Most of Earth's volcanoes erupt unseen, on the ocean floor
Picture a volcano and you probably imagine a smoking cone. In fact most volcanic activity happens underwater along mid-ocean ridges, quietly building new seafloor. The biggest known volcano is not even on Earth: Olympus Mons on Mars holds that title, and some moons erupt ice instead of molten rock.
The word comes from Vulcano, an island in Italy's Aeolian chain, named after Vulcan, the Roman god of fire, and entered English in the early 17th century. A volcano needs three things: a reservoir of magma, a conduit through the crust and a vent. On Earth they cluster at plate boundaries. Earth's rigid shell is broken into sixteen large plates and several smaller ones, creeping as the mantle below convects, and because most boundaries lie beneath the sea, so do most volcanoes.
Setting governs behaviour. Where plates pull apart, rising mantle rock melts as pressure falls, producing gentle eruptions of runny basalt; Iceland is a ridge that pokes above the waves. Where an ocean plate dives under a continent, water released from the sinking slab lowers the melting point of the mantle above, a process called flux melting. The resulting magma is rich in silica and sticky, so these arc volcanoes, like those of the Pacific Ring of Fire, tend to be violently explosive. Volcanoes seldom form where plates simply slide past each other.
Away from boundaries, hotspots are thought to be fed by mantle plumes rising from the core-mantle boundary about 3,000 kilometres down, though the idea has been questioned. As a plate drifts over a plume, each volcano goes quiet and a new one forms, building chains like Hawaii; Yellowstone sits over such a hotspot now. Continental rifts such as East Africa's can also erupt unusual lavas, including carbonatites.
Shapes vary. Broad shield volcanoes grow from fluid lava flowing long distances, as in Hawaii and Iceland, while steep lava domes rise from very viscous lava. Craters may hold lakes, like Lake Taupō in New Zealand. Volcanoes are called active, dormant if they have not erupted in the roughly 12,000 years of the Holocene, or extinct, though the categories overlap. Large eruptions can cool the lower atmosphere with ash and sulfuric acid droplets, and past volcanic winters brought devastating famines. A 2009 paper proposed widening the definition to cover cryovolcanoes on icy moons.
Source: Volcano
