Volcanism, Types and Active Volcanoes

Why Volcanism Matters for CDS

Volcanism is an endogenic (internal) process — it is driven by heat and pressure deep inside the Earth, unlike weathering or erosion which act on the surface. CDS examiners regularly test it because it connects several syllabus areas at once: the interior of the Earth, plate tectonics, rock formation and natural hazards.

You will usually see one to two questions every year, often as match-the-following (volcano with country) or as a direct fact about lava type, the Ring of Fire, or a recently erupting volcano in the news. The questions are rarely calculation-heavy, which means a candidate who memorises the right examples and a few classifications can score full marks here with very little effort. That is exactly why this chapter rewards focused revision more than almost any other in physical geography.

Volcanism is also closely tied to the theory of plate tectonics, the rock cycle, and the formation of mountains and plateaus. When the examiner frames a question around "the interior of the Earth" or "endogenic forces", volcanism is almost always part of the intended answer set. Treat it as a hub topic that links many smaller ones together.

Magma, Lava and the Meaning of Volcanism

Molten rock stored below the Earth’s surface is called magma. The moment it reaches the surface and flows out, the same material is renamed lava. This one-word switch is a favourite trick in objective papers.

• Magma → underground molten material, rich in dissolved gases.
• Lava → magma that has erupted onto the surface.
• Volcanism → all processes by which magma and gases rise and reach (or approach) the crust.

Lava is broadly of two kinds. Basic (basaltic) lava is low in silica, very hot, runny and flows far before solidifying. Acidic lava is high in silica, sticky (viscous), cooler and solidifies quickly, often blocking the vent and causing violent explosions. The viscosity of the lava — how easily it flows — therefore controls almost everything about an eruption: how far the lava travels, how steep the resulting cone is, and how dangerous the blast becomes.

Along with molten rock, a volcano throws out large volumes of gases (mainly water vapour, carbon dioxide and sulphur compounds) and solid fragments collectively called pyroclastic material — ash, cinders, lapilli and large volcanic bombs. In an explosive eruption it is often this ash and gas, not the lava, that causes the greatest loss of life. The trapped gases inside sticky acidic magma are the very reason such eruptions are so sudden and forceful.

Anatomy of a Volcano

Knowing the parts of a volcano helps you answer landform and diagram questions quickly.

• Magma chamber − the underground reservoir of molten rock.
• Vent / Pipe − the central passage through which material rises.
• Crater − the bowl-shaped mouth at the top of the cone.
• Caldera − a very large, collapsed depression formed when the chamber empties and the top falls in.
• Ejecta − gases, ash, cinders and larger fragments thrown out during eruption.

Types of Volcanoes by Activity

The first classification is based on how recently and how often a volcano erupts.

• Active volcano — erupts frequently or has erupted in recorded history and may erupt again. Examples: Etna (Italy), Stromboli, Kilauea (Hawaii), Barren Island (India’s only active volcano, Andaman Sea).
• Dormant volcano — ‘sleeping’; quiet now but has erupted in the past and could erupt again. Examples: Vesuvius (Italy), Fujiyama (Japan).
• Extinct (dead) volcano — has not erupted in historic time and is unlikely to erupt; the vent is often sealed. Examples: Kilimanjaro (Tanzania), Aconcagua, Popa (Myanmar).

Types of Volcanoes by Shape and Eruption

The second classification is based on the form of the cone and the nature of the eruption. This is the section CDS tests most.

1. Shield Volcano

Built almost entirely of fluid basaltic lava. They are the largest volcanoes, with broad, gently sloping sides like a warrior’s shield. Eruptions are mild and explosive only when water enters the vent. Mauna Loa in Hawaii is the classic example.

2. Composite (Strato) Volcano

Built of alternating layers of cooler, viscous lava plus large amounts of ash and pyroclastic material. They are tall, steep and prone to violent, explosive eruptions. Examples: Vesuvius, Fujiyama, Mayon.

3. Caldera

The most explosive of all. The eruption is so violent that the volcano collapses into itself, leaving a huge cauldron-like hollow rather than a tall cone.

4. Flood Basalt Province

Highly fluid lava flows out of long cracks (fissures) and spreads over vast areas, building lava plateaus. The Deccan Traps of peninsular India are a textbook example.

5. Mid-Ocean Ridge Volcano

Occur along the centre of oceans at divergent plate boundaries, where the crust pulls apart and lava wells up to form the ocean floor. This is a continuous, system-wide form of volcanism stretching for thousands of kilometres along the ocean floor. New oceanic crust is constantly created here, which is why mid-ocean ridges are central to the idea of sea-floor spreading.

6. Cinder Cone

The simplest type, built when gas-charged lava is blown into the air and falls back as cinders around a single vent, forming a small, steep, cone-shaped hill. Cinder cones are usually short-lived and often appear on the flanks of larger volcanoes.

Intrusive Volcanic Landforms

Not all magma reaches the surface. When it cools and solidifies inside the crust, it produces intrusive landforms. These names appear often in match-the-following questions.

• Batholith — a large dome-shaped mass of cooled magma deep in the crust; the granite cores of many mountains.
• Laccolith — a dome-shaped intrusion with a flat base, connected by a pipe below; resembles a surface volcano but lies underground.
• Sill — a sheet of solidified lava lying horizontally, parallel to rock layers.
• Dyke — a sheet of solidified lava cutting vertically/across the rock layers. Dykes are common in the Deccan region.

Global Distribution and the Ring of Fire

Volcanoes are not scattered randomly. They cluster along plate boundaries, so their distribution mirrors the world map of earthquakes.

• Pacific Ring of Fire — the horseshoe-shaped belt around the Pacific Ocean. It holds about two-thirds of the world’s active volcanoes (Andes, Rockies, Japan, Philippines, Indonesia).
• Mid-Atlantic belt — along the mid-ocean ridge, e.g. Iceland and the Azores.
• Mediterranean belt — Vesuvius, Etna, Stromboli.
• East African Rift — Kilimanjaro region, linked to crustal stretching.

The reason for this clustering is plate tectonics. At convergent boundaries, one plate is forced beneath another (subduction); the descending plate melts, and this magma rises to feed explosive composite volcanoes — the dominant story of the Ring of Fire. At divergent boundaries, plates move apart and gentle basaltic volcanism creates new crust, as in Iceland. A third type, hotspot volcanoes, sits far from any plate edge over a fixed plume of rising magma; Hawaii is the textbook example. Knowing these three settings lets you explain why a volcano is where it is, not just where it is.

Volcanism in India

India is largely free of present-day volcanic activity because it sits in the stable interior of the Indo-Australian plate. But two facts are heavily tested.

• Barren Island in the Andaman Sea is India’s only active volcano; Narcondam Island nearby is considered dormant.
• The Deccan Traps formed about 65 million years ago from massive fissure eruptions of basaltic lava, producing the black, fertile regur (black cotton) soil ideal for cotton.

Effects of Volcanism — Good and Bad

Volcanism is destructive yet also constructive, a balance examiners like to probe.

Harmful effects

• Loss of life, property and farmland from lava flows and ash.
• Volcanic ash clouds disrupt air travel and lower temperatures.
• Underwater eruptions can trigger tsunamis.

Beneficial effects

• Lava weathers into highly fertile soils (e.g. regur soil over the Deccan).
• Volcanic regions yield minerals, sulphur and building stone.
• Geothermal energy and hot springs come from underground heat.
• New land and islands are created (Iceland, Hawaii).

Because of this dual nature, many fertile and densely populated regions of the world — the slopes of Vesuvius, the volcanic soils of Indonesia and Japan, the Deccan of India — sit right next to serious volcanic danger. People accept the hazard for the reward of rich soil and energy. For CDS, remember the balanced verdict: volcanism is destructive in the short term but constructive over geological time.

Worked Example

Let’s reason through a typical classification question step by step.

Notice how each physical clue (viscosity, slope, eruption style) points to one answer. Building this chain in your head turns a guess into a sure mark.

Common Mistakes to Avoid

Previous-Year Style Question

Quick Revision

Revise this page once before the exam with a world map open, and volcanism questions become almost guaranteed marks.