volcano brief

8/6/2019 Volcano Brief

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VolcanoFrom Wikipedia, the free encyclopediaThis article is about the geological feature. For other uses, see Volcano (disambiguation) .

Cleveland Volcano in the Aleutian Islands of Alaskaphotographed from the International Space Station , May2006

Cross-section through a stratovolcano (vertical scale is exaggerated):1. Large magma chamber 2. Bedrock 3. Conduit (pipe)4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank

9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater 15. Ash cloud


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Pinatubo ash plume reaching a height of 19 km, 3 days before the climactic eruption of 15 June 1991

A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma ,volcanicash and gases to escape from below the surface.

Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge,for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonicplates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonicplates coming together. By contrast, volcanoes are usually not created where two tectonic plates slidepast one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust inthe interiors of plates, e.g., in the East African Rift, the Wells Gray-Clearwater volcanic field andthe Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "Platehypothesis" volcanism.[1]

Intraplate volcanism has also been postulated to be caused by mantle plumes . These so-called"hotspots ", for example Hawaii, are postulated to arise from upwelling diapirs from the core-mantle

boundary, 3,000 km deep in the Earth.EtymologyThe word v olcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italywhose name in turn originates from Vulcan, the name of a god of fire in Roman mythology.[2]The studyof volcanoes is called volcanology, sometimes spelled v ulcanology .

Plate tectonics

Map showing the divergent plate boundaries (OSR Oceanic Spreading Ridges) and recent sub aerialvolcanoes.

Divergent plate boundar iesM ain article: Di v ergent boundary

At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is beingformed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridgesdue to the pull of the tectonic plates. The release of pressure due to the thinning of the crust leadsto adiabatic expansion, and the partial melting of the mantle causing volcanism and creating newoceanic crust. Most divergent plate boundaries are at the bottom of the oceans, therefore mostvolcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example


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of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands areformed, for example, Iceland.

Mount Rinjani eruption in 1994, inLombok, Indonesia

Lava enters the Pacific at the Big Island of Hawaii

C onvergent plate boundar iesM ain article: Con v ergent boundary

Subduction zones are places where two plates, usually an oceanic plate and a continental plate,collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming adeep ocean trench just offshore. Water released from the subducting plate lowers the meltingtemperature of the overlying mantle wedge, creating magma. This magma tends to bevery viscous due to its high silica content, so often does not reach the surface and cools at depth.When it does reach the surface, a volcano is formed. Typical examples for this kind of volcanoare Mount Etna and the volcanoes in the Pacific Ring of Fire.

"H otspots " M ain article: Hotspot (geology)

"Hotspots" is the name given to volcanic provinces postulated to be formed by mantle plumes . Theseare postulated to comprise columns of hot material that rise from the core-mantle boundary. They aresuggested to be hot, causing large-volume melting, and to be fixed in space. Because the tectonicplates move across them, each volcano becomes dormant aftera while and a new volcano is thenformed as the plate shifts over the postulated plume. The Hawaiian Islands have been suggested tohave been formed in such a manner, as well as the Snake River Plain , with the YellowstoneCaldera being the part of the North American plate currently above the hot spot. This theory iscurrently under criticism, however. [1]


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Volcanic features

Conical Mount Fujiin Japan , at sunrise from Lake Kawaguchi (2005)

The most common perception of a volcano is of a conical mountain, spewing lava andpoisonous gases from acrater at its summit. This describes just one of many types of volcano, and thefeatures of volcanoes are much more complicated. The structure and behavior of volcanoes dependson a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than asummit crater, whereas others present landscape features such as massive plateaus . Vents that issuevolcanic material (lava, which is what magma is called once it has escaped to the surface, and ash )and gases (mainly steam and magmatic gases ) can be located anywhere on the landform. Many of these vents give rise to smaller cones such as Pu u on a flank of Hawaii's K lauea.

Lakagigar fissure vent in Iceland, source of the major world climate alteration of 178384.

Skjaldbreiur , a shield volcano whose name means "broad shield"

January 2009 image of the rhyolitic lava dome of Chaitn Volcano , southern Chile during its 20082009 eruption


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