Volcano

A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot, molten rock, ash and gases to escape from below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time.

Volcanoes are generally found where tectonic plates are pulled apart or come together. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by "divergent tectonic plates" pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by "convergent tectonic plates" coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the Wells Gray-Clearwater Volcanic Field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes.

Volcanoes can be caused by "mantle plumes". These so-called "hotspots" , for example at Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons.
Volcanoes can be classified by their eruptive habits and are generally arranged by progressive increases in the explosiveness of their eruptions. The type of volcanic eruption also plays an important role in the evolution of a volcanic landform, thus forming a significant link between eruptive habit and volcanic structure.

Less explosive eruptions involve the effusive outpouring of basaltic magma that is relatively low in viscosity and in gas content. (The term lava is applied to molten magma after it has erupted to the surface.) More explosive eruptions generally involve magma that is more viscous and has a higher gas content. Such magma is shattered into pyroclastic fragments by explosive boiling during an eruption.

Types of eruptions
In classification schemes based on character of eruption, volcanic activity and volcanic areas are commonly divided into six major types in order of increasing degree of explosiveness: (1) Icelandic, (2) Hawaiian, (3) Strombolian, (4) Vulcanian, (5) Pelean, and (6) Plinian.

The Icelandic type is characterized by effusions of molten basaltic lava that flow from long parallel fissures. Such outpourings often build lava plateaus.

The Hawaiian type is similar to the Icelandic variety. In this case, however, fluid lava flows from summit and radial fissures to form shield volcanoes.

The Strombolian type involves moderate bursts of expanding gases that eject clots of incandescent lava in cyclical or nearly continuous small eruptions. Because of such small intermittent outbursts, Stromboli Volcano (off the west coast of Italy) has been called the "lighthouse of the Mediterranean."

The Vulcanian type, named for the island of Vulcano that neighbours Stromboli, generally involves moderate explosions of gas laden with volcanic ash (i.e., fine volcanic particles). This mixture forms dark, turbulent eruption clouds that rapidly ascend and expand in convoluted shapes.

The Pelean type is associated with explosive outbursts that generate dense mixtures of hot volcanic fragments and gas. It is named for the destructive eruption of Mt. Pelee in Martinique in 1902. The fluidized slurries produced by Pelean-type eruptions are heavier than air but are of low viscosity and pour down valleys and slopes at velocities exceeding 100 kilometres per hour (60 miles per hour). Variously called nuées ardentes, glowing avalanches, ash flows, or pyroclastic flows, such hot fluidized flows of volcanic gases and fragments are extremely destructive.

The Plinian type is an intensely violent kind of volcanic eruption exemplified by the outburst of Mt. Vesuvius in AD 79 that killed the famous Roman scholar Pliny the Elder, from whom the name is taken. In this type, gases boiling out of gas-rich magma generate enormous, nearly continuous jetting blasts that rip apart and core out the magma column. The uprushing gases and volcanic fragments appear like a gigantic rocket blast directed vertically upward. Plinian eruption clouds can rise into the stratosphere and are sometimes sustained for several hours.