Chapter 7: Volcanoes

In the north-western United States there is a lake so deep and so still that visitors often take it for an ordinary mountain lake. It is not. Crater Lake is about 10 km wide, held inside a ring of steep walls, and it is the deepest lake in the USA. The walls are the clue. They are what is left of a volcano. The mountain that once stood here erupted again and again. Every eruption drew molten rock out of the huge chamber lying beneath it. In the end the chamber could no longer push up enough magma to fill the space it had emptied, and the summit had nothing left to rest on. The cone collapsed inward, leaving a wide sunken hollow where a peak had been. Rain and melting snow did the rest, and the hollow slowly filled with water. So a lake now marks the spot where a mountain destroyed itself from below. That is the strange double life of a volcano. The same vent that smothers fields under ash also builds plateaus, warms the springs that people travel a long way to bathe in, and hollows out a basin deep enough to hold a lake like this one.

Diagram

Fig. 7.A1 Caption: A caldera lake is the record of a summit that collapsed into the space its own magma had emptied. Brief: Three-panel cross-section — full cone above a full magma chamber; then the emptied chamber with the summit collapsing inward, arrows pointing down; then the hollow filled with water — label cone, magma chamber, caldera and lake in each panel.

Curiosity Corner

Q1. Why do most volcanoes erupt near the edges of the earth’s plates rather than in the middle of them? Q2. A volcano may be called active, dormant or extinct. What decides which of the three names it gets? Q3. How can a lake come to sit in the place where a mountain summit once stood? Q4. Volcanic ash can bury a farmer’s field and ruin the crop, so why is volcanic land so often called fertile? This chapter will let you answer all four for yourself.

7.1 Causes Of Volcanic Eruptions

The forces that arise from the interior of the earth are called endogenic forces. There are two types of endogenic forces — diastrophic movements and sudden movements. This chapter concerns itself only with the sudden movements, which bring about instant changes on the surface of the earth, and that is exactly how they get their name.

A volcanic eruption is one of these sudden movements, and it is driven by forces working inside the earth rather than on its surface. A volcano is a vent in the earth’s crust from which fiery hot magma erupts from the earth’s interior. The word therefore names an opening, not a mountain; the mountain is only what later collects around the opening.

The chapter gives three main causes of volcanic eruptions: (a) Heat and pressure inside the earth melt rock at depth and build up a pressure that must eventually find a way out. (b) Plate tectonics weakens the crust along the margins where the earth’s movable plates meet, giving that pressure its escape route. (c) A magma chamber collects the molten rock under the crust and pushes it steadily upwards until a route to the surface opens.

Heat That Cannot Escape

Temperature and pressure both increase as you go from the surface of the earth towards its centre. Rocks, however, are bad conductors of heat, which means heat passes through them very slowly. The earth’s heat therefore does not escape on its own.

Trapped where it is made, that heat melts the rocks around it and builds up great pressure. The molten rock material inside the earth is known as magma. The pressure then forces the heat to find an escape route through the fissures and cracks that run through the rocks above.

Where The Crust Is Weakest

Most volcanic eruptions take place near plate margins. The earth’s crust is made up of a series of movable plates, and the intense heat inside the earth’s core keeps these plates in motion. When plates slide past each other, or collide against each other, their edges produce faults — fractures along which blocks of rock have shifted — and these faults follow the lines of weakness in the crust.

It is along these plate boundaries that the crust is weakened, and it is through them that the hot magma gushes out, leading to a volcanic eruption. This explains something you may already have met: the same forces that cause earthquakes also cause volcanic eruptions. Fig. 7.1 sets the world’s volcanoes against the outlines of the plates, and the two patterns are hard to tell apart.

Map

Fig. 7.1 Caption: Tectonic plates and distribution of volcanoes Brief: World map with plate boundaries drawn as lines and individual volcanoes marked as triangles; label the major oceans and continents as printed; key to show volcanoes and tectonic plates; every map of India must show the full extent of India as officially defined.

Read It

Fig. 7.1 The triangles are volcanoes; the lines are plate boundaries. Read the two together and the triangles crowd along the lines, thinning out in plate interiors. The map does not tell you which of these volcanoes is active now, nor when any of them last erupted — a triangle marks a site, not an event.

The Chamber Beneath The Volcano

While it is still under the earth’s crust, the molten material melts the weak rocks around it and creates a huge chamber for itself. Fresh magma continues to pour into this chamber from below. Because magma contains silicate material, gases and water vapours, its pressure acts vertically upwards, pressing against the roof of rock above it.

Once a route is found, the magma rises to the surface. This is why many volcanoes are found along plate boundaries, where such routes already exist. All major volcanoes have beneath them a huge magma chamber, or reservoir, joined to the earth’s surface by a conduit known as the volcanic vent.

Curiosity Corner

Q1. Why do most volcanoes erupt near the edges of the earth’s plates rather than in the middle of them?

A. Rising magma needs a weakness to travel through. Plate edges grind and collide, producing faults along lines of weakness, so the crust there is already broken and offers the easiest route to the surface.

The three causes are not three separate stories. They are one chain, and it helps to see it whole before moving on.

Flowchart

Fig. 7.A2 Caption: One chain runs from trapped heat to an eruption at the surface. Nodes: heat and pressure increase with depth → rocks are bad conductors, so heat cannot escape → rocks melt into magma → magma melts weak rock and forms a magma chamber → gases and water vapour make the pressure act upwards → is there a line of weakness above? yes → magma rises through faults at a plate boundary and erupts; no → magma stays trapped and pressure keeps building

7.2 Products Of A Volcano

An eruption is not a single substance coming out of a single hole. A volcano erupts a number of products, including steam, gases, molten rock, dust, ashes and other liquid and solid matter. Sorting them into groups makes the eruption much easier to picture, and the book sorts them into two.

Magma And Lava

Magma and lava are the same material, and the only thing that separates them is where the material happens to be. The molten rock material inside the earth is known as magma. When that same molten rock reaches the surface of the earth, it is known as lava. So the moment magma passes out through the vent it changes its name, though nothing about its make-up has changed.

Lava consists of many materials and gases. Steam is the most abundant gas that comes out of a volcano, and when steam is ejected in huge quantities it forms clouds in the atmosphere, which then give rainfall. Other gases ejected by volcanoes include hydrogen sulphide, sulphur dioxide, carbon dioxide and hydrogen.

Solid Matter

The solid fragments thrown out by a volcano are pieces of rock, and they are known as pyroclasts. They are not all of one size. The dust particles go on increasing in size, and the book names the fragments by how large they have grown.

Pyroclasts are usually described in three sizes: (a) The finest particles are called dust, light enough to be carried far from the volcano by the wind. (b) The small stone-sized particles are called lapilli, too heavy to drift far and so they settle nearer the vent. (c) Violent eruptions throw out material larger still, known as bombs, and eject more of it than a quiet eruption does.

One point about these fragments is easy to get wrong. The rock fragments that a volcano erupts do not originate from the magma chamber. They are picked up by the magma in the course of its upward movement, torn from the walls of the passage as the molten rock forces its way to the surface.

What If

Magma Without Its Gases Remove: the gases and water vapours that magma carries. Then: the pressure inside the chamber would no longer act so strongly upwards, the magma would rise sluggishly rather than gush, and there would be no steam clouds and far less material blasted out as dust, lapilli and bombs. Which tells us: the violence of an eruption comes from the gases trapped in the magma, not from the molten rock alone.

Flowchart

Fig. 7.A3 Caption: Everything a volcano throws out belongs to one of three states of matter. Nodes: products of a volcano → liquid: magma, called lava once it reaches the surface / gaseous: steam, the most abundant, then hydrogen sulphide, sulphur dioxide, carbon dioxide and hydrogen / solid: pyroclasts → dust → lapilli → bombs, increasing in size

7.3 Structure Of A Volcano

The very hot material that a volcano pours out creates structures both inside the earth and on its surface, and the structures are typical of a particular type of eruption. Some eruptions take place quietly, with lava spreading out of the opening, while others take place violently, blasting material into the air. Because the two kinds of eruption leave different material behind, no two volcanoes are built quite alike.

The Opening At The Top

The lava flows out through an opening in the earth’s crust called the vent, which sits at the top of the central conduit channel leading up from the magma chamber. The opening is usually like a circular pipe. There may be only one opening at the summit of the volcano.

Repeated eruptions, however, do not always use the same route. Subsidiary vents are formed during repeated eruptions, so an old volcano may pour out material from openings on its flanks as well as from its summit. Fig. 7.2 shows the whole arrangement in cross-section, from the chamber at depth to the material leaving the summit.

Diagram

Fig. 7.2 Caption: Structure of a volcano Brief: Cut-away cross-section of a cone; label eruption of lava, smoke, steam, pyroclastic material, vent, central conduit channel, magma chamber, solidified lava layer and solidified ash layer, with arrows running upward from the chamber through the conduit to the vent.

Read It

Fig. 7.2 Follow the arrows upward: chamber, conduit, vent, surface. Inside the cone, lava layers and ash layers alternate, which tells you the volcano was built by many eruptions, not one. The figure shows structure, not size or time — it cannot tell you how tall the cone is or how long it took to grow.

How The Cone Is Built

The erupted fluid material is hot when it leaves the vent, and it collects around the opening. There it begins to cool gradually and solidifies. Each eruption therefore leaves a fresh layer on top of the last one, which is why the cut-away view shows solidified lava and solidified ash stacked in bands.

The accumulation of material around the vent gives the volcano its typical cone-like shape, known as the volcanic cone. A cone is not something a volcano starts with; it is something the volcano manufactures for itself out of its own products. Cones are landforms made by volcanoes, and the landforms of volcanoes are taken up later in this chapter.

Flowchart

Fig. 7.A4 Caption: The cone is the record of every eruption the volcano has had. Nodes: magma chamber → central conduit channel → vent at the summit → lava and pyroclasts collect around the vent → material cools and solidifies → one new layer added to the cone → next eruption → back to the magma chamber, with the cone one layer taller each time; where the old route is blocked → subsidiary vent opens on the flank

7.4 Types Of Volcanoes

Volcanoes are not classified by their height, their shape or the damage they do. They are classified on the basis of the frequency of their eruption — that is, on how recently and how often they have erupted. On this basis, volcanoes are divided into three main types: active, dormant and extinct volcanoes.

The three types are told apart like this: (a) An active volcano is presently in an active state, because it has erupted in the recent past. (b) A dormant volcano has not erupted in the recent historical period, but it has not lost the ability to erupt and may become active at any time. (c) An extinct volcano has not erupted in the present geological period and is not likely to become active again.

Active Volcanoes

There are at present about 500 known active volcanoes on the earth. About 20 of these may erupt at any time, which means only a small share of the active list is close to going off in any given season. A few active volcanoes are also in a state of permanent eruption, so material leaves them more or less continuously rather than in separate bursts.

Mt. Stromboli and Mt. Etna, both in Italy in southern Europe, are examples of active volcanoes. So are Pinatubo in the Philippines, an island country in south-east Asia, and Mauna Loa in Hawaii, a group of islands in the Pacific Ocean.

Curiosity Corner

Q2. A volcano may be called active, dormant or extinct. What decides which of the three names it gets?

A. Only one thing — how recently it has erupted. Erupted in the recent past means active; silent through recent historical time means dormant; silent through the present geological period, with no likelihood of returning, means extinct.

Dormant Volcanoes

Dormant volcanoes are the ones that have not erupted in the recent historical period. They are regarded as ‘sleeping volcanoes’, and the word sleeping is exact: they may become active at any time. Mt. Kilimanjaro of Africa is an example, and many other volcanoes in the Philippines and in Japan are dormant too.

Mt. Vesuvius shows how fine the line can be. It is commonly regarded as active, yet it is actually rated as dormant, because there has been no major eruption since 1944. The label follows the eruption record, not the volcano’s reputation.

Extinct Volcanoes

An extinct volcano has fallen silent for far longer. It has not erupted in the present geological period, and it is not likely to become active again. Mt. Aconcagua in South America and Mt. Kenya in Africa are examples of such volcanoes.

The difference between a dormant and an extinct volcano is the one most easily confused, and it is worth stating plainly. Both are quiet now, and neither is erupting; what separates them is the length of the silence and what that silence is expected to mean. A dormant volcano is between eruptions and can wake, while an extinct volcano is judged to have finished erupting altogether.

How Do We Know

Telling A Sleeping Volcano From A Dead One Evidence: the record of past eruptions, historical for dormant volcanoes and geological for extinct ones. Reasoning: a longer silence, stretching beyond the present geological period, makes a return to activity unlikely. Limit: the record shows only what has happened, so a volcano can always break the pattern, as a dormant one may erupt at any time.

Flowchart

Fig. 7.A5 Caption: Three questions about the eruption record place any volcano in its type. Nodes: a volcano → has it erupted in the recent past? yes → active volcano, about 500 known, about 20 able to erupt at any time; no → has it erupted in the recent historical period? no, but it may become active again → dormant volcano; no, and not in the present geological period, with no likelihood of returning → extinct volcano

7.5 Effects Of Volcanoes

A volcanic eruption is one of the few natural events that build and destroy in the same afternoon. Volcanic eruptions have both constructive and destructive effects, and a student of Geography has to hold the two together rather than choose between them. The same lava that buries a field is the lava that will one day make the soil of that field rich.

7.5.1 Constructive Effects

Volcanic eruptions are a sudden force, and they are also termed ‘constructive forces’, because they have a number of constructive effects on the surface of the earth. Constructive here means building — adding new material to the surface, or shaping something new out of what is already there.

Extrusive Landforms

Numerous types of landforms are created by the cooling and solidification of two things: magma below the earth’s surface, and lava on the earth’s surface. The landforms created on the earth’s surface are called extrusive landforms, from the idea of material being pushed out. These include volcanic plateaus, volcanic mountains and volcanic plains.

The three extrusive landforms are built in three different ways: (a) Volcanic plateaus are derived from lava which flows out during volcanic eruptions and spreads widely before it cools, leaving a high, broadly level surface. (b) Volcanic mountains are built from material ejected from the fissures in the earth’s crust, and they are the most diverse of all mountains, because eruptions differ greatly and so do the materials they throw up. (c) Volcanic plains are vast and smooth, formed by extensive volcanic flooding from volcanic centres, where lava spreads out instead of piling up.

Each type has well known examples. The plateau of Peninsular India, especially the north-western Deccan, is an example of a lava plateau, and the South African plateau, the Columbian Plateau and the Ethiopian plateau are others. Among volcanic mountains, Mauna Loa in Hawaii rises 9 km from the ocean floor; volcanic mountains are common in the Circum Pacific Belt and include such peaks as Mt. Fuji in Japan, Mt. Mayon in the Philippines, Mt. Merapi in Sumatra, Mt. Agung in Bali and Mt. Cotopaxi in Ecuador.

The finest examples of volcanic plains are the Western Victorian Plains in Victoria, in south-eastern Australia. These plains were formed by volcanoes over the last six million years, the most recent eruption being at Mount Napier, 7200 years ago, and over 400 volcanic sites have been found in this region. A plain built by 400 separate centres over six million years is a reminder that a landform of this size is the work of very many eruptions, not one.

Diagram

Fig. 7.3 Caption: Saint Helena, in eruption Brief: Photograph of a snow-covered volcanic cone in eruption, with a tall column of ash and steam rising vertically from the summit; keep the full height of the plume in frame.

An Eruption After 12,000 Years

Hayli Gubbi is a shield volcano in the Afar Basin of Ethiopia, in north-eastern Africa. It lies in the East African Rift zone, a region where the Nubian, Somalian and Arabian plates are actively diverging, that is, moving apart from one another. This region is one of the hottest and lowest places on earth and is a hotspot for geological activity.

On 23 November 2025 this volcano erupted after 12,000 years. It erupted unexpectedly, for the first time since the end of the last ice age, which makes it an extremely rare geological event. It underscores continental drift and suggests that the Arabian Sea and the rift valley might eventually form a new ocean.

The eruption produced thick ash plumes of smoke up to 15 km high, which drifted eastwards as far as China, covering Oman, Yemen, India, Pakistan and Southeast Asia. Along with the usual rock particles, this thick column of ash contained volcanic glass, mineral crystals and about 220,000 tons of sulphur dioxide.

Diagram

Fig. 7.A6 Caption: The ash column from Hayli Gubbi rose over a dry, low-lying landscape in the East African Rift. Brief: Photograph of a tall, dense grey ash column rising from a low volcanic ridge under a clear sky, with the flat rift-floor landscape visible at the base for scale.

Caldera Lakes

During repeated eruptions the summit of a volcano may be blown up, and in its place a large depression called a caldera is formed. Calderas are generally formed when the magma chamber is no longer able to emit sufficient magma, and this results in the collapse of a cone either partly or wholly. The cone, in other words, falls into the space its own magma has emptied.

This depression eventually gets filled with water and forms a lake. Crater Lake in the USA is a caldera lake. It is about 10 km wide and is the deepest lake in the USA.

Curiosity Corner

Q3. How can a lake come to sit in the place where a mountain summit once stood?

A. Repeated eruptions empty the magma chamber below. With nothing to rest on, the cone collapses and leaves a wide depression, the caldera, which fills with water over time.

Hot Springs And Geysers

The movement of magma in the interior of the earth heats up underground water, particularly around magma chambers. When that water turns into steam it gushes up to the surface. Hot springs, also known as thermal springs, are the result, and they contain many dissolved minerals picked up from the rocks the water has passed through.

Since hot springs are a rich source of sulphur, they are considered good for health, especially for curing certain skin diseases. The sulphur hot springs at Manikaran, in the Kullu valley of Himachal Pradesh in the western Himalayas, are a great tourist attraction for this reason.

A geyser is a fountain of hot water and steam that escapes when underground water comes into contact with hot volcanic rocks. The difference from a hot spring is in the delivery: a hot spring is water that reaches the surface and stays there, while a geyser throws its water and steam upward in bursts. The ‘Old Faithful’ geyser in Yellowstone National Park in Wyoming, USA, is the best known example, and during an eruption its water temperature at the vent has been measured at 95.6 °C. Fig. 7.4 shows how the underground water and the hot rock are arranged beneath both.

Diagram

Fig. 7.4 Caption: Geysers and hot springs Brief: Cross-section through hot rock showing water-filled cracks and channels below the surface; label hot springs where water reaches the surface quietly and geysers where it is thrown up in a fountain, with arrows showing water moving down into the rock and steam moving up.

Read It

Fig. 7.4 The blue channels are underground water, the surrounding pink is hot volcanic rock. Read the depth: water sinks, meets the hot rock, and returns as steam or a fountain. The figure does not show the magma chamber doing the heating, and it gives no depth scale, so you cannot tell how far down the water travels.

Flowchart

Fig. 7.A7 Caption: One supply of heated groundwater gives two different surfaces features. Nodes: magma moves in the interior → underground water near the magma chamber is heated → water turns to steam and gushes upward → does it reach the surface steadily? yes → hot spring, carrying dissolved minerals and sulphur; does the water meet hot volcanic rocks and flash suddenly? yes → geyser, a fountain of hot water and steam

Other Constructive Effects

Besides the landforms, volcanoes leave behind three further benefits: (a) The lava and ash deposited during an eruption break down to provide valuable nutrients for the soil, and thus create fertile soil which is good for agriculture. (b) Places that have a high level of heat due to volcanic activity inside the earth have a higher potential for generating geothermal energy, which is power drawn from the earth’s own heat. (c) The dramatic scenery created by volcanic eruptions attracts tourists, and this brings income to the area.

Curiosity Corner

Q4. Volcanic ash can bury a farmer’s field and ruin the crop, so why is volcanic land so often called fertile?

A. Because time changes the ash. Fresh layers smother a standing crop, but as the lava and ash break down they release valuable nutrients into the soil, leaving land that is good for agriculture.

7.5.2 Destructive Effects

The same eruption that will one day enrich the soil arrives as a disaster. Some of the destructive effects of volcanoes are the following: (a) When volcanoes erupt they destroy life and property, because the hot lava together with ash and dust destroys not only human beings but animal and plant life as well. (b) Agricultural fields are covered with layers of volcanic ash and dust, making them unsuitable for cultivation, so a family may lose not only one harvest but the use of the land. (c) Huge clouds are formed after the eruption of volcanoes, causing heavy rains which result in floods and landslides, sometimes well away from the volcano itself. (d) Volcanoes also emit poisonous gases which pollute the environment and cause health problems for people living downwind of the eruption. (e) Explosive volcanoes in ocean islands are followed by high tsunami waves, which flood the area and destroy property, people, animals and crops.

Notice how many of these effects travel. The lava stays close to the vent, but ash, cloud, gas and wave carry the eruption outward, which is why the harm from a volcano is often felt by people who never see the mountain.

7.6 Important Volcanic Zones Of The World

Volcanoes are not scattered evenly over the earth. They gather into belts, and the belts follow the places where the crust is weakest. Two of these belts hold most of the world’s volcanoes, and both of them run along lines that a student can trace on a map.

The Circum Pacific Belt

The volcanoes of the Circum Pacific Belt occur near earthquake belts, around the young fold mountains that ring the Pacific Ocean. The belt is also called the Pacific Ring of Fire, because over 80% of the total number of active volcanoes are concentrated in this one region. The name is a description of the map, not a flourish: the volcanoes really do form a ring around an ocean.

Among the volcanoes of this belt are 40 active volcanoes in the Andes, the long mountain chain down the western edge of South America, over 100 in the Philippines, 70 in Indonesia and 40 in Japan. Map 7.1 shows the belt as a band of colour, with the plate names printed on either side of it.

Map

Map 7.1 Caption: The Circum Pacific Belt, or Pacific Ring of Fire, traced around the margins of the Pacific Ocean. Brief: World map centred on the Pacific; shade the Ring of Fire as a continuous brown band around the ocean margin; label the Eurasian, North American, Pacific, South American, Nazca, Indian-Australian and Antarctic plates; show plate boundaries as lines; every map of India must show the full extent of India as officially defined.

Read It

Map 7.1 The brown band is the volcanic belt; the printed names are the plates. Read the two together and the band sits where the Pacific Plate meets its neighbours. The map shows where volcanoes cluster, not how many stand at each point, and it says nothing about which are active now.

Why Here

Volcanoes Around The Pacific Factors: the belt follows earthquake belts, young fold mountains and the margins where plates meet. The decisive one: the plate margins, because it is there that faults open along lines of weakness and give magma its route to the surface. Move it: the ring would sit elsewhere only if the plate boundaries themselves lay elsewhere — the mountains and the earthquake belts follow the plates, not the other way round.

Worked Case

WC 7.A1 — How Much Of The World’s Activity Sits In One Belt The situation: this chapter states that there are at present about 500 known active volcanoes, and that over 80% of all active volcanoes lie in the Circum Pacific Belt. It also gives counts for four areas inside the belt. The working: \[\text{Volcanoes in the belt} = 80\% \text{ of } 500 = \frac{80}{100} \times 500 = 400 \text{ active volcanoes (at least)}\] then adding the four areas the chapter names, in numbers of active volcanoes: \[40 \text{ (Andes)} + 100 \text{ (Philippines)} + 70 \text{ (Indonesia)} + 40 \text{ (Japan)} = 250\] and since the Philippines figure is given as over 100, the true total for these four is more than 250.

Both results are worth pausing on. The belt holds at least 400 of the world’s roughly 500 active volcanoes, which leaves fewer than 100 for the whole of the rest of the earth. And four named areas alone account for more than half of the belt’s own share, so even inside the Ring of Fire the volcanoes are bunched rather than spread evenly around the ocean.

The Mid-world Mountain Belt

The second belt is the Mid-world Mountain Belt. Mt. Vesuvius, Mt. Etna and Mt. Stromboli, the famous active volcanoes met earlier in this chapter, are all located in it. So is the only active volcano in India, which lies in the Andaman and Nicobar Islands in the Bay of Bengal.

The belt reaches well beyond the Mediterranean. The Hawaiian Islands, the Canary Islands in the Atlantic off north-western Africa, and Mt. Kenya are all in the Mid-world Mountain Belt. A single belt can therefore contain an active volcano, a dormant one and an extinct one at the same time, because the belt describes where volcanoes are, while active, dormant and extinct describe when they last erupted.

New Words

Plate Tectonics — the theory that explains the movements of plates of the lithosphere and the resultant landforms Magma — molten material moving under the earth’s crust Lava — magma when it comes to the surface of the earth Extrusive forces — forces acting on the surface of the earth

7.7 Summary

The forces arising from the interior of the earth are called endogenic forces, and they are of two types — diastrophic movements and sudden movements. Volcanic eruptions belong to the sudden movements, because they bring about instant changes on the surface of the earth.
A volcano is a vent in the earth’s crust from which fiery hot magma erupts from the earth’s interior.
Volcanic eruptions have three main causes, and together they form a single chain.
1. Heat and pressure inside the earth rise towards the centre, and because rocks are bad conductors the trapped heat melts rock and builds great pressure.
2. Plate tectonics weakens the crust, since moving plates produce faults along lines of weakness at their margins, which is why most eruptions take place near plate margins.
3. A magma chamber forms under the crust, and the gases and water vapours in the magma make its pressure act vertically upwards until a route to the surface is found.
A volcano erupts steam, gases, molten rock, dust, ashes and other liquid and solid matter.
1. Molten rock inside the earth is magma, and the same material at the surface is lava.
2. Steam is the most abundant gas, and in huge quantities it forms clouds that give rainfall; other gases include hydrogen sulphide, sulphur dioxide, carbon dioxide and hydrogen.
3. Solid fragments are called pyroclasts, and they grow in size from dust to lapilli to the bombs thrown out by violent eruptions.
4. These fragments do not come from the magma chamber but are picked up by the magma on its way up.
All major volcanoes have a magma chamber joined to the surface by a conduit, and the lava flows out through an opening called the vent, with subsidiary vents forming during repeated eruptions.
Erupted material collects around the vent, cools and solidifies, and this accumulation gives the volcano its typical cone-like shape, known as the volcanic cone.
On the basis of the frequency of their eruption, volcanoes are of three types.
1. Active volcanoes have erupted in the recent past, and about 500 are known at present, of which about 20 may erupt at any time.
2. Dormant volcanoes have not erupted in the recent historical period and are regarded as sleeping volcanoes that may become active at any time.
3. Extinct volcanoes have not erupted in the present geological period and are not likely to become active again.
Volcanic eruptions have constructive effects, and the landforms they build on the surface are called extrusive landforms.
1. Volcanic plateaus are formed from lava that flows out and spreads, as in the north-western Deccan of Peninsular India.
2. Volcanic mountains are built from material ejected from fissures and are the most diverse of all mountains.
3. Volcanic plains are vast and smooth, formed by extensive volcanic flooding from volcanic centres.
4. A caldera is the depression left when a cone collapses because the magma chamber can no longer emit sufficient magma, and it fills with water to form a caldera lake.
5. Hot springs, also called thermal springs, form where magma heats underground water, and geysers are fountains of hot water and steam thrown up where that water meets hot volcanic rocks.
Volcanoes also help people in three further ways: lava and ash break down into fertile soil, volcanic heat gives a high potential for geothermal energy, and dramatic volcanic scenery brings tourist income.
The destructive effects are severe. Eruptions destroy life and property, bury farmland under ash and dust, form clouds that bring heavy rain with floods and landslides, release poisonous gases, and in ocean islands are followed by high tsunami waves.
Volcanoes are concentrated in two belts.
1. The Circum Pacific Belt, also called the Pacific Ring of Fire, holds over 80% of all active volcanoes, including 40 in the Andes, over 100 in the Philippines, 70 in Indonesia and 40 in Japan.
2. The Mid-world Mountain Belt contains Mt. Vesuvius, Mt. Etna and Mt. Stromboli, the Hawaiian and Canary Islands, Mt. Kenya, and India’s only active volcano in the Andaman and Nicobar Islands.

Flowchart

Fig. 7.A8 Caption: One chain carries the whole chapter, from heat locked inside the earth to the good and the harm an eruption leaves behind. Nodes: endogenic forces → heat and pressure increase with depth and cannot escape through poorly conducting rock → rock melts into magma → magma chamber forms and its gases press upwards → is there a line of weakness above, usually at a plate margin? no → pressure keeps building; yes → magma rises through the conduit and out of the vent → eruption, throwing out lava, gases and pyroclasts → material collects and solidifies around the vent → volcanic cone, and with repeated eruptions → back to the vent for the next eruption, the cone growing each time → over time the eruption record decides the type: recent past → active / not in recent historical period → dormant / not in the present geological period → extinct → and every eruption leaves two kinds of result: constructive → plateaus, mountains, plains, calderas and caldera lakes, hot springs, geysers, fertile soil, geothermal energy, tourism / destructive → loss of life and property, buried fields, floods and landslides, poisonous gases, tsunami waves

What This Chapter Cannot Tell You

The classification of volcanoes rests entirely on the eruption record, so it describes the past and not the future. It can say that a dormant volcano may become active at any time, but it cannot say which one will wake, or when, and Mt. Vesuvius shows how uneasily even the labels themselves sit.

The counts have limits of their own. About 500 known active volcanoes is a count of the ones known, so it can only grow as more are found, and a world map of volcanic sites marks positions without saying how large, how frequent or how dangerous each eruption is. At the scale of a whole world map, a triangle covering hundreds of kilometres hides the single valley where an eruption would actually be felt.

7.8 Check Your Concepts

Look at Fig. 7.4 and trace the path taken by the underground water. Then state, in your own words, how the water behaves differently at a hot spring and at a geyser.
A village lies many kilometres away from an erupting volcano, yet it is hit by a landslide a few days later. Set out the steps that link the eruption to that landslide, in the order in which they happen.
This chapter states that there are at present about 500 known active volcanoes, and that over 80% of them lie in the Circum Pacific Belt. Using only these two figures, work out roughly how many active volcanoes lie outside this belt, showing each step of your working.
Mt. Vesuvius is commonly regarded as active, but the chapter rates it as dormant.
1. Which single piece of evidence decides that rating?
2. Explain why evidence of this kind can never tell us that a volcano has finished erupting for good.
The shaded area on Map 7.1 is referred to as the Pacific Ring of Fire. Why is it so called?
What is the difference between a dormant volcano and an extinct volcano?
Describe the main causes of volcanic eruptions.
Give a geographical reason for each of the following:
1. Many volcanoes are found along plate boundaries.
2. Volcanic mountains are the most diverse mountains in the world.
3. Hot springs have medicinal properties that help cure skin diseases.
Draw a well labelled diagram showing the different parts of a volcano and the products erupted by it.
Explain briefly the landforms created by volcanoes on the surface of the earth.