VOLCANOES ARE VENTS OR FISSURES in the Earth’s crust through which magma (molten rock that originates from deep beneath the crust) is forced on to the surface as lava. They occur most commonly along the boundaries of crustal plates; most volcanoes lie in a belt called the “Ring of Fire,” which runs along the edge of the Pacific Ocean. Volcanoes can be classified according to the violence and frequency of their eruptions. Nonexplosive volcanic eruptions generally occur where crustal plates pull apart. These eruptions produce runny basaltic lava that spreads quickly over a wide area to form relatively flat cones. The most violent eruptions take place where plates collide. Such eruptions produce thick rhyolitic lava and may also blast out clouds of dust and pyroclasts (lava
fragments). The lava does not flow far before cooling and therefore builds up steep-sided, conical volcanoes. Some volcanoes produce lava and ash eruptions, which build up composite volcanic cones. Volcanoes that erupt frequently are described as active; those that erupt rarely are termed dormant; and those that have stopped erupting altogether are termed extinct. As well as the volcanoes themselves, other features associated with volcanic regions include geysers, hot mineral springs, solfataras, fumaroles, and bubbling mud pools.
