Tukuk-OSAlmost all volcanoes sit in one of three settings: where plates split apart (mid-ocean ridges and continental rifts), where one plate sinks beneath another (subduction zones — the Pacific Ring), and where a deep mantle plume burns through the middle of a plate (hotspots — Hawaii, Iceland, Yellowstone). A fourth fact dominates the statistics: most volcanoes are under the sea, and most eruptions on Earth are unnoticed submarine ones building new crust.
Temperament is chemistry. Runny basaltic lava, low in gas, oozes and fountains: Hawaii’s Kīlauea has erupted continuously for decades with lava lakes and rivers that move slower than a walking human — spectacular, mostly avoidable. Thick, gas-charged magma from subduction zones is another animal: it shatters itself apart in explosive blasts of ash and pumice, and collapses down its flanks as pyroclastic flows — superheated avalanches of gas and rock travelling over 100 km/h at several hundred degrees, which is what destroyed Pompeii and buried Montserrat’s capital in 1995. Lava rarely kills whole cities; flows, ash, lahars (mud rivers down volcano gullies) and gas do.
The global cooling exceptions are real but rare: only truly colossal explosive eruptions inject enough sulphur gas into the stratosphere to shade the planet for a year or two. Tambora in 1815 made 1816 “the year without a summer”; Pinatubo in 1991 cooled Earth about half a degree for a year and was the first big eruption whose climate effect scientists predicted and measured with modern instruments. Most eruptions are far too small for that — and far too dangerous up close, which is where the vigilance belongs.