A volcano is an opening where molten rock, ash, and gas escape from below. Magma forms when mantle or crust melts, often where plates pull apart, where one plate dives and waters the mantle above it, or where a hot plume rises, as under Hawaii. If the melt is runny and low in gas, it can fountain and flow as lava, building broad shields. If it is sticky and charged with steam and carbon dioxide, pressure grows until the mountain explodes, throwing ash columns that can circle the globe and collapse as searing density currents down the slopes.

People live near volcanoes because the soils can be rich, the views are dramatic, and eruptions may be centuries apart. That calm is misleading. Pompeii, Krakatoa, and Mount St. Helens showed how sudden the change can be. Ash wrecks engines and roofs. Gas can kill in low valleys. Mudflows of volcanic debris, lahars, race down riverbeds when heat melts ice or rain remobilizes ash.

Volcanoes also make land. The ocean floor is largely volcanic. Island arcs and plateaus exist because lava piled up. Climate has felt their breath: a large sulfur-rich eruption can cool the surface for a year or two by shading sunlight. The mountain that looks asleep is a valve on the planet’s heat, sometimes dripping, sometimes shouting.
