Volcano

A volcano is an opening in Earth's surface through which molten rock, hot gases, and rocks are ejected. Volcanoes create new land and islands. They can also produce economically important mineral deposits, fertile soils, and beautiful landscapes. However, volcanoes can also destroy lives and property. Therefore, they constitute significant geologic hazards in many parts of the world.

With regard to the hazard that they present, volcanoes can be classified as active, dormant, or extinct. Active volcanoes are those that have erupted within recorded history. Dormant volcanoes are those that have not erupted during recorded history but may erupt again, whereas extinct volcanoes are those for which there is little or no chance of future eruptions.

Controls on the explosivity of volcanic eruptions

Generally, the viscosity of a magma controls the type and violence of eruptions. Viscosity is the resistance of a fluid to flow. The more viscous the magma, the more explosive its eruption is likely to be. Very viscous magmas tend to resist eruption, and so gas pressure builds within the magma pipe leading to the volcanic vent. By the time sufficient pressure builds to displace a viscous magma, the force released by the eruption will be much greater than for a fluid magma. This leads to explosive eruptions. The most important controls on viscosity are the silica content of the magma and its temperature. Basaltic (mafic) lavas are very fluid due to their low silica content. Conversely, rhyolitic lavas are very viscous due to their high silica content. Magma and lava viscosity is also a function of temperature; therefore, a the viscosity of a lava flow will increase as its temperature decreases.

Volatile substances are elements or compounds—hydrogen sulfide, water, carbon dioxide, radon, and other gasses—that escape during eruptions. The Latin root for volatile means "winged." Volatile compounds in magma can cause violent explosive eruptions.

Calderas

The most violent large volcanic eruption is the collapse of a composite volcano. This normally happens on the active margins of tectonic plates, that is, at subduction zones or along a continental rift valley (where a continent is breaking apart). The process is part of the evolution of a composite volcano, which starts with a reservoir of molten rock, several miles wide and under high pressure. This magma rises in the earth's crust and forces its way to the surface. A composite volcano is born in clouds of ash, supersonic steam explosions filling the air with hot rock, ash, and various gases.

After a series of eruptions, perhaps over millions of years, the volcano forms a mountain of lava and pyroclastic material as much as 2-3 mi (3-4 km) high. Eventually, there is one last eruption of ash and pyroclastic flows. The magma begins to boil, gas bubbles expand the magma to many times its original volume, and it explodes upward. The magma chamber rapidly empties its contents onto the landscape above and the volcano collapses into the void, forming a depression known as a caldera.