Volcano simulator
Erupt a real volcano at any size on the explosivity scale and see how far the pyroclastic flows and the ash would reach.
Open the volcano simulatorWhat this simulator does
Click one of the red markers on the map — each is a real volcano from the Smithsonian Institution's Global Volcanism Program, 1,560 of them — or click a city near one, and the simulator snaps to the nearest volcano within 40 km. Set the eruption size on the Volcanic Explosivity Index from 1 to 8, and it draws three zones:
- Pyroclastic flows — avalanches of gas and rock at several hundred degrees, which almost nobody inside survives
- 10 cm of ash — enough weight to collapse roofs, especially if it rains
- 1 cm of ash — the light fall that stops aircraft, contaminates water and ruins crops
Towns inside each zone are counted from OpenStreetMap and turned into an estimated death toll.
What VEI actually means
The Volcanic Explosivity Index measures how much rock and ash an eruption throws out. From VEI 2 upward, each step is ten times more material than the one below it. That makes the scale deceptive: the difference between VEI 5 and VEI 7 is not "somewhat bigger", it is a hundredfold.
- VEI 4 — Eyjafjallajökull, 2010. Small, but it grounded European aviation for a week.
- VEI 5 — Mount St. Helens, 1980. About 1 km³ of material, 57 dead.
- VEI 6 — Pinatubo, 1991. Cooled the entire planet by about 0.5 °C for two years.
- VEI 7 — Tambora, 1815. About 100 km³; the following year had no summer in Europe and North America, and the famine killed far more people than the eruption.
- VEI 8 — Toba, Yellowstone. Over 1,000 km³. There has not been one in 26,000 years.
Why the zones are circles, and why reality is not
This is the model's biggest simplification, and it matters. Real pyroclastic flows do not spread evenly — they are dense and they pour downhill, following valleys for tens of kilometres on one side of a volcano while the ridge on the other side stays untouched. Ash does not fall in a circle either; it drifts downwind in a long plume, so one town 40 km east can be buried while another 40 km west gets nothing.
Read the circles as "how far this effect can reach", not "where it will land". Two things are missing entirely: lahars, the mudflows that follow rain or melting snow and have killed more people in the last century than the flows themselves, and volcanic gases.
Volcanoes worth erupting
- Vesuvius at VEI 6 — three million people live inside the red zone
- Campi Flegrei at VEI 7 — the caldera under the western suburbs of Naples
- Mount Fuji at VEI 5 — last erupted in 1707, with Tokyo downwind
- Popocatépetl at VEI 5 — 70 km from Mexico City
- Yellowstone at VEI 8 — the supereruption everyone asks about
Why you can't erupt a volcano just anywhere
Because eruptions only happen where there is a volcano. Earlier versions of this simulator let you put one under any city, which produced dramatic pictures and taught nothing. Now a click within 40 km of a catalogued volcano uses that volcano; further away, the site tells you the nearest real one and offers to take you there.
New volcanoes do appear — Parícutin grew out of a Mexican cornfield in 1943 and reached 424 metres — but far too rarely to be worth simulating.
Questions
Where do the volcano positions come from?
The Smithsonian Institution's Global Volcanism Program, which catalogues volcanoes with known eruptions in the Holocene — the last ~12,000 years.
Would people really die if nobody evacuated?
The casualty estimate assumes exactly that, which makes it pessimistic: most real eruptions give days or weeks of warning through earthquakes and ground deformation, and modern monitoring has saved tens of thousands of lives. Pinatubo in 1991 was evacuated in time; Nevado del Ruiz in 1985 was not, and 23,000 people died in a lahar.
What is the most dangerous volcano in the world?
By people at risk rather than by size, Vesuvius and Campi Flegrei are usually named first — both have hundreds of thousands to millions of people living on ground that pyroclastic flows have covered before.
Can a supervolcano end civilisation?
A VEI 8 would not destroy much by blast, but the ash and sulphate aerosols would cut global temperatures for years and wreck harvests worldwide. The damage would be agricultural, not geological — and it is the rarest event on this site by a wide margin.
Further reading: The biggest volcanic eruptions in history — and the VEI scale
Open the volcano simulator