Lava gushes from a spatter cone at Kilauea, Hawaii, Volcanic eruptions are partly driven by pressure from dissolved gas, much as escaping gases force the cork out of a bottle of champagne. As the magma rises, pressure decreases, allowing the gases to form bubbles.
Volcano , vent in the crust of the Earth or another planet or satellite, from which issue eruptions of molten rock, hot rock fragments, and hot gases. Yet while eruptions are spectacular to watch, they can cause disastrous loss of life and property, especially in densely populated regions of the world. Sometimes beginning with an accumulation of gas-rich magma molten underground rock in reservoirs near the surface of the Earth, they can be preceded by emissions of steam and gas from small vents in the ground. Swarms of small earthquakes , which may be caused by a rising plug of dense, viscous magma oscillating against a sheath of more-permeable magma, may also signal volcanic eruptions, especially explosive ones. In some cases, magma rises in conduits to the surface as a thin and fluid lava, either flowing out continuously or shooting straight up in glowing fountains or curtains. In other cases, entrapped gases tear the magma into shreds and hurl viscous clots of lava into the air. In more violent eruptions, the magma conduit is cored out by an explosive blast, and solid fragments are ejected in a great cloud of ash-laden gas that rises tens of thousands of metres into the air.
A volcano is a rupture in the crust of a planetary-mass object , such as Earth , that allows hot lava , volcanic ash , and gases to escape from a magma chamber below the surface. Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. For example, a mid-oceanic ridge , such as the Mid-Atlantic Ridge , has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can also form where there is stretching and thinning of the crust's plates, e. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism. Volcanoes are usually not created where two tectonic plates slide past one another. Large eruptions can affect ambient temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's troposphere ; historically, large volcanic eruptions have been followed by volcanic winters which have caused catastrophic famines. The word volcano is derived from the name of Vulcano , a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan , the god of fire in Roman mythology. At the mid-oceanic ridges , two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Because the crust is very thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion without transfer of heat or matter and the partial melting of the mantle , causing volcanism and creating new oceanic crust.
Science Explorer. Multimedia Gallery. Park Passes. Technical Announcements. Employees in the News. Emergency Management. Survey Manual. There are about 1, potentially active volcanoes worldwide, aside from the continuous belts of volcanoes on the ocean floor at spreading centers like the Mid-Atlantic Ridge. Many of those are located along the Pacific Rim in what is known as the " Ring of Fire. There are potentially active volcanoes in the United States.