Volcanoes

The first principle

Rock in the mantle is hot enough to melt but is kept solid by the enormous pressure above it, so magma forms only when that balance is upset: when pressure falls, when water lowers the melting point, or when extra heat arrives. Once formed, magma is lighter than the rock around it and rises, and how violently it erupts depends on two things, how much silica makes it sticky and how much dissolved gas it carries. Almost everything about volcanoes, from their shape and location to their effect on climate, follows from these few physical facts.

The core

How magma forms

• Decompression melting. When hot mantle rises, as under a mid ocean ridge or in a mantle plume, the pressure on it drops faster than it cools, so it crosses its melting point without any extra heat; this produces runny basaltic magma.
• Flux melting. At a subduction zone the sinking ocean plate carries water locked in its minerals; at depths of about 100 km the water is driven off into the mantle wedge above, where it lowers the melting point of rock just as salt lowers the melting point of ice.
• Heat transfer. Basaltic magma ponding under a continent can melt the crust around it, which yields silica rich, gas rich magma and explains the violent eruptions of continental arcs.

Where volcanoes occur

• Convergent margins hold most of the world's explosive volcanoes, and the Pacific Ring of Fire, from the Andes through Japan and Indonesia to New Zealand, contains a large majority of the active volcanoes on land.
• Divergent margins such as the Mid Atlantic Ridge and the East African Rift erupt quietly; Iceland sits where a ridge crosses a hotspot, so it has unusually high volcanic output.
• Hotspots like Hawaii and Reunion lie in the middle of plates, and the age of islands increasing away from the active volcano records the direction of plate movement.

Explosive versus effusive eruptions

Silica molecules link into chains, so magma with about 70 per cent silica (rhyolite) is thousands of times more viscous than basalt with about 50 per cent. Gas cannot escape from sticky magma, so pressure builds until the magma shatters into ash and pumice, much as a shaken soda bottle bursts when opened. Runny basalt lets gas bubble out, so it flows out as lava rivers and fountains, which is why Hawaii and Iceland erupt effusively while Pinatubo and Vesuvius explode.

Dimensions

• Landforms. Extrusive features include cones, craters, calderas, lava plateaus and crater lakes, while intrusive bodies form when magma cools inside the crust: a batholith is a huge dome shaped mass of granite at depth, a laccolith is a mushroom shaped body that arches the beds above it, a sill runs parallel to the rock layers and a dyke cuts across them. These are exposed only after erosion removes the cover, as the granite tors of Karnataka and the dolerite dykes of the Deccan show.
• Hazards near the volcano. Pyroclastic flows are avalanches of hot gas and ash moving at well over 100 km an hour, and they cause most deaths; lahars are mudflows of ash and water that follow river valleys long after an eruption; ash collapses roofs, ruins crops and contaminates water.
• Wider effects. Ash clouds close airspace because ash melts inside jet engines, as the 2010 Eyjafjallajokull eruption showed when it shut much of European airspace for days. Sulphur dioxide causes acid rain and volcanic smog (vog) downwind.
• Global climate. Only eruptions that push sulphur dioxide into the stratosphere change world climate, because there the gas turns into long lived sulphate droplets that reflect sunlight.

In practice

India's volcanoes

• Barren Island in the Andaman Sea is India's only active volcano; it lies on the arc where the Indian plate subducts beneath the Burma microplate, and it became active in 1991 after a long quiet spell and has erupted on and off since.
• Narcondam, to its north on the same arc, is dormant, while the mud volcanoes of Baratang in the Andamans are driven by gas from buried sediments rather than magma.
• The Deccan Traps are flood basalts erupted from fissures about 66 million years ago as India passed over the Reunion hotspot; they still cover about 5 lakh sq km and weathered into the black cotton soil of Maharashtra.

The eruptions of 2021 and their regional effects

The Smithsonian Global Volcanism Program records a few dozen volcanoes in eruption in any year, and 2021 was a typical year with several eruptions that hit people hard.

• Cumbre Vieja, La Palma (Canary Islands, Spain), erupted from 19 September to 13 December 2021, the longest eruption on the island on record. Lava flows buried about 3,000 buildings, roads and banana plantations that are the island's economy, forced about 7,000 people out, and built a new lava delta into the sea, while ash closed the airport.
• Nyiragongo, Democratic Republic of the Congo (DR Congo), erupted in May 2021 and sent lava to the edge of Goma; about 30 people died, and swarms of earthquakes under the city then drove a mass evacuation of hundreds of thousands for fear of a fresh eruption beneath Goma or Lake Kivu.
• Semeru, Java (Indonesia), collapsed in December 2021 and sent pyroclastic flows and lahars down river valleys that buried villages, killing dozens and destroying a bridge.
• La Soufriere, St Vincent, exploded in April 2021 after a timely evacuation of about 20,000 people, and its ash fell on neighbouring Barbados.
• Fagradalsfjall, Iceland, began a gentle fissure eruption in March 2021, the first on the Reykjanes peninsula in about 800 years, which posed mainly a gas hazard and drew tourists.
• Taal, Philippines, erupted in July 2021 and its sulphur dioxide haze reached Metro Manila.
• Hunga Tonga Hunga Ha'apai began erupting in December 2021 and exploded on 15 January 2022, sending a pressure wave round the globe, a Pacific wide tsunami and, by NASA's estimate, enough water vapour into the stratosphere to raise its content by about 10 per cent. Because water vapour traps heat, this eruption may have slightly warmed rather than cooled the climate, which makes it unusual.

Past cases that changed climate

• Tambora, Indonesia (1815) produced the "year without a summer" in 1816, with crop failures across Europe and North America.
• Laki, Iceland (1783) released sulphur that poisoned Icelandic livestock and caused a famine.
• Pinatubo, Philippines (1991) put about 15 million tonnes of sulphur dioxide into the stratosphere and cooled the globe by about 0.5°C for nearly two years, and timely evacuation based on monitoring saved thousands of lives.

Issues and way forward

• Benefits that keep people near volcanoes. Volcanic soils rich in potash and phosphorus support dense farming in Java and the Deccan; geothermal energy supplies about a quarter of Iceland's electricity, and India has explored it at Puga in Ladakh; volcanic systems also concentrate sulphur, copper and gold. This is why millions live in danger zones, so the aim is to live safely with volcanoes, not to leave them.
• Monitoring gaps. Many volcanoes in poorer countries have no seismometers or gas sensors, so eruptions surprise people; volcanologists and the United States Geological Survey's assistance programmes recommend basic monitoring and satellite gas and ground deformation tracking for every dangerous volcano.
• Weak land use rules. Settlements grow on lahar paths and old lava fields, as at Goma; hazard zone maps tied to building rules and drilled evacuation plans, as in the Pinatubo response, cut deaths sharply.
• Aviation risk. Nine Volcanic Ash Advisory Centres set up under the International Civil Aviation Organization track ash clouds, but better ash concentration forecasts would allow fewer blanket closures.
• India. Barren Island is uninhabited, but a large eruption or flank collapse could send a local tsunami, so the National Centre for Seismology and the Geological Survey of India need continuous monitoring there.

Common confusions

• Magma and lava. Magma is molten rock below ground; lava is the same material once it reaches the surface and loses much of its gas.
• Crater and caldera. A crater is the vent opening, usually less than a kilometre across; a caldera is a much larger basin formed when the summit collapses into an emptied magma chamber, as at Toba.
• Sill and dyke. A sill lies along the rock layers like a sheet in a book; a dyke cuts across them like a wall.
• Ash and aerosol. Ash is rock that falls out in days and causes local harm; sulphate aerosols stay aloft for a year or more and cause global cooling.
• Active, dormant and extinct. Active volcanoes have erupted in historical times or show unrest; dormant ones could erupt again; extinct ones have no magma supply, and the line between the last two is often uncertain.

For the exam

• Keyword bank: decompression melting, flux melting, hotspot, Ring of Fire, viscosity, silica, pyroclastic flow, lahar, caldera, flood basalt, sill, dyke, sulphate aerosols, stratospheric water vapour, geothermal energy.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): The 85 day eruption of Cumbre Vieja on La Palma in 2021 destroyed about 3,000 buildings and the island's banana farms, showing how a single volcano can reshape a regional economy.
  • Introduction (report): The United States Geological Survey estimates that Pinatubo's 1991 eruption put about 15 million tonnes of sulphur dioxide into the stratosphere and cooled the earth by about 0.5°C.
  • Conclusion: Volcanoes destroy locally and briefly but enrich soils and supply heat and minerals for centuries, so the task is monitoring and zoning that let people live safely beside them.
  • Conclusion: The contrast between Pinatubo's cooling and Hunga Tonga's water vapour shows that an eruption's climate effect depends on what it puts into the stratosphere, not only on its size.