all about volcanoes - GENE RIC FEATURES A volcanic vent is...

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GENE RIC FEATURES A volcanic vent is an opening exposed on the earth's surface where volcanic material is emitted. All volcanoes contain a central vent underlying the summit crater of the volcano. The volcano's cone-shaped structure, or edifice , is built by the more-or-less symmetrical accumulation of lava and/or pyroclastic material around this central vent system. The central vent is connected at depth to a magma chamber , which is the main storage area for the eruptive material. Because volcano flanks are inherently unstable, they often contain fractures that descend downward toward the central vent, or toward a shallow-level magma chamber. Such fractures may occasionally tap the magma source and act as conduits for flank eruptions along the sides of the volcanic edifice. These eruptions can generate cone-shaped accumulations of volcanic material, called parasitic cones . Fractures can also act as conduits for escaping volcanic gases , which are released at the surface through vent openings called fumaroles . Summit Crater Parasitic Cones Fumarole MAIN VOLCANO TYPES Although every volcano has a unique eruptive history, most can be grouped into three main types based largely on their eruptive patterns and their general forms. The form and composition of the three main volcano types are summarized here: VOLCANO TYPE VOLCANO SHAPE COMPOSITION ERUPTION TYPE SCORIA CONE Straight sides with steep slopes; large summit crater Basalt tephra; occasionally andesitic Strombolian SHIELD VOLCANO Very gentle slopes; convex upward Basalt lava flows Hawaiian
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STRATOVOLCANO Gentle lower slopes, but steep upper slopes; concave upward; small summit crater Highly variable; alternating basaltic to rhyolitic lavas and tephra with an overall andesite composition Plinian SUBORDINATE VOLCANO TYPES -- Lava and tephra can erupt from vents other than these three main volcano types. A fissure eruption , for example, can generate huge volumes of basalt lava; however, this type of eruption is not associated with the construction of a volcanic edifice around a single central vent system. Although point-source eruptions can generate such features as spatter cones and hornitos , these volcanic edifices are typically small, localized, and/or associated with rootless eruptions (i.e., eruptions above the surface of an active lavaflow, unconnected to an overlying magma chamber) . Vent types related to hydrovolcanic processes generate unique volcanic structures, discussed separately under hydrovolcanic eruptions . For a description of each of the main volcano types , see: SCORIA CONES SHIELD VOLCANOES STRATOVOLCANOES WHEN IS A VOLCANO CONSIDERED ACTIVE, DORMANT, OR EXTINCT? Classifying a volcano as active, dormant, or extinct is a subjective and inexact exercise. A volcano is generally considered active if it has erupted in historic time. This definition, however, is rather ambiguous, because recorded history varies from thousands of years in Europe and the Middle East, to only a few
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This note was uploaded on 10/23/2010 for the course BIOL 2308 taught by Professor Boxer during the Spring '10 term at University of Houston - Downtown.

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all about volcanoes - GENE RIC FEATURES A volcanic vent is...

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