Volcanoes | ICSE Class 9 Geography Notes

This note covers the meaning and structure of volcanoes, erupted materials, active, dormant and extinct volcanoes, constructive and destructive effects, volcanic landforms, important volcanic zones, and examples from Hawaii and the Deccan Traps.

What is a volcano, and how are magma and lava related?

Definition: A volcano is a place where gases, ash (fine volcanic particles) and/or molten rock escape to the Earth's surface. An eruption is the release of these materials. Volcanism is the movement of molten rock and associated materials within the Earth and towards its surface.

The crust is the Earth's outermost solid layer. Beneath it is the mantle, the thick layer below the crust. The asthenosphere is a weaker zone in the upper mantle and an important source of molten rock involved in volcanic activity. The core is the Earth's innermost layer.

Magma is molten rock material below the Earth's surface. Lava is the name used for molten rock that emerges at the surface. The distinction helps trace the material's journey from the interior to a surface flow.

Why is a volcano more than a mountain?

A landform is a natural feature of the Earth's surface. A volcanic mountain is one possible landform produced by eruptions, but the definition of a volcano concerns the place where material escapes. Volcanic activity also occurs on ocean floors and produces extensive lava-covered regions.

Molten rock does not have to build a tall cone. A cone is a mound tapering towards its summit. Lava may spread over a broad area, and violent activity may leave a depression, meaning a lowered or sunken part of the surface.

Volcanic material also provides evidence about the Earth's interior. Material brought to the surface can be collected and analysed. However, it is difficult to ascertain the depth from which a particular magma originated; a surface sample does not directly reveal every condition underground.

The lithosphere comprises the crust and uppermost mantle. It is divided into moving slabs of solid rock called tectonic plates. Their movement helps explain the geographical pattern of volcanic activity.

Note: Do not picture the entire mantle as a liquid ocean. It is mostly solid, although a weaker, partially molten zone helps explain the movement of plates and the origin of magma.

How does material reach the surface during an eruption?

A magma chamber is an underground accumulation of magma. A vent is an opening through which volcanic material escapes. A crater is a depression around a volcanic opening, commonly at the summit of a cone.

A fissure is a crack through which molten material may rise. Material can emerge through a concentrated opening or through cracks. These different pathways help explain why some eruptions build cones while others spread lava widely across the surrounding surface.

What is the basic sequence?

1. Molten rock material is present below the crust, with the weaker upper mantle providing an important source of magma.
2. Magma moves upwards through available openings or weaknesses in the overlying rocks towards the surface.
3. Material escapes through a vent or fissure; the eruption may release molten rock, gases and broken fragments.
4. Lava flows and erupted fragments accumulate at the surface, while cooling turns molten material into solid rock.

Igneous rock is rock formed when molten material cools and solidifies. Cooling at the surface produces volcanic rock. Cooling within the crust produces intrusive rock, meaning rock formed below the surface rather than from a surface lava flow.

The sequence describes the broad process, rather than a fixed timetable. Different volcanoes release different mixtures of material. The amount of lava, the character of the fragments and the manner of eruption influence the form that develops.

Draw a cut-away view with a magma chamber below the surface, a passage leading to a vent, a crater at the top and a lava flow on the slope. Label magma below the surface and lava outside it.

Use arrows to show the direction of upward movement. A section diagram shows what would be visible if the volcano were cut through; the underground chamber and passage are explanatory features, not objects visible from outside a real volcanic cone.

What materials erupt, and why do eruptions differ?

Volcanic eruptions release molten material, solid fragments and gases. Pyroclastic material means broken material thrown out during an eruption. Volcanic ash consists of fine volcanic particles, rather than the residue left by burning wood. Larger pieces are also thrown out during eruptions.

Volcanic bombs are larger pieces of molten or partly molten material thrown out during an eruption and solidifying during flight or afterwards. Ash, dust, bombs and lava flows are different products, so an account of an eruption should not describe everything as lava.

How does the character of lava matter?

Viscosity is resistance to flow. More viscous lava flows less readily. Basalt is an igneous rock formed from lava that can be very fluid when erupted. Fluid lava can spread widely and help create gently sloping volcanic forms.

A shield volcano is a broad, gently sloping volcanic form. Shield volcanoes are mostly made of basalt and are characterised by low explosivity unless water enters the vent. Explosivity describes the tendency of an eruption to release material violently. Low explosivity does not mean that the lava is cold or harmless.

Composite volcanoes contain accumulated layers of lava and erupted fragments. Their lava is cooler and more viscous than basaltic lava, and their eruptions often become explosive. Keep the word often: a tendency is not a statement that every eruption behaves identically.

These descriptions classify volcanoes by their form and eruption character. They describe how material erupts and what it builds, rather than whether a volcano is currently active or expected to erupt again. A description of shape should therefore not substitute for a description of activity.

How do active, dormant and extinct volcanoes differ?

Classification by activity considers eruptions and the possibility of further activity. The three terms are active, dormant and extinct. They do not refer to the colour, height or outline of a volcanic mountain.

What does each activity class mean?

An active volcano is releasing volcanic material now or has released it in the recent past. It need not be erupting continuously. A quiet interval between eruptions therefore does not, by itself, establish that a volcano is extinct.

A dormant volcano has been inactive for a long period but may erupt again. Dormant means temporarily inactive or sleeping. The possibility of renewed eruption is the crucial part of the definition, rather than the absence of visible lava on a particular day.

An extinct volcano is one that is not expected to erupt again. The phrase not expected matters. The category expresses a judgement about future activity, rather than a claim that someone can directly observe all future conditions.

What evidence can a description support?

If a description explicitly states that material is being released, the active category follows directly. If it states both long inactivity and the possibility of renewed eruption, dormant is appropriate. If it states that further eruption is not expected, extinct is the appropriate term.

Note: There is no fixed number of quiet years used in these definitions here. Do not invent an age limit or decide that every volcano without a current lava flow is extinct.

What constructive effects can volcanic activity produce?

Constructive effects are effects that build landforms or provide useful resources. Volcanic activity can create rock and relief, meaning differences in surface height and shape. Its constructive role becomes clear when lava cools and remains as part of the landscape.

How does volcanism build land?

Repeated eruptions can accumulate material around a vent and build volcanic hills or mountains. Broad flows can produce a lava plateau, an extensive raised region built from layers of solidified lava. The Deccan Traps illustrate widespread accumulation of basaltic flows.

At spreading oceanic boundaries, places where plates move apart beneath the sea, rising molten material forms new crust. A mid-ocean ridge is a long submarine mountain system associated with such spreading. Submarine means beneath the sea. Thus, constructive volcanic activity is not restricted to visible mountains on land.

Volcanic material can also build land above sea level to form islands. An island is land surrounded by water. Formation of new rock and formation of useful soil are different stages and should not be treated as simultaneous events.

How can volcanic regions provide resources?

Weathering means the breakdown or alteration of rock in place. Over time, weathering of volcanic material can contribute to fertile soils, meaning soils able to support plant growth. Fresh ash burial can nevertheless damage existing crops.

Geothermal energy is heat, or electricity generated using heat, from the Earth's interior. In suitable areas, groundwater absorbs heat from hot rocks. Steam can drive a turbine, a rotating machine used in electricity generation.

Volcanic regions can offer this heat resource where conditions are suitable. A geothermal resource is not a promise that every volcano can generate electricity. It depends on accessible heat and the conditions needed to use it.

Volcanic landscapes can also attract visitors, while volcanic rocks can supply building material. These benefits concern the resources and landscape left by geological activity. They do not remove the destructive effects of a new eruption or make nearby settlements automatically safe.

What destructive effects can volcanic eruptions cause?

Destructive effects are effects that damage life, property or the environment. The same eruption that adds rock to the surface can threaten people living nearby. A complete explanation connects each damaging agent with the harm it can cause.

How do lava, ash and gases cause damage?

Lava flows can burn or bury buildings, roads, crops and vegetation in their path. The damage is caused by hot material moving across the surface and then remaining as solid rock. Landform creation and destruction of existing land use can therefore happen together.

Ash fall is the settling of airborne volcanic particles onto the ground. Ash can cover cultivated land, contaminate water and damage buildings. A heavy accumulation on roofs can cause collapse, while suspended fine particles can make breathing difficult.

Volcanic gases can harm living organisms. Explosively ejected fragments can also injure people and damage property. The hazard is not limited to the visible path of a lava flow, because falling fragments and airborne material behave differently from flowing lava.

How can eruptions disturb slopes and settlements?

A landslide is the downslope movement of rock or other earth material. Volcanic eruptions can trigger landslides by shaking the ground and weakening slopes. Loose material and steep slopes can increase the risk of slope failure.

Damage to homes, transport routes and cultivated land can disrupt daily life and livelihoods, meaning the activities through which people support themselves. An explanation should distinguish the physical process, such as ash burial, from its consequences for people.

Photograph: Deposition by volcanic eruption (NCERT Class 9 Figure 2.6). Caption: Photograph showing the deposition by volcanic eruption. Grey volcanic deposits cover the ground and slopes around buildings and trees, partly burying the settlement.

Where are the important volcanic zones of the world?

A volcanic zone is a region with a concentration of volcanic activity. Volcanoes are unevenly distributed. Many occur in belts related to moving plates, rather than being scattered evenly over every continent and ocean.

Which broad zones should be recognised?

The Circum-Pacific belt follows much of the Pacific Ocean's margin. Circum-Pacific means around the Pacific. It is commonly called the Ring of Fire because of its concentration of volcanoes. The name describes a broad belt, not an unbroken ring of erupting cones.

The Mediterranean-Asiatic belt is a conventional name for a broad zone extending from the Mediterranean region towards parts of Asia. Its volcanic areas are discontinuous. Do not infer that every part of the Alpine-Himalayan mountain system has active volcanoes.

The mid-ocean ridge system is another important volcanic zone. It includes the Mid-Atlantic Ridge and extends through the ocean basins. Eruptions are frequent in central parts of the ridges, even though much of this activity is beneath ocean water.

East Africa also has volcanic activity associated with a rift, a region where the crust is being pulled apart. Volcanoes also occur away from plate margins, as in Hawaii. The distribution therefore requires more than one geographical explanation.

Black triangles mark active volcanoes and red dots mark earthquake origins. Many symbols cluster around the Pacific margins. Other volcanic symbols appear in the Atlantic, the Mediterranean region and eastern Africa. The two symbols represent different phenomena.

See Fig. 2.4 in your NCERT textbook

How do moving plates explain much of volcanic distribution?

The lithosphere is the rigid outer part of the Earth, comprising the crust and uppermost mantle. A tectonic plate is a large moving slab of lithosphere. Plate tectonics is the explanation of Earth's surface changes through the movement and interaction of these plates.

What happens where plates separate?

A divergent boundary is a boundary where plates move apart. Molten material rises into the opening and forms new crust. The Mid-Atlantic Ridge provides an example of a spreading boundary, linking the location of volcanism with the movement of plates.

Sea-floor spreading is the formation and outward movement of oceanic crust from spreading ridges. Repeated additions of molten material contribute to new crust as the ocean floor moves away on either side. This explains why oceanic volcanic activity can build crust.

What happens where plates approach one another?

A convergent boundary is a boundary where plates move towards each other. Subduction is the sinking of one plate beneath another. Where an oceanic plate sinks beneath a continental plate, the interaction can lead to volcanic activity and earthquakes.

Not every convergent boundary produces the same landforms. Collision between continental plates can form fold mountains, meaning mountains produced by compression and bending of rock layers. The Himalayas illustrate this distinction; a mountain belt should not automatically be labelled a chain of active volcanoes.

A transform boundary is one where plates slide past each other. Such movement mainly causes earthquakes. An earthquake is shaking of the Earth caused by a release of energy. Earthquake distribution and volcanic distribution overlap considerably, but the two phenomena are not identical.

A hot spot is a localised source of volcanic activity that can occur within a plate.

The map uses separate symbols for volcanic eruptions, shallow earthquake centres, deep earthquake zones and hot spots. The Ring of Fire is labelled around the Pacific, while Hawaii is labelled within the ocean.

See Fig. 4.2 in your NCERT textbook

Hawaii helps show why the statement that most volcanoes occur near plate boundaries should not be changed into a claim that all do.

What does the Hawaii case study show about volcanic form?

Case study: Hawaiian shield volcanoes

A shield volcano is a broad volcanic form with gentle slopes, built largely from fluid lava. Hawaiian volcanoes are well-known examples. Their shape provides a useful link between the properties of erupted material and the landform produced.

These volcanoes are mostly made of basalt. Their lava is very fluid when erupted, so it spreads rather than remaining concentrated in a steep pile beside the opening. The gently sloping form follows from this spreading behaviour.

Shield volcanoes are characterised by low explosivity unless water enters the vent. This condition should remain attached to the description. Calling them incapable of explosive eruption would remove a qualification that changes the meaning of the explanation.

How does this compare with a composite volcano?

A composite volcano involves cooler, more viscous lava and often explosive eruptions. Lava, ash and other fragments accumulate in layers around openings. The resulting layered structure explains the word composite, meaning made up of different components.

Hawaii also contributes to the geographical explanation. It lies within the Pacific Ocean, while the Ring of Fire follows much of the ocean's margin. Its mapped hot-spot position helps show that plate-margin belts do not account for every volcanic location.

Keep activity class separate from this comparison. Shield describes the form and associated eruption characteristics. The term does not by itself say whether a particular volcano is erupting now, temporarily inactive or not expected to erupt again.

What does the Deccan Traps case study show about lava landforms?

Case study: The Deccan Traps of India

A flood basalt province is a large region built from extensive basaltic lava flows. The Deccan Traps, presently covering most of the Maharashtra plateau, are an important example. They show that volcanic construction can produce broad landscapes rather than isolated cones.

Highly fluid lava can travel long distances. Repeated flows cool and accumulate as layers. The resulting landscape records successive additions of volcanic material. A plateau formed in this way is therefore explained through the spread and solidification of lava.

Dimensions of flood basalt flows: Some flood basalt provinces contain extensive, thick lava flows. The following measurements describe flood basalt flows in general.

It is believed that the original Deccan trap formations covered a much larger area than the present.

What does the example help distinguish?

A volcanic cone concentrates attention on material accumulating around an opening. A flood basalt province concentrates attention on lava spreading over an extensive area. Both are volcanic products, but they differ in the arrangement of the material and the surface form produced.

A caldera is a large collapsed volcanic depression. Caldera-forming volcanoes are usually so explosive that they tend to collapse rather than build a tall structure. This provides another reminder that volcanic activity can lower part of a surface as well as build it up.

The link between process and result can be followed in order: eruption supplies fluid lava; lava spreads; cooling produces solid rock; repeated flows accumulate. These steps connect the origin of the molten material with the final landform. The explanation depends on repeated accumulation, rather than treating the entire plateau as a single volcanic cone.

Draw several broad, superimposed layers above a labelled ground line. Label them successive solidified lava flows and identify the resulting lava plateau. This is a simplified process sketch, not a measured cross-section of the Deccan Traps.

The constructive effect illustrated here is the formation of a widespread rock landscape. It should be distinguished from the immediate effects on a settlement during an eruption: geological construction and damage to human land use describe different consequences.

Glossary

• Volcano — A place where gases, ash and/or molten rock escape to the Earth's surface.
• Magma — Molten rock material below the surface that may rise towards volcanic openings.
• Lava — Molten rock emerging at the surface and capable of cooling into solid rock.
• Vent — An opening through which volcanic gases, fragments and molten material escape.
• Crater — A depression around a volcanic opening, commonly located at a cone's summit.
• Active volcano — A volcano releasing material now or having released it in the recent past.
• Dormant volcano — A volcano inactive for a long period but capable of erupting again.
• Extinct volcano — A volcano judged to be no longer expected to erupt again.
• Viscosity — Resistance to flow, greater in material that moves less readily.
• Pyroclastic material — Broken material thrown out during an eruption, including volcanic ash and larger fragments.
• Lava plateau — A broad raised region built from accumulated layers of solidified lava.
• Caldera — A large volcanic depression produced when part of a volcano collapses.
• Subduction — The sinking of one tectonic plate beneath another at a convergent boundary.
• Ring of Fire — The major volcanic belt following much of the margin of the Pacific Ocean.
• Geothermal energy — Heat or electricity obtained by using heat from the Earth's interior.

Common errors and misconceptions

• Misconception: Every volcano is a steep mountain. Correct: Volcanic activity can produce broad shield forms, extensive lava plateaus and collapsed depressions, as well as cones.
• Misconception: Magma and lava identify different unrelated substances. Correct: Both refer to molten rock material; the distinction used here is whether it remains underground or has emerged at the surface.
• Misconception: An active volcano must erupt continuously. Correct: A volcano is active if it is releasing material now or has done so in the recent past.
• Misconception: Dormant and extinct mean the same thing. Correct: Dormant volcanoes may erupt again; extinct volcanoes are not expected to do so.
• Misconception: Volcanic ash is ash from burning fuel. Correct: It consists of fine volcanic particles and is distinct from a continuous lava flow.
• Misconception: All volcanoes occur on plate boundaries. Correct: Most are associated with boundaries, but hot-spot activity such as Hawaii shows that this is not universal.
• Misconception: Shield volcanoes cannot erupt explosively. Correct: They are characterised by low explosivity unless water enters the vent; that condition must remain in the explanation.
• Misconception: Constructive effects mean an eruption is harmless. Correct: Lava may create new rock while destroying buildings or crops, and useful soil can develop only over time.

Exam-style questions with model answers

Q1. Molten rock is stored beneath the surface and later emerges in a surface flow. Name the material at each of these two stages. [2 marks]

1. The molten rock below the Earth's surface is called magma.
2. Once the molten rock emerges at the surface, it is called lava.

Q2. Classify each description and explain your choice: a volcano releasing material now; a volcano long inactive but capable of renewed eruption; and a volcano not expected to erupt again. [3 marks]

1. The first volcano is active because it is currently releasing volcanic material. Continuous eruption over a long period is not required by this definition.
2. The second volcano is dormant because its long period of inactivity is combined with the possibility of renewed eruption in the future.
3. The third volcano is extinct because further eruption is not expected. This is different from merely observing that it is quiet today.

Q3. Fluid lava repeatedly emerges, spreads across a wide area and cools. Explain in four steps how these processes can build a lava plateau. [4 marks]

1. The eruption brings fluid lava to the surface, supplying the molten material from which the landform can be built.
2. The lava spreads over a broad area rather than remaining concentrated in a steep mound close to an opening.
3. Cooling turns the spread lava into solid igneous rock, leaving a layer of volcanic material across the surface.
4. Repeated flows add further layers of solidified lava, building the broad raised region described as a lava plateau.

Q4. Consider these possible results of volcanic activity: accumulation of lava into landforms; weathering of volcanic material into fertile soil; use of underground heat for electricity; lava entering farms and roads; and heavy ash falling on roofs. Explain three constructive and two destructive effects, one point for each result. [5 marks]

1. Accumulation and cooling of lava construct new rock landforms. Repeated flows can create extensive lava plateaus, adding material to the Earth's surface.
2. Weathering of volcanic material can contribute to fertile soil over time. This can support agriculture, although fresh deposits are not immediately equivalent to cultivated soil.
3. Accessible underground heat can provide geothermal energy in suitable areas. Heat transferred to groundwater can produce steam used to drive turbines for electricity.
4. Lava entering farms and roads can burn or bury crops and transport routes. This damages both existing land use and the activities dependent on it.
5. Heavy ash deposits place a load on roofs and can cause them to collapse. This threatens buildings and the people using them.

Q5. Two plates move apart at the Mid-Atlantic Ridge; an oceanic plate sinks beneath a continental plate elsewhere; and two plates slide past one another at a third boundary. Name and explain each boundary type, including its relationship to crust formation, volcanic activity or earthquakes. [3 marks]

1. The Mid-Atlantic example is a divergent boundary. The plates separate, allowing molten material to rise and cool into new crust along the spreading ridge.
2. The sinking oceanic plate marks a convergent boundary with subduction. This interaction between oceanic and continental plates can produce volcanic activity and earthquakes.
3. The sliding plates form a transform boundary. They move past each other without creating or destroying crust, and this movement mainly causes earthquakes.

Q6. Compare shield and composite volcanoes using these observations: shield volcanoes are mostly basaltic, with very fluid lava, gentle slopes and low explosivity unless water enters the vent; composite volcanoes have cooler, more viscous lava, often explosive eruptions and layers of lava and fragments around openings. Give three paired comparisons. [3 marks]

1. Shield volcanoes are mostly basaltic and have very fluid lava when it erupts; composite volcanoes have cooler lava that is more viscous and flows less readily.
2. Shield volcanoes have gentle slopes associated with spreading lava; composite volcanoes contain layers of lava and erupted fragments accumulated around their openings.
3. Shield volcanoes show low explosivity unless water enters the vent; composite volcanoes often produce explosive eruptions, without that being a universal rule for every eruption.

Q7. A distribution map labels volcanoes around the Pacific margin, along the Mid-Atlantic Ridge and at Hawaii within the Pacific Ocean. Identify the Pacific belt and explain why these observations do not support the statement that every volcano lies on an ocean's margin. [2 marks]

1. The Pacific marginal belt is the Circum-Pacific belt, commonly called the Ring of Fire.
2. The Mid-Atlantic Ridge and Hawaii are within ocean basins, so the stated locations include volcanoes away from ocean margins.

Q8. The Deccan Traps presently cover most of the Maharashtra plateau and are a flood basalt province. Highly fluid lava can spread far, cool into rock and accumulate through repeated flows. It is believed that the original trap formations covered a much larger area. Explain these features in five points, preserving the qualification about earlier extent. [5 marks]

1. The Deccan Traps are an Indian volcanic landscape presently covering most of the Maharashtra plateau, providing an example of a widespread lava formation.
2. The term flood basalt province identifies an extensive region built from basaltic lava flows, rather than simply describing one steep cone around a vent.
3. Highly fluid lava can spread over long distances before solidifying, helping explain how volcanic material can cover a broad area of land.
4. Cooling produces solid rock, and repeated flows add successive layers. Their accumulation helps explain the construction of a broad volcanic plateau.
5. It is believed that the original trap formations covered a much larger area than today. The qualification must remain because this describes their interpreted earlier extent.

Key takeaways

• A volcano is a place where gases, ash and/or molten rock escape; volcanic activity does not always build a steep mountain.
• Magma is molten rock below the surface, while lava names the material that emerges at the Earth's surface.
• Active, dormant and extinct classify activity; shield and composite describe different volcanic forms and eruption characteristics.
• Volcanism constructs rock landscapes and can provide fertile soils and geothermal resources, while eruptions can damage lives and property.
• Most volcanoes occur near plate boundaries, especially around the Pacific, but Hawaii illustrates activity within a plate.
• Divergent boundaries allow new crust to form; subduction at some convergent boundaries is associated with volcanic activity.
• Hawaiian shield volcanoes illustrate the connection between very fluid basaltic lava, spreading flows and gently sloping volcanic forms.
• The Deccan Traps illustrate widespread basalt flows; it is believed that their original extent was much larger than today's.

Test yourself

Why is the absence of a lava flow today insufficient to call a volcano extinct?

An active volcano may be between eruptions, while a dormant volcano may erupt again after a long quiet period.

What distinguishes a vent from a crater?

A vent is an opening through which material escapes; a crater is a depression around a volcanic opening.

What does viscosity mean, and how does it affect flow?

Viscosity is resistance to flow. Material with greater viscosity flows less readily than more fluid material.

Why must “often” remain in the description of composite eruptions?

Composite volcanoes often erupt explosively, but this describes a tendency rather than stating that every eruption must be explosive.

How can the same lava flow have constructive and destructive effects?

It can cool into new rock while burning or burying buildings, crops and roads already present in its path.

What gives the Ring of Fire its name?

It is the concentration of volcanoes around much of the Pacific Ocean's margin, forming a major volcanic belt.

Why is the Mid-Atlantic Ridge associated with new crust?

Plates move apart there, allowing molten material to rise and cool into new oceanic crust.

What qualification belongs with the statement about the Deccan Traps' original extent?

It is believed that the original trap formations covered a much larger area than they do at present.