Volcanoes: How They Form, Types, Volcanic Landforms, World Distribution and India’s Volcanoes (UPSC Notes + PYQs)

Background: what a volcano is

A volcano is an opening in the Earth’s crust through which molten rock, gases and fragments escape to the surface. Below the crust, in the upper mantle, there is a weak zone called the asthenosphere, where rock is partly molten. When pressure drops, water is added or temperature rises, rock melts and forms magma, which is lighter than the solid rock around it and rises. It collects in a magma chamber and erupts through a vent when pressure is high enough. Once on the surface it is called lava. Volcanoes are not spread randomly over the globe. Most lie along plate boundaries, where plates either pull apart or one plate dives under another, and a few lie over hotspots in the middle of plates. Studying them explains much of physical geography: the shape of continents, island arcs, mountain chains, fertile soils and even short-term changes in climate.

Where volcanoes occur

• Convergent boundaries: oceanic plate subducts, melts and feeds explosive volcanoes (Andes, Japan, Indonesia).
• Divergent boundaries: magma wells up at mid-ocean ridges and rift valleys (Iceland, East African Rift).
• Hotspots: plumes from the mantle (Hawaii, Yellowstone, Reunion; the Deccan Traps are linked to the Reunion hotspot).

Types of volcanoes

Volcanic landforms

Extrusive: lava plateaus, cones, craters, calderas. Intrusive (magma cools underground): batholiths (large granite bodies), laccoliths (dome-shaped), lopoliths (saucer-shaped), phacoliths (wavy, in folds), sills (horizontal) and dykes (vertical). Dykes are common in the Deccan region.

India’s volcanoes

Barren Island (Andaman) erupted in 1991 and several times since; Narcondam is considered dormant. The Deccan Traps formed from huge fissure eruptions around 66 million years ago; weathering of that basalt produced black cotton (regur) soil. Dhinodhar hill in Kutch and the Dhosi hill in Haryana are remnants of ancient volcanic activity.

Materials that come out

The type of lava decides the type of eruption. Basaltic lava, low in silica, is runny and lets gas escape gently. Andesitic and rhyolitic lava, rich in silica, is thick and traps gas, so eruptions are explosive.

Effects

Destruction and lahars, but also fertile soils (Java, Deccan), geothermal energy (Iceland), and short-term global cooling from sulphur aerosols (Pinatubo, 1991, cooled the earth by about 0.5°C).

Plate boundaries and volcanoes

Continental-continental collisions, such as India with Asia, produce fold mountains but few volcanoes, because neither plate sinks deep enough to melt.

Volcanoes and climate

Large explosive eruptions can cool the planet for one or two years. They inject sulphur dioxide into the stratosphere, where it turns into tiny sulphate droplets that reflect sunlight. Tambora in Indonesia (1815) led to the ‘year without a summer’ in 1816 in Europe and North America, with crop failures. Mount Pinatubo in the Philippines (1991) cooled global average temperature by about half a degree Celsius for a year or so. Over much longer periods, volcanic carbon dioxide has warmed the climate, and huge outpourings such as the Deccan Traps are linked by many scientists to mass extinctions. These ideas also lie behind proposals for solar geoengineering, which would mimic volcanic aerosols, a topic UPSC has touched in questions on climate.

Hazards and benefits

Volcanoes kill through pyroclastic flows, lahars, ash fall, gases and tsunamis. Krakatoa (1883) generated tsunamis that killed tens of thousands. Ash grounds flights, as in the Eyjafjallajokull eruption in Iceland in 2010. But volcanoes also bring benefits. Volcanic soils are very fertile: the black regur soil of the Deccan, derived from basalt, is ideal for cotton. Geothermal energy powers much of Iceland and parts of New Zealand, Kenya and Indonesia. Volcanic regions hold valuable minerals and attract tourism. In India, geothermal potential exists at Puga valley in Ladakh and in several hot spring belts. A balanced answer mentions both sides and the role of monitoring, hazard mapping and early warning in reducing risk.

Mains and Prelims tips

For Prelims, link each Indian volcano with its location: Barren Island and Narcondam in the Andaman Sea, the Deccan Traps in peninsular India, and the Dhinodhar hills of Kutch with old volcanic rocks. For Mains, a world map sketch showing the Ring of Fire, the Mid-Atlantic Ridge and Hawaii, with arrows for plate movement, earns marks in questions on the distribution of volcanoes and earthquakes.

Volcanoes: UPSC angle

Volcanoes UPSC PYQs (Prelims)

Questions UPSC has actually asked, from the official papers. Pick an option to check yourself.

Mains PYQs on this topic

No UPSC Mains question from 2023-2026 is directly on this topic. Older papers (2013-2022) will be added as the Mains bank grows.

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