning System	Model intrusion locations and sizes, detect ash clouds
	Photogrammetic and structure from motion	Map and identify or measure morphologic changes
	Lidar	Precision mapping, detect ash and aerosol heights
	Radar	Quantify rapid surface movements and velocities of ballistic pyroclasts
Gas	Miniature differential optical absorption spectrometer	Detect sulfur species concentrations and calculate gas flux
	Open-path Fourier transform infrared spectroscopy	Quantify gas concentration ratios
	Ultraviolet imagers	Detect plume sulfur
	Gigenbach-type sampling and multiGAS sensors	Determine chemical and isotopic compositions and make in situ measurements of gas species
	Portable laser spectrometer	Measure stable isotopic ratios of gases
Thermal	Infrared thermal camera	Detect dome growth, lava breakouts, and emissions of volcanic ash and gas
Thermal	In situ thermocouple	Monitor fumarole temperatures
Hydrologic	Temperature probe	Detect changes in hydrothermal sources
	Discharge measurements	Detect changes in pressure or permeability
	Sampling for chemical and isotopic composition	Detect magma movement
Potential fields	Gravimeter	Detect internal mass movement
	Self-potential, resistivity	Detect fluids and identify fractures and voids
	Magnetotellurics	3D location of fluids and magma in shallow crust
Other	Cosmic ray muon detector	Tomography
	High-speed camera	Image explosion dynamics
	Drones	Visually observe otherwise inaccessible surface phenomena
	Lightning detection array	Locate lightning and identify ash emissions

including seismometers and infrasound sensors, are used to detect vibrations caused by breakage of rock and movement of fluids and to assess the evolution of eruptive activity. Ambient seismic noise monitoring can image subsurface reservoirs and document changes in wave speed that may reflect stress. changes. Deformation monitoring tools, including tiltmeters, borehole strainmeters, the Global Navigation Satellite System (GNSS, which includes the Global Positioning System [GPS]), lidar, radar, and gravimeters, are used to detect the motion of magma and other fluids in the subsurface. Some of these tools, such as GNSS and lidar, are also used to detect erupted products, including ash clouds, pyroclastic density currents, and volcanic bombs. Gas monitoring tools, including a range of sensors ( Table 1.1 ), and direct sampling of gases and fluids are used to detect magma intrusions and changes in magma–hydrothermal interactions. Thermal monitoring tools, such as infrared cameras, are used to detect dome growth and lava breakouts. Continuous video or photographic observations are also commonly used and, despite their simplicity, most directly document volcanic activity. Less commonly used monitoring technologies, such as self-potential, electromagnetic techniques, and lightning detection are used to constrain fluid movement and to detect

ash clouds. In addition, unmanned aerial vehicles (e.g., aircraft and drones) are increasingly being used to collect data. Rapid sample collection and analysis is also becoming more common as a monitoring tool at volcano observatories. A schematic of ground-based monitoring techniques is shown in Figure 1.6 .

Monitoring Volcanoes from Space

Satellite-borne sensors and instruments provide synoptic observations during volcanic eruptions when collecting data from the ground is too hazardous or where volcanoes are too remote for regular observation. Repeat-pass data collected over years or decades provide a powerful means for detecting surface changes on active volcanoes. Improvements in instrument sensitivity, data availability, and the computational capacity required to process large volumes of data have led to a dramatic increase in “satellite volcano science.”

Although no satellite-borne sensor currently in orbit has been specifically designed for volcano monitoring, a number of sensors measure volcano-relevant

TABLE 1.2 Satellite-Borne Sensor Suite for Volcano Monitoring

Measurement	Purpose	Examples
High-temporal/low-spatial-resolution multispectral thermal infrared	Detect eruptions and map ash clouds	GOES
Low-temporal/moderate-spatial-resolution multispectral thermal infrared	Detect eruptions and map ash clouds with coverage of high latitudes; infer lava effusion rate	AVHRR, MODIS
Low-temporal/high-spatial-resolution multispectral visible infrared	Map detailed surface and plumes; infer lava effusion rate	Landsat, ASTER, Sentinel-2
Hyperspectral ultraviolet	Detect and quantify volcanic SO , BrO, and OClO	OMI
Hyperspectral infrared	Detect and quantify volcanic SO and H S in nighttime and winter	IASI, AIRS
Microwave limb sounding	Detect volcanic SO and HCl in the upper troposphere and stratosphere	MLS
Visible–near-infrared multiangle imaging	Determine volcanic ash cloud altitudes and plume speed	MISR
Ultraviolet–visible limb scattering	Measure aerosol vertical profiles	OMPS-LP
Ultraviolet–near-infrared solar occultation	Measure stratospheric aerosol	SAGE III
Spaceborne lidar	Develop vertical profiles of volcanic clouds	CALIPSO
Spaceborne W-band radar	Measure volcanic hydrometeors	CloudSat
Multiband (X-, C-, L-band) synthetic aperture radar	Measure deformation globally	Sentinel-1a/b, ALOS-2, COSMO-SkyMed, TerraSAR-X, TanDEM-X, Radarsat-2

NOTE: AIRS, Atmospheric Infrared Sounder; ALOS, Advanced Land Observing Satellite; ASTER, Advanced Spaceborne Thermal Emission and Reflection Radiometer; AVHRR, Advanced Very High Resolution Radiometer; CALIPSO, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation; COSMO-SkyMed, Constellation of Small Satellites for Mediterranean Basin Observation; GOES, Geostationary Operational Environmental Satellite; IASI, Infrared Atmospheric Sounding Interferometer; MISR, Multi-angle Imaging SpectroRadiometer; MLS, Microwave Limb Sounder; MODIS, Moderate Resolution Imaging Spectroradiometer; OMI, Ozone Monitoring Instrument; OMPS, Ozone Mapping and Profiler Suite; SAGE, Stratospheric Aerosol and Gas Experiment.

parameters, including heat flux, gas and ash emissions, and deformation ( Table 1.2 ). Thermal infrared data are used to detect eruption onset and cessation, calculate lava effusion rates, map lava flows, and estimate ash column heights during explosive eruptions. In some cases, satellites may capture thermal precursors to eruptions, although low-temperature phenomena are challenging to detect. Both high-temporal/low-spatial-resolution (geostationary orbit) and high-spatial/low-temporal-resolution (polar orbit) thermal infrared observations are needed for global volcano monitoring.

Satellite-borne sensors are particularly effective for observing the emission and dispersion of volcanic gas and ash plumes in the atmosphere. Although several volcanic gas species can be detected from space (including SO 2 , BrO, OClO, H 2 S, HCl, and CO; Carn et al., 2016 ), SO 2 is the most readily measured, and it is also responsible for much of the impact of eruptions on climate. Satellite measurements of SO 2 are valuable for detecting eruptions, estimating global volcanic fluxes and recycling of other volatile species, and tracking volcanic clouds that may be hazardous to aviation in near real time. Volcanic ash cloud altitude is most accurately determined by spaceborne lidar, although spatial coverage is limited. Techniques for measuring volcanic CO 2 from space are under development and could lead to earlier detection of preeruptive volcanic degassing.

Interferometric synthetic aperture radar (InSAR) enables global-scale background monitoring of volcano deformation ( Figure 1.7 ). InSAR provides much higher spatial resolution than GPS, but lower accuracy and temporal resolution. However, orbit repeat times will diminish as more InSAR missions are launched, such as the European Space Agency’s recently deployed Sentinel-1 satellite and the NASA–Indian Space Research Organisation synthetic aperture radar mission planned for launch in 2020.

1.5 ERUPTION BEHAVIOR

Eruptions range from violently explosive to gently effusive, from short lived (hours to days) to persistent over decades or centuries, from sustained to intermittent, and from steady to unsteady ( Siebert et al., 2015 ). Eruptions may initiate from processes within the magmatic system ( Section 1.3 ) or be triggered by processes and properties external to the volcano, such as precipitation, landslides, and earthquakes. The eruption behavior of a volcano may change over time. No classification scheme captures this full diversity of behaviors (see Bonadonna et al., 2016 ), but some common schemes to describe the style, magnitude, and intensity of eruptions are summarized below.

Eruption Magnitude and Intensity

The size of eruptions is usually described in terms of total erupted mass (or volume), often referred to as magnitude, and mass eruption rate, often referred to as intensity. Pyle (2015) quantified magnitude and eruption intensity as follows:

magnitude = log 10 (mass, in kg) – 7, and

intensity = log 10 (mass eruption rate, in kg/s) + 3.

The Volcano Explosivity Index (VEI) introduced by Newhall and Self (1982) assigns eruptions to a VEI class based primarily on measures of either magnitude (erupted mass or volume) or intensity (mass eruption rate and/or eruption plume height), with more weight given to magnitude. The VEI classes are summarized in Figure 1.8 . The VEI classification is still in use, despite its many limitations, such as its reliance on only a few types of measurements and its poor fit for small to moderate eruptions (see Bonadonna et al., 2016 ).

Smaller VEI events are relatively common, whereas larger VEI events are exponentially less frequent ( Siebert et al., 2015 ). For example, on average about three VEI 3 eruptions occur each year, whereas there is a 5 percent chance of a VEI 5 eruption and a 0.2 percent chance of a VEI 7 (e.g., Crater Lake, Oregon) event in any year.

Eruption Style

The style of an eruption encompasses factors such as eruption duration and steadiness, magnitude, gas flux, fountain or column height, and involvement of magma and/or external source of water (phreatic and phreatomagmatic eruptions). Eruptions are first divided into effusive (lava producing) and explosive (pyroclast producing) styles, although individual eruptions can be simultaneously effusive and weakly explosive, and can pass rapidly and repeatedly between eruption styles. Explosive eruptions are further subdivided into styles that are sustained on time scales of hours to days and styles that are short lived ( Table 1.3 ).

Classification of eruption style is often qualitative and based on historical accounts of characteristic eruptions from type-volcanoes. However, many type-volcanoes exhibit a range of eruption styles over time (e.g., progressing between Strombolian, Vulcanian, and Plinian behavior; see Fee et al., 2010 ), which has given rise to terms such as subplinian or violent Strombolian.

1.6 ERUPTION HAZARDS

Eruption hazards are diverse ( Figure 1.9 ) and may extend more than thousands of kilometers from an active volcano. From the perspective of risk and impact, it is useful to distinguish between near-source and distal hazards. Near-source hazards are far more unpredictable than distal hazards.

Near-source hazards include those that are airborne, such as tephra fallout, volcanic gases, and volcanic projectiles, and those that are transported laterally on or near the ground surface, such as pyroclastic density currents, lava flows, and lahars. Pyroclastic density currents are hot volcanic flows containing mixtures of gas and micron- to meter-sized volcanic particles. They can travel at velocities exceeding 100 km per hour. The heat combined with the high density of material within these flows obliterates objects in their path, making them the most destructive of volcanic hazards. Lava flows also destroy everything in their path, but usually move slowly enough to allow people to get out of the way. Lahars are mixtures of volcanic debris, sediment, and water that can travel many tens of kilometers along valleys and river channels. They may be triggered during an eruption by interaction between volcanic prod-

TABLE 1.3 Characteristics of Different Eruption Styles

Eruption Style	Characteristics
Hawaiian	Sustained fountaining of magmatic gas and pyroclasts (up to ~1,000 m) often generating clastogenic, gas-charged lava flows from single vents or from fissures
Strombolian	Short-duration, low-vigor, episodic, small (<100s of meters) explosions driven by escape of pockets of gas and ejecting some bombs and spatter
Vulcanian	Short-duration, moderately vigorous, magma-fragmenting explosions producing ash-rich columns that may reach heights >1,000 m
Surtseyan	Short duration, weak phreatomagmatic explosive eruptions where fluid magma interacts with standing water
Phreatoplinian	Prolonged powerful phreatomagmatic explosions where viscous magma interacts with surface water or groundwater
Dome collapse	Dome collapse pyroclastic flows occur at unstable gas-charged domes either with an explosive central column eruption (e.g., Mount Pelee) or without (e.g., Unzen, Montserrat, and Santiaguito)
Plinian	Very powerful, sustained eruptions with columns reaching the stratosphere (>15 km) and sometimes generating large pyroclastic density currents from collapsing eruption columns

ucts and snow, ice, rain, or groundwater. Lahars can be more devastating than the eruption itself. Ballistic blocks are large projectiles that typically fall within 1–5 km from vents.

The largest eruptions create distal hazards. Explosive eruptions produce plumes that are capable of dispersing ash hundreds to thousands of kilometers from the volcano. The thickness of ash deposited depends on the intensity and duration of the eruption and the wind direction. Airborne ash and ash fall are the most severe distal hazards and are likely to affect many more people than near-source hazards. They cause respiratory problems and roof collapse, and also affect transport networks and infrastructure needed to support emergency response. Volcanic ash is a serious risk to air traffic. Several jets fully loaded with passengers have temporarily lost power on all engines after encountering dilute ash clouds (e.g., Guffanti et al., 2010 ). Large lava flows, such as the 1783 Laki eruption in Iceland, emit volcanic gases that create respiratory problems and acidic rain more than 1,000 km from the eruption. Observed impacts of basaltic eruptions in Hawaii and Iceland include regional volcanic haze (“vog”) and acid rain that affect both agriculture and human health (e.g., Thordarson and Self, 2003 ) and fluorine can contaminate grazing land and water supplies (e.g., Cronin et al., 2003 ). Diffuse degassing of CO 2 can lead to deadly concentrations with fatal consequences such as occurred at Mammoth Lakes, California, or cause lakes to erupt, leading to massive CO 2 releases that suffocate people (e.g., Lake Nyos, Cameroon).

Secondary hazards can be more devastating than the initial eruption. Examples include lahars initiated by storms, earthquakes, landslides, and tsunamis from eruptions or flank collapse; volcanic ash remobilized by wind to affect human health and aviation for extended periods of time; and flooding because rain can no longer infiltrate the ground.

1.7 MODELING VOLCANIC ERUPTIONS

Volcanic processes are governed by the laws of mass, momentum, and energy conservation. It is possible to develop models for magmatic and volcanic phenomena based on these laws, given sufficient information on mechanical and thermodynamic properties of the different components and how they interact with each other. Models are being developed for all processes in volcanic systems, including melt transport in the mantle, the evolution of magma bodies within the crust, the ascent of magmas to the surface, and the fate of magma that erupts effusively or explosively.

A central challenge for developing models is that volcanic eruptions are complex multiphase and multicomponent systems that involve interacting processes over a wide range of length and time scales. For example, during storage and ascent, the composition, temperature, and physical properties of magma and host rocks evolve. Bubbles and crystals nucleate and grow in this magma and, in turn, greatly influence the properties of the magmas and lavas. In explosive eruptions, magma fragmentation creates a hot mixture of gas and particles with a wide range of sizes and densities. Magma also interacts with its surroundings: the deformable rocks that surround the magma chamber and conduit, the potentially volatile groundwater and surface water, a changing landscape over which pyroclastic density currents and lava flows travel, and the atmosphere through which eruption columns rise.

Models for volcanic phenomena that involve a small number of processes and that are relatively amenable to direct observation, such as volcanic plumes, are relatively straightforward to develop and test. In contrast, phenomena that occur underground are more difficult to model because there are more interacting processes. In those cases, direct validation is much more challenging and in many cases impossible. Forecasting ash dispersal using plume models is more straightforward and testable than forecasting the onset, duration, and style of eruption using models that seek to explain geophysical and geochemical precursors. In all cases, however, the use of even imperfect models helps improve the understanding of volcanic systems.

Modeling approaches can be divided into three categories:

Reduced models make simplifying assumptions about dynamics, heat transfer, and geometry to develop first-order explanations for key properties and processes, such as the velocity of lava flows and pyroclastic density currents, the height of eruption columns, the magma chamber size and depth, the dispersal of tephra, and the ascent of magma in conduits. Well-calibrated or tested reduced models offer a straightforward ap-

proach for combining observations and models in real time in an operational setting (e.g., ash dispersal forecasting for aviation safety). Models may not need to be complex if they capture the most important processes, although simplifications require testing against more comprehensive models and observations.

Multiphase and multiphysics models improve scientific understanding of complex processes by invoking fewer assumptions and idealizations than reduced models ( Figure 1.10 ), but at the expense of increased complexity and computational demands. They also require additional components, such as a model for how magma in magma chambers and conduits deforms when stressed; a model for turbulence in pyroclastic density currents and plumes; terms that describe the thermal and mechanical exchange among gases, crystals, and particles; and a description of ash aggregation in eruption columns. A central challenge for multiphysics models is integrating small-scale processes with large-scale dynamics. Many of the models used in volcano science build on understanding developed in other science and engineering fields and for other ap-

plications. Multiphysics and multiscale models benefit from rapidly expanding computational capabilities.

Laboratory experiments simulate processes for which the geometry and physical and thermal processes and properties can be scaled ( Mader et al., 2004 ). Such experiments provide insights on fundamental processes, such as crystal dynamics in flowing magmas, entrainment in eruption columns, propagation of dikes, and sedimentation from pyroclastic density currents ( Figure 1.11 ). Experiments have also been used successfully to develop the subsystem models used in numerical simulations, and to validate computer simulations for known inputs and properties.

The great diversity of existing models reflects to a large extent the many interacting processes that operate in volcanic eruptions and the corresponding simplifying assumptions currently required to construct such models. The challenge in developing models is often highlighted in discrepancies between models and observations of natural systems. Nevertheless, eruption models reveal essential processes governing volcanic eruptions, and they provide a basis for interpreting measurements from prehistoric and active eruptions and for closing observational gaps. Mathematical models offer a guide for what observations will be most useful. They may also be used to make quantitative and testable predictions, supporting forecasting and hazard assessment.

Volcanic eruptions are common, with more than 50 volcanic eruptions in the United States alone in the past 31 years. These eruptions can have devastating economic and social consequences, even at great distances from the volcano. Fortunately many eruptions are preceded by unrest that can be detected using ground, airborne, and spaceborne instruments. Data from these instruments, combined with basic understanding of how volcanoes work, form the basis for forecasting eruptions—where, when, how big, how long, and the consequences.

Accurate forecasts of the likelihood and magnitude of an eruption in a specified timeframe are rooted in a scientific understanding of the processes that govern the storage, ascent, and eruption of magma. Yet our understanding of volcanic systems is incomplete and biased by the limited number of volcanoes and eruption styles observed with advanced instrumentation. Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing identifies key science questions, research and observation priorities, and approaches for building a volcano science community capable of tackling them. This report presents goals for making major advances in volcano science.

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ENCYCLOPEDIC ENTRY

A volcano is an opening in a planet or moon’s crust through which molten rock and gases trapped under the surface erupt, often forming a hill or mountain.

Volcanic eruption

Volcanic eruptions can create colorful and dramatic displays, such as this eruption of this volcano in the Virunga Moutains of the Democratic Republic of the Congo.

Photograph by Chris Johns

A volcano is an opening in a planet or moon’s crust through which molten rock, hot gases, and other materials erupt . Volcanoes often form a hill or mountain as layers of rock and ash build up from repeated eruptions .

Volcanoes are classified as active, dormant, or extinct. Active volcanoes have a recent history of eruptions ; they are likely to erupt again. Dormant volcanoes have not erupted for a very long time but may erupt at a future time. Extinct volcanoes are not expected to erupt in the future.

Inside an active volcano is a chamber in which molten rock, called magma , collects. Pressure builds up inside the magma chamber, causing the magma to move through channels in the rock and escape onto the planet’s surface. Once it flows onto the surface the magma is known as lava .

Some volcanic eruptions are explosive, while others occur as a slow lava flow. Eruptions can occur through a main opening at the top of the volcano or through vents that form on the sides. The rate and intensity of eruptions, as well as the composition of the magma, determine the shape of the volcano.

Volcanoes are found on both land and the ocean floor. When volcanoes erupt on the ocean floor, they often create underwater mountains and mountain ranges as the released lava cools and hardens. Volcanoes on the ocean floor become islands when the mountains become so large they rise above the surface of the ocean.

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