Volcanoes and Volcanic Landforms

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. Molten rock below the surface is called magma; once it reaches the surface it is lava. The material comes mainly from the asthenosphere, the weak, partly molten zone of the upper mantle. Volcanoes build new land, create fertile soils and supply minerals and geothermal energy, but they can also kill thousands of people and even cool the global climate. In GS Paper 1, UPSC asks about types of volcanoes, their distribution, volcanic landforms and recent eruptions in the news.

Why volcanoes occur where they do

• Convergent (subduction) boundaries: when an oceanic plate sinks beneath another plate, it carries water down; this lowers the melting point of the mantle above, producing sticky, gas-rich magma and explosive volcanoes – the Andes, Japan, Indonesia and the Cascades.
• Divergent boundaries: at mid-ocean ridges and rift valleys, plates pull apart and runny basaltic magma wells up gently – Iceland, the East African Rift (Kilimanjaro, Nyiragongo).
• Hotspots: plumes of hot mantle rise beneath the middle of plates – Hawaii, Yellowstone, and the Réunion hotspot that produced the Deccan Traps.

About 75 per cent of the world's active volcanoes lie in the Pacific Ring of Fire. Other zones are the Mid-Atlantic Ridge, the Mediterranean belt (Vesuvius, Etna, Stromboli – the "Lighthouse of the Mediterranean") and the East African Rift.

Types of volcanoes by activity

Types of volcanoes by form and eruption

• Shield volcanoes – broad, gentle slopes built by runny basaltic lava; the largest volcanoes on Earth; mostly non-explosive. Mauna Loa and Kilauea in Hawaii.
• Composite (strato) volcanoes – tall, steep cones of alternating layers of lava and ash; viscous andesitic magma traps gas, so eruptions are explosive. Fuji, Vesuvius, Mount St Helens, Pinatubo, Merapi.
• Cinder cones – small, steep cones of loose fragments around a single vent; e.g. Parícutin in Mexico, which grew from a cornfield in 1943.
• Calderas – huge depressions formed when a volcano collapses after emptying its magma chamber in a massive eruption; e.g. Yellowstone, Toba in Sumatra (a super-eruption about 74,000 years ago), Krakatoa. Hunga Tonga–Hunga Ha'apai (January 2022) was the largest explosion recorded by modern instruments.
• Flood basalt provinces – not single cones but vast outpourings of fluid lava from long fissures, covering huge areas: the Deccan Traps in India, the Columbia Plateau in the USA, the Siberian Traps.
• Mid-ocean ridge volcanoes – mostly underwater, building new crust.

Volcanic Explosivity Index (VEI) measures eruption size from 0 (gentle) to 8 (super-eruption), based on the volume of material thrown out and the height of the eruption column.

Materials thrown out

• Lava flows – basaltic (runny, fast-moving, low silica) or rhyolitic/andesitic (thick, slow, high silica).
• Pyroclastic material – volcanic bombs, lapilli, ash and dust. Pyroclastic flows – fast avalanches of hot gas and ash, travelling at over 100 km/h – are the deadliest hazard; they buried Pompeii in AD 79.
• Gases – water vapour, carbon dioxide, sulphur dioxide. Sulphur dioxide forms aerosols in the stratosphere that reflect sunlight; Pinatubo (1991) cooled the world by about 0.5°C for a year or two, and Tambora (1815) caused the "Year Without a Summer" in 1816.
• Lahars – mudflows of volcanic debris and water, which killed about 23,000 people at Nevado del Ruiz, Colombia, in 1985.

Volcanic landforms

Intrusive landforms form when magma cools and hardens inside the crust:

Extrusive landforms form on the surface: volcanic cones, craters (the funnel at the top), calderas, lava plateaus (Deccan), crater lakes (Crater Lake, Oregon; Lonar is an impact crater, not volcanic), and features of continuing heat such as geysers (Old Faithful in Yellowstone), hot springs and fumaroles.

Volcanoes and India

• Barren Island in the Andaman Sea is India's only confirmed active volcano; it erupted in 1787, has been active again on and off since 1991, and showed activity in recent years (including reported eruptions in 2017 and minor activity in 2022). It lies on the boundary between the Indian and Burma plates.
• Narcondam Island, to its north, is usually described as dormant; it is also home to the endemic Narcondam hornbill.
• The Deccan Traps are one of the largest flood basalt provinces on Earth, erupted about 66 million years ago, around the time of the mass extinction of the dinosaurs; they gave India its black soils.
• Geothermal energy potential exists at Puga valley (Ladakh), Manikaran (Himachal) and Tattapani (Chhattisgarh); ONGC began a geothermal pilot project at Puga.
• Hot springs such as Manikaran, Tapovan (Uttarakhand), Rajgir (Bihar) and Bakreshwar (West Bengal) show residual heat.

Benefits and hazards

Benefits: very fertile soils (Java, the Deccan, the slopes of Etna), new land (Iceland, Hawaii), minerals such as sulphur, copper and gold, building stone and pumice, geothermal power (Iceland meets most of its heating needs this way; Kenya, New Zealand), and tourism.

Hazards: lava flows, pyroclastic flows, ash falls that collapse roofs and ground aircraft (the Eyjafjallajökull eruption of 2010 shut European airspace for days), lahars, toxic gases, tsunamis (Krakatoa 1883) and short-term climate cooling.

Monitoring and mitigation: seismographs to track magma movement, tiltmeters and satellite radar to detect ground swelling, gas sensors, hazard maps, evacuation plans and aviation ash advisories. Successful warnings, as at Pinatubo in 1991, saved tens of thousands of lives.

Mud volcanoes and recent eruptions in the news

Not every "volcano" erupts molten rock. Mud volcanoes release mud, water and gases such as methane from underground pressure in sedimentary basins. India's best-known examples are on Baratang Island in the Andamans, which erupted noticeably after the 2004 earthquake. Large mud volcanoes are also found in Azerbaijan, Pakistan's Makran coast and Indonesia, where the Lusi mud flow in East Java has buried villages since 2006.

Eruptions that appeared in the news in recent years include Kilauea in Hawaii (repeated lava fountains in 2024–25), Etna in Sicily, the Reykjanes peninsula in Iceland (a series of fissure eruptions near the town of Grindavík since 2023), Mount Lewotobi Laki-laki and Ruang in Indonesia (2024), and Mount Spurr in Alaska and Kanlaon in the Philippines, which showed unrest in 2024–25. UPSC often picks such places and asks you to locate them or name the plate setting, so link each news item to the map and to the theory above. Remember that eruption reports change quickly, so confirm dates before quoting them.

Mains practice question

Explain the relationship between plate tectonics and the global distribution of volcanoes. How do volcanic eruptions affect the global climate? (250 words)

Answer framework

• Intro: definition; asthenosphere source.
• Distribution: convergent (Ring of Fire), divergent (Iceland, East Africa), hotspots (Hawaii, Réunion).
• Climate: SO₂ aerosols cool (Pinatubo, Tambora); CO₂ long-term; ash.
• Indian examples: Barren Island, Deccan Traps.
• Conclusion: monitoring and preparedness.