How do volcanoes erupt?
A volcano is a mountain built by its own eruptions. Cut one open, melt solid mantle by taking the pressure off, shake a bottle to see why magma explodes, turn runny lava sticky, race a pyroclastic flow to a village, and watch the ground swell before an eruption. With Barren Island and the Deccan Traps.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Inside a volcano. A volcano is an opening where molten rock, ash and gas escape, and the mountain is the pile its own eruptions have built: layers of lava and ash. Magma collects in a chamber 2 to 10 km down. Five kilometres of rock press on it with about 132 MPa. When fresh magma pushes the chamber's pressure past what the roof can hold, the rock cracks and it erupts.
- Where magma comes from. The mantle is solid: pressure raises its melting point by about 4 °C for every kilometre of depth. Rock melts in three settings. Where plates pull apart it rises and the pressure falls. Where a plate sinks, water driven out of it lowers the melting point of the mantle above. And over a hotspot, a plume 100 to 250 °C hotter than normal rises from deep down.
- Why it rises and explodes. Magma is lighter than solid rock, so buoyancy pushes it up. Deep down, pressure keeps gas dissolved in it, like the fizz in a sealed bottle. As it rises the pressure falls, bubbles form and swell, and when they fill about three quarters of the magma it tears itself into ash and pumice. Volcanic ash is shattered bubble walls: tiny shards of glass.
- Runny or sticky. Silica makes magma sticky. Runny basalt (about 50% silica) lets its gas escape: lava fountains, flows and broad shield volcanoes. Sticky dacite and rhyolite trap gas until they shatter: ash columns and steep stratovolcanoes. The Volcanic Explosivity Index runs from 0 to 8 by the volume thrown out, ten times more for each step. Tambora in 1815 was a 7.
- What comes out. Lava at 1,100 to 1,200 °C usually creeps at under 1 km/h. Pyroclastic flows of hot ash and gas race downhill at more than 80 km/h at 200 to 700 °C, and lahars, floods of ash and water, follow rivers for tens of kilometres. A plume that reaches the stratosphere leaves a sulphur haze for one to three years: Pinatubo cooled the Earth by about 0.5 °C, and Tambora brought the year without a summer.
- India's volcanoes, and watching them. Barren Island in the Andaman Sea is India's only active volcano, erupting on and off since 1991. The Deccan Traps are lava flows 66 million years old, more than 2 km thick, across about 500,000 km². Moving magma gives itself away through small earthquakes, swelling ground and gas, so eruptions can often be forecast days ahead. Follow evacuation orders, keep out of river valleys, and shelter from ash. Emergency 112, NDMA 1078.
| Term | Meaning |
|---|---|
| Magma and lava | Molten rock: magma while it is underground, lava once it reaches the surface. |
| Decompression melting | Solid mantle melting because it rises and the pressure on it falls, not because it is heated. |
| Subduction | One plate sinking under another. Water from the sinking plate lowers the melting point of the mantle above it. |
| Fragmentation | Bubbly magma tearing into ash and pumice when gas bubbles fill about three quarters of it. |
| Viscosity | How strongly a liquid resists flowing. Silica-rich magma is millions of times stickier than basalt. |
| Volcanic Explosivity Index | A scale from 0 to 8 based on the volume an eruption throws out. Each step from 2 upward is ten times more. |
| Pyroclastic flow | A fast, ground-hugging avalanche of hot ash, rock and gas: the deadliest volcanic hazard. |
| Lahar | A flood of volcanic ash and rock mixed with water that flows down river valleys like wet concrete. |
| Flood basalt | A vast stack of runny lava flows from long cracks, such as India's Deccan Traps. |
From a letter about Vesuvius to satellites watching Barren Island: how people learned what a volcano is, what it can do, and how to see an eruption coming.
- 79 · Vesuvius, and the first eyewitness report (Pliny the Younger and Pliny the Elder, Bay of Naples)
- 1787 · Barren Island's first recorded eruption (Seen from passing ships, Barren Island, Andaman Sea)
- 1815 · Tambora: the largest eruption in recorded history (The people of Sumbawa, Sumbawa)
- 1883 · Krakatoa is heard 4,800 km away (The people of the Sunda Strait, Sunda Strait)
- 1902 · Mont Pelée and the glowing cloud (Alfred Lacroix; the people of Saint-Pierre, Saint-Pierre, Martinique)
- 1912 · The Hawaiian Volcano Observatory (Thomas Jaggar, Kīlauea, Hawaii)
- 1980 · Mount St Helens blows out sideways (David Johnston and the US Geological Survey team, Washington State)
- 1985 · Nevado del Ruiz: a warning not acted on (The people of Armero, Tolima)
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