Volcanoes are not ranked on a scale of one to ten the way a hotel is. The Volcanic Explosivity Index is logarithmic: every rung up means roughly ten times more rock thrown into the sky — and, very roughly, ten times longer to wait for one. The eruption that closed European airspace in 2010 sits near the bottom of the ladder. The ladder goes six rungs higher.
Estimated probability of a magnitude‑7 eruption — Tambora‑sized, the kind that rearranges a year's weather — happening somewhere on Earth in the next 100 years, based on ice‑core records of past eruptions.
Cassidy & Mani, Nature, 2022
Christopher Newhall and Stephen Self built the index in 1982 out of two things you can measure long after the fact: how much material came out, and how high the column went. Above VEI 2, each step multiplies erupted volume by about ten.
The ladder has a second axis nobody puts on the poster: waiting time. Small eruptions are a weekly fact of the planet. Each rung up multiplies the gap between events by about an order of magnitude — which is why the biggest rung has no entry in all of recorded history.
Eruptions catalogued in the last 11,700 years. About 90% of them are VEI 3 or smaller.
Ten confirmed magnitude‑7 events in 11,700 years — and not one confirmed VEI 8.
Volcanoes with ongoing eruptions at any given moment. The Smithsonian counts 64–89 eruptions a year since 2000.
Plotted linearly, only one bar would be visible. Below, bar length runs on a logarithmic scale of erupted volume — so equal steps mean tenfold jumps. Even so, the eruption that grounded a continent barely registers.
Volcanoes are not scattered. They trace the edges of tectonic plates — above all the horseshoe of subduction zones around the Pacific, where roughly 75% of the world's active volcanoes and 90% of its earthquakes occur.
Of the world's active volcanoes stand on the Pacific rim, along with about 90% of the planet's earthquakes.
People live within 100 km of one of Indonesia's 142 Holocene volcanoes — more than any other country.
Holocene volcanoes are spread across 86 countries. Half of all fatal volcanic incidents since 1500 happened within 10 km of the vent.
A big eruption's real leverage isn't the lava. It's the sulphur and water it injects above the weather, into the stratosphere, where nothing rains it out for a year or more.
The drop in average global temperature in 1816 — the "year without a summer." Snow fell in New England in June; harvests failed across Europe. Roughly 10,000 people were killed by the eruption itself, and tens of thousands more by the famine and disease that followed.
Sulphur dioxide injected into the stratosphere, where it became a sulphate haze that dimmed the sun. Global mean temperature fell about 0.5 °C, peaking roughly 18 months after the eruption.
Plume height — the tallest ever measured by satellite, punching into the mesosphere. It threw 146 ± 5 teragrams of water vapour into the stratosphere, about 10% more water than that layer already held.
Eyjafjallajökull was a small eruption by any geological measure. It was also the most expensive week in the history of European aviation — a preview of what a modern world does when it meets even a modest column of ash.
Across eight days of European airspace closure, affecting about 10 million passengers.
IATA's estimate of industry losses from a single VEI 4 eruption.
14.3% of the world's 2015 population lived within 100 km of a Holocene volcano — and that zone has been filling faster than the world as a whole since 1975.
Recorded volcanic fatalities across 635 incidents — about 540 a year, with deaths logged as far as 170 km from the vent.
A magnitude‑7 eruption is likelier this century than a civilisation‑threatening asteroid strike — and far less prepared for. NASA spent about $300 million testing whether it could nudge an asteroid; pandemic preparedness draws on the order of $10 billion a year; volcano monitoring, as Cassidy and Mani put it, gets next to nothing. The world of 1815 had a billion people and almost no aviation. Ours has eight billion, a global food trade, and a sky full of jet engines that ash melts shut.
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