VOLCANISM

VolcanismVolcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface.

TYPES OF VOLCANOES

COMPOSITE VOLCANOES Composite volcanoes are constructed from multiple eruptions,

sometimes recurring over hundreds of thousands of years, sometimes over a few hundred. Andesite magma, the most common but not the only magma type forming composite cones, produces lava more brittle than basaltic lava because of its higher viscosity. Although andesitic composite cones are constructed dominantly of fragmental debris, some of the magma intrudes the cones as dike or sills. In this way, multiple intrusive events build a structural framework that knits together the voluminous accumulation of volcanic rubble, which can stand higher than cones composed solely of fragmental material. Composite cones can grow to such heights that their slopes become unstable and susceptible to collapse from the pull of gravity. Famous examples of composite cones are Mayon Volcano Philippines, Mount Fuji in Japan, and Mount Rainier, Washington, U.S.A. Some composite volcanoes attain two to three thousandmeters in height above their bases. Most composite volcanoes occur in chains and are separated by several tens of kilometers. There are numerous composite volcano chains on earth, notably around the Pacific rim, known as the "Rim of Fire".

Shield volcanoes are large volcanoes that are built almost entirely of fluid lava flows. It has broad sloping sides and is usually surrounded by gently sloping hills in a circular or fan shaped pattern, that looks like a warrior's shield.

The volcano is produced by the action of the gas (steam or water vapor) with heat from the earth's core. This action melts rock turning it into magma. The pressure from the heat of the gas pushes the magma upwards till it explodes. Molten magma shoots upward from deep below the ocean floor and breaks through the drifting plates to form shield volcanoes. Lava flows gently and continuously out of the central volcanic vent or group of vents. This lava is very runny, and can't be piled up into steep mounds. It gradually accumulates and cools around the volcano. The eruptions are characterized by low explosivity lava-fountaining that forms cinder cones and spatter cones at the vent. The volcanoes are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava. The lava spread widely over great distances, then cools as thin gently dipping sheets. Lavas also erupt from vents along fractures (rift zones) that form on the flanks of the cone. Some of the largest volcanoes in the world are Shield volcanoes.

Shield Volcanoes

Cinder Cone VolcanoesCinder cone volcanoes are the most

common kind of volcanoes. They are steep sided cones of basaltic fragments and are smaller and simpler than composite volcanoes. Streaming gases carry liquid lava blobs into the atmosphere that fall back to earth around a single vent to form the cone. The volcano forms when ash, cinders and bombs pile up around the vent to form a circular or oval cone.

Spatter ConesA spatter cone is formed of molten lava ejected from a vent

somewhat like taffy. Expanding gases in the lava fountains tear the liquid rock into irregular gobs that fall back to earth, forming a heap around the vent. The still partly liquid rock splashes down and over the sides of the developing mound is called spatter. Because spatter is not fully solid when it lands, the individual deposits are very irregular in shape and weld together as they cool, and in this way particularly differ from cinder and ash. Spatter cones are typical of volcanoes with highly fluid magma, such as those found in the Hawaiian Islands. The spatter that builds up the cone can either be agglutinated or welded, the former meaning that the individual spatters pose one above each other with a lesser degree of welding occurring, while welded spatter is almost fluid when it lands and therefore welds easily.

Complex VolcanoesComplex volcanoes are mixed landforms. In

most cases, they occur because of changes either in eruptive habit or in location of the principal vent area. A stratovolcano may form a large explosion crater that later becomes filled by a lava dome, or several new cones and craters may develop on a caldera's rim. One stratovolcano cone may overlap another and have multiple summits.

Cinder Cones Spatter Cones

Complex Volcano

Kinds of volcanic eruption

Strombolian EruptionStrombolian eruptions are often associated with small lava lakes,

which can build up in the conduits of volcanoes. They are one of the least violent of the explosive eruptions, although they can still be very dangerous if bombs or lava flows reach inhabited areas. Strombolian eruptions are named for the volcano that makes up the Italian island of Stromboli, which has several erupting summit vents. These eruptions are particularly spectacular at night, when the lava glows brightly. 

Vulcanian EruptionA Vulcanian eruption is a short, violent, relatively small

explosion of viscous magma (usually andesite, dacite, or rhyolite). This type of eruption results from the fragmentation and explosion of a plug of lava in a volcanic conduit, or from the rupture of a lava dome (viscous lava that piles up over a vent). Vulcanian eruptions create powerful explosions in which material can travel faster than 350 meters per second (800 mph) and rise several kilometers into the air. They produce tephra, ash clouds, and pyroclastic density currents (clouds of hot ash, gas and rock that flow almost like fluids). 

Plinian EruptionThe largest and most violent of all the types of volcanic

eruptions are Plinian eruptions. They are caused by the fragmentation of gassy magma, and are usually associated with very viscous magmas (dacite and rhyolite). They release enormous amounts of energy and create eruption columns of gas and ash that can rise up to 50 km (35 miles) high at speeds of hundreds of meters per second. Ash from an eruption column can drift or be blown hundreds or thousands of miles away from the volcano. The eruption columns are usually shaped like a mushroom (similar to a nuclear explosion) or an Italian pine tree; Pliny the Younger, a Roman historian, made the comparison while viewing the 79 AD eruption of Mount Vesuvius, and Plinian eruptions are named for him.

Lava DomesLava domes form when very viscous, rubbly lava (usually

andesite, dacite or rhyolite) is squeezed out of a vent without exploding. The lava piles up into a dome, which may grow by inflating from the inside or by squeezing out lobes of lava (something like toothpaste coming out of a tube). These lava lobes can be short and blobby, long and thin, or even form spikes that rise tens of meters into the air before they fall over. Lava domes may be rounded, pancake-shaped, or irregular piles of rock, depending on the type of lava they form from. 

Surtseyan EruptionSurtseyan eruptions are a kind of hydromagmatic eruption,

where magma or lava interacts explosively with water. In most cases, Surtseyan eruptions occur when an undersea volcano has finally grown large enough to break the water's surface; because water expands when it turns to steam, water that comes into contact with hot lava explodes and creates plumes of ash, steam and scoria. Lavas created by a Surtseyan eruption tend to be basalt, since most oceanic volcanoes are basaltic. 

Products of Volcanic Eruptions

Sources:http://geology.com/volcanoes/types-of-volcanic-eruptions

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www.britannica.com/EBchecked/topic/130036/complex-volcano

http://www.k12.hi.us/~kapunaha/student_projects/volc_blowout/cinder_cone_volcano.htm