Volcanoes are mountains but they are very
different from other mountains; they are not formed by folding and
crumpling or by uplift and erosion. Instead, volcanoes are built by the
accumulation of their own eruptive products -- lava, bombs (crusted over
ash flows, and tephra (airborne ash and dust). A volcano is most
commonly a conical hill or mountain built around a vent that connects
with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the
molten rock and associated gases are expelled.
Driven by
buoyancy and gas pressure the molten rock, which is lighter than the
surrounding solid rock forces its way upward and may ultimately break
though zones of weaknesses in the Earth's crust. If so, an eruption
begins, and the molten rock may pour from the vent as non-explosive lava
flows, or if may shoot violently into the air as dense clouds of lava
fragments. Larger fragments fall back around the vent, and accumulations
of fall-back fragments may move downslope as ash flows under the force
of gravity. Some of the finer ejected materiaIs may be carried by the
wind only to fall to the ground many miles away. The finest ash
particles may be injected miles into the atmosphere and carried many
times around the world by stratospheric winds before settling out.
