The World's Volcanoes

What a volcano is

A volcano is a vent where molten rock, ash and gas reach the surface from deep inside the Earth. Most sit along the boundaries between tectonic plates, where one plate dives beneath another or two plates pull apart, so the world map of volcanoes is really a map of those boundaries.

• Ring of Fire: the horseshoe of subduction zones around the Pacific, home to most of the world's active volcanoes.
• Holocene: the current geological epoch, the last roughly 11,700 years. This catalogue holds every volcano known to have been active in that window.
• Dormant, not extinct: a volcano quiet for centuries is still a volcano; the record here reaches back thousands of years for exactly that reason.

How eruptions are measured

The size of an explosive eruption is graded on the Volcanic Explosivity Index, a scale from 0 to 8 based on how much material is thrown out and how high the plume rises.

• VEI is logarithmic: each step up is roughly ten times more ejected material, so a VEI 5 is about ten times the size of a VEI 4.
• The frequency falls fast: small eruptions happen constantly; a VEI 4 arrives somewhere on Earth most years, a VEI 6 only a few times a century.
• Old eruptions lose their number: VEI is often unknown for events dated only from ash layers or ice cores, so many rows in the record carry no VEI at all.

Why big is not the same as deadly

The most dangerous volcano is rarely the largest. It is the one with people, aircraft or insured assets in the way.

A modest eruption beside a city can kill and cost far more than a giant one in a remote arc. Lava is the least of it: ashfall collapses roofs and closes airspace across whole regions, while pyroclastic flows and lahars move faster than anyone can run.

For risk readers the lesson is exposure, not explosivity. A working volcano with an international airport and a million residents on its slopes is a civic risk long before it is a geological spectacle.