How an eruption works
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Rising
An explosive eruption, stage by stage
Magma rises because it is lighter than the rock around it.The gas does the damage, not the heat.Every eruption ends by building the ground it stands on.
Volcanoes do not explode because they are hot. They explode because the gas dissolved in their magma has nowhere left to go. Scroll to take one eruption from a quiet crater to a cooling lava field, a stage at a time.
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01 Repose
A crater glowing at dusk is a volcano venting heat and gas without erupting. Magma sits in a reservoir below, less dense than the rock around it, slowly giving up water vapour, carbon dioxide and sulphur dioxide through cracks. Most volcanoes spend most of their lives here.
02 Unrest
As magma rises, dissolved gas comes out of solution — the same way a bottle fizzes when the cap turns. Bubbles expand, pressure builds against the rock above, and the ground answers: small earthquakes, a flank that swells by centimetres, more gas and more heat at the surface.
03 Fragmentation
When the bubbles occupy more volume than the melt between them, the magma tears itself apart. That instant is fragmentation: liquid becomes a jet of gas carrying shards of glass and rock, and it leaves the vent at hundreds of metres per second. This is the moment an eruption becomes explosive.
04 The column
The lowest few kilometres are pushed up by sheer momentum. Above that the jet has entrained so much air, heated it and made it buoyant, that the column rises on its own — sometimes into the stratosphere, where it flattens out and spreads sideways as an umbrella cloud.
05 Tephra fall
Everything the column carries is tephra: bombs land near the vent, lapilli further out, ash furthest of all. Grain size sorts itself by distance, so a single ashfall deposit records both how big the eruption was and which way the wind was blowing that day.
06 Flow field
Lava chills against the air and forms a crust within minutes. That crust is an excellent insulator, so the interior stays molten and keeps flowing — often through tubes roofed over by rock. What looks like a cooling plain is a landscape with a working plumbing system underneath.
An explosive eruption, stage by stage
Volcanoes do not explode because they are hot, but because the gas dissolved in their magma has nowhere left to go. Here is one eruption in six stages, from a quiet crater to a cooling lava field. The imagery is AI-generated and illustrative.
Six stages · without the scroll
Swipe through the phases
Before
A volcano between eruptions still degasses, still heats groundwater, still glows at the vent on a cold night. Unrest is the shift from that steady state to something building: quakes clustering under the edifice, the flank inflating, sulphur dioxide output climbing.
During
Gas expansion does the work here, not heat. Fragmented magma leaves the vent as a jet, feeding a fountain near the ground and a convecting column above it, while coarser material falls straight back to build cones and spatter ramparts around the opening.
After
Effusion often outlasts the explosion. Basaltic lava spreads as sheets and lobes, crusts over, and keeps advancing beneath its own roof. Weeks later the surface is walkable while the interior is still above a thousand degrees.
How it is measured
The Volcanic Explosivity Index rates an eruption from 0 to 8 on how much it erupted and how high the column went. Each step is roughly a tenfold increase, which is why the largest events on the scale are separated from ordinary ones by orders of magnitude rather than degrees.
Mildest to most violent
Volcanologists sort eruptions by style, and style comes down to two things: how stiff the magma is, and how much gas it is still holding. Everything else — the height of the column, the shape of the mountain, how long it lasts — follows from those.
01
Runny basalt, gas that escapes easily
The least explosive style. Low-viscosity basalt lets gas bubble out instead of building pressure, so the eruption is fountains and flows rather than blasts. It builds broad, gently sloping shields rather than steep cones.
02
Gas slugs bursting at the surface
Large bubbles rise through the conduit and burst one at a time, each throwing incandescent scoria a few hundred metres. It can carry on in the same rhythm for years, which is why the volcano it is named after has been called the lighthouse of the Mediterranean.
03
A plugged vent clearing itself
Stickier magma seals the vent between eruptions. Pressure builds under the plug until it fails, and the result is a short, violent, ash-rich blast — cannon-like, minutes long, with a column a few kilometres high.
04
Sustained column, stratospheric reach
The most violent of the four and the rarest. Gas-rich, viscous magma sustains a column for hours, tens of kilometres high, blanketing whole regions in pumice and ash. Named for Pliny the Younger, who described exactly this at Vesuvius in AD 79.
Not evenly spread
Volcanoes are not scattered at random. Almost all of them sit in one of three situations, and which one decides how the eruption behaves long before anything reaches the surface.
01
One plate sinks beneath another and carries water down with it. Water lowers the melting point of the mantle above, producing gas-rich, sticky magma — which is why the most explosive volcanoes on Earth ring the Pacific.
02
Where plates pull apart, mantle rises and melts simply because the pressure on it drops. It erupts as basalt, quietly and almost entirely underwater. By volume this is the largest volcanic system on the planet, and almost nobody ever sees it.
03
A persistent source of melt sits under a moving plate, so the plate carries each volcano away from it and a new one grows behind. The result is a chain that gets older in one direction — the Hawaiian islands being the textbook case.
The whole sequence, in order
Read left to right and the whole cycle runs in the space of a paragraph: a crater venting quietly, gas pressure finding the surface, the vent clearing, a column building its own weather, then ash falling and lava spreading into ground that did not exist an hour earlier.
Six words worth knowing
Things people get wrong
Same material, different address. It is magma while it is underground, where it still holds its dissolved gas. It is lava once it has erupted and lost most of that gas — which is precisely why lava flows and magma explodes.
Usually, yes. Most flows advance at walking pace or slower, and they destroy property rather than kill people. The thing you cannot outrun is a pyroclastic flow — a ground-hugging avalanche of hot gas and ash that moves at highway speeds and is what makes explosive eruptions lethal.
Volcanic ash is not soft. It is pulverised rock and volcanic glass, sharp and abrasive, and it does not dissolve in rain. Wet ash is heavy enough to collapse roofs, and it melts inside jet engines, which is why eruptions close airspace hundreds of kilometres downwind.
Forecast, more than predicted. Magma on the move announces itself: swarms of small earthquakes, ground that inflates by centimetres, rising sulphur dioxide, new heat at the surface. Monitored volcanoes give warning. The difficulty is that unrest does not always end in an eruption.
Briefly, the big ones do. Sulphur dioxide reaching the stratosphere forms a haze of droplets that reflects sunlight, and a large eruption can measurably lower global temperatures for a year or two before it settles out. The carbon dioxide volcanoes emit is a rounding error next to what people do.
Every image here is AI-generated and illustrative. None of it is documentary footage, none of it shows a real volcano, and details will not survive close geological reading. For the real thing, the USGS Volcano Hazards Program and the Smithsonian’s Global Volcanism Program are where to go.