How a Volcano Works
Written by Viktor Jonas · Reading time about 6 minutes
It is not the heat that blows the mountain apart, but the gas coming out of solution as the magma rises.
A volcano is not a hole with hot Earth pouring out of it. Almost everything beneath our feet is solid. For molten rock to exist at all, special conditions have to come together — which is exactly why volcanoes are not scattered at random but line up in narrow belts along plate boundaries.
Magma — molten rock while it is still underground.
Lava — the same material once it has reached the surface.
Magma chamber — the reservoir a few kilometres down where melt collects.
Viscosity — how thick and sticky the melt is. It decides whether a volcano flows quietly or explodes.
Pyroclastic flow — an avalanche of hot gas, ash and rock travelling at hundreds of kilometres per hour; the deadliest part of an eruption.
How rock turns liquid
The mantle is well over a thousand degrees and still solid. The reason is the enormous pressure: it holds the atoms so tightly together that they cannot slip into the disordered motion of a liquid. There are essentially three ways to tip that balance.
The first is decompression. Where two plates pull apart, mantle material rises. Pressure falls, the melting point falls with it, and a small fraction of the rock turns liquid. This builds practically all of the ocean crust — most volcanism on Earth happens unnoticed on the sea floor. Iceland is simply the spot where the process sticks out above sea level.
The second route is water. A subducting ocean plate carries seawater locked into its minerals. At depth the water is driven out and rises into the mantle above. Water lowers the melting point of rock dramatically, much as road salt lowers the freezing point of ice. That is why every subduction zone carries a line of volcanoes above it: the Andes, the Japanese islands, the Cascades.
The third route is hotspots — stationary upwellings from great depth that ignore plate boundaries entirely. Hawaii is the textbook case: the Pacific plate drifts across the hotspot and leaves behind a chain of islands that get older and more eroded towards the north-west.
Why some volcanoes flow and others explode
Whether an eruption is a tourist attraction or a catastrophe comes down to the chemistry of the melt. The more silica it contains, the stickier it is. In sticky melt the gas bubbles cannot escape — they stay trapped, pressure builds, and eventually the plug gives way. In runny melt the gas leaks out along the way and the lava simply pours out.
| Runny | Sticky | |
|---|---|---|
| Silica | low (about 50 %) | high (over 65 %) |
| Rock | basalt | rhyolite, andesite |
| Eruption | quiet lava flows | explosion, ash cloud |
| Shape | flat shield volcano | steep stratovolcano |
| Example | Kilauea, Hawaii | Vesuvius, Pinatubo |
That is why you can stand next to a lava flow in Hawaii and take photographs, while Vesuvius in AD 79 buried entire towns under ash within hours. Same planet, chemically two different worlds.
How big was the eruption?
Each step means ten times as much.
VEI 2 → about 0.001 km³ — small eruption, happens somewhere most years
VEI 4 → about 0.1 km³ — Eyjafjallajökull 2010, grounded European air traffic
VEI 5 → about 1 km³ — Mount St. Helens 1980
VEI 6 → about 10 km³ — Pinatubo 1991, cooled the planet by 0.5 °C
VEI 7 → about 100 km³ — Tambora 1815, the "year without a summer"
💬 Mia asks Grandpa Theo
Why would anyone live right next to a volcano?
Because volcanic ash makes the best farmland in the world. It contains exactly the minerals plants need. Millions of people around Vesuvius and on Java live off it — they know the risk and stay anyway.
Can you predict when it will go off?
Better than we used to. Before an eruption a volcano swells slightly, small quakes start, and more sulphur dioxide escapes. Days or weeks of warning are often possible now. Nobody will give you the exact day, though.
Common mistakes
- "Lava is the dangerous part."
- Lava flows are usually slow enough to walk away from. Most deaths come from pyroclastic flows, mudflows and famine after crop failure.
- Mixing up magma and lava
- Magma below ground, lava above — same material, different place.
- "An extinct volcano cannot erupt."
- Active, dormant and extinct are assessments, not guarantees. Volcanoes have woken up after thousands of years of silence.
- Volcanoes as the main source of CO₂
- All the world's volcanoes together release only a small fraction of what humanity emits each year.
A VEI 6 eruption ejects roughly 10 km³ of material. How many VEI 4 eruptions would you need for the same amount? Then explain why the VEI still says nothing about the number of casualties.
Show solution
From VEI 4 to VEI 6 is two steps, so 10 × 10 = 100 times as
much. You would need 100 eruptions of VEI 4.
The index says nothing about casualties because that depends on how many people live
nearby, how well they are warned, and which way the wind carries the ash. A small
eruption in a crowded area can kill more people than a large one in the wilderness.
Volcanoes and the climate
A large eruption cools the planet, measurably. What matters is not the ash, which settles within weeks, but the sulphur dioxide. If it reaches the stratosphere it forms fine droplets of sulphuric acid that scatter sunlight back into space. That veil lasts one to three years. After Pinatubo erupted in 1991 the global average temperature fell by about half a degree.
Tambora in 1815 was many times larger. The following year went down in history as the "year without a summer": snow in June in New England, failed harvests across central Europe, famine, waves of emigration. That a single mountain in Indonesia could double grain prices in Germany shows better than any diagram how tightly the Earth system is coupled.
Over very long timescales, though, volcanoes do the opposite. They are the return path for carbon: what was deposited as limestone on the sea floor over millions of years and dragged down in subduction zones comes back out as CO₂. Without that cycle the atmosphere would slowly run out of carbon — and with it the natural greenhouse effect that keeps the Earth warm enough for liquid water in the first place.
About 1,500 volcanoes are considered potentially active, and some 50 to 70 erupt in an average year. Hardly any of them make the news — and by far the greatest share of volcanism happens invisibly on the ocean floor anyway.