A volcanoe is any vent in the crust of the Earth or other planet or satellite (e.g., Jupiter's Io), from which molten rock, pyroclastic debris, and steam issue.
Volcanoes are commonly divided into two broad types: fissure and central. Each type is associated with a different mode of eruption and surface structure.
Fissure volcanoes are much more common than those of the central type. They occur along fractures in the crust and may extend for many kilometres. Lava, usually of basaltic composition, is ejected relatively quietly and continuously from the fissures and forms enormous plains or plateaus of volcanic rock. Submarine fissure eruptions are common along the crests of mid-ocean ridges and are pivotal in seafloor spreading. When molten rock is extruded under water, pillow lava--piles of sack-shaped rock masses measuring up to several metres in diameter--are often formed.
Central volcanoes have a single vertical lava pipe and tend to develop a conical profile. The volcanic cone is generally built up of a succession of lavas, ignimbrites, and welded tuffs (porous rock formed by the cementation of solidified volcanic ash and dust particles). Lava flows from the throat of a central volcano following the easiest path downhill, its flow pattern strongly influenced by the topography.
The shape of any given volcanic landform depends on a variety of local circumstances and on the relative abundances of lavas, tuffs, and ignimbrites. This in turn depends on the composition of the magma arriving at the surface. The lower the viscosity, the more readily the lava flows away from the throat or fissure. There is, as a consequence, relatively little tendency to build up a steep-sided cone. The more viscous the magma, however, the greater the tendency to chill and solidify close to the source and to form a cone. In many cases, highly viscous lava also may clog the throat of the volcano, causing a pressure buildup that can only be relieved by violent explosions and nues ardentes. Such eruptions, exemplified by those of Mount St. Helens in southwestern Washington state during the early 1980s and Vesuvius in AD 79, may completely remove the top of a volcanic cone and occasionally part of the interior of the cone as well. The resultant roughly circular hollow is called a caldera. Further eruption may lead to the formation of a lava lake within the cone, and if the lava cools and solidifies, the inward drainage of rainwater may produce a water lake on the surface of the lava lake. A caldera may also form without an explosion by the collapse of the top of the cone into an underlying accumulation of magma. Kilauea on southeastern Hawaii Island is an excellent example of a large volcanic cone with a well-developed caldera produced by collapse.
Excerpt from the Encyclopedia Britannica without permission.