VOLCANISM.docx

\VOLCANISM * The processes by which magma and gases are transferred from the Earth’s interior to near or on the surface. * 2 basic types: extrusive vs intrusive volcanism Extrusive: when magma (lava) flows out onto the Earth’s surface and then cools to form igneous rock * volcano: a landform with a vent, or fissure(crack) on the surface which is the end of a conduit originating from below the crust in the Asthenosphere * often a mountain landform (fissures usually do not form features of high relief) * magma originating in theAsthenosphere is expelled through the conduit & out the vent * magma often collects in a magma chamber below the volcano before being expelled * crater: the surface depression at the summit of the volcano * a volcano may be categorized as Active: currently erupting, or has erupted during recorded history Dormant: an active volcano that is in repose but is expected to erupt in the future Extinct: a volcano that is not expected to erupt again * 75% of volcanoes are on the sea floor * most convergent and divergent plate boundaries have high volcanic activity * Hot spots are also areas of high volcanic and earthquake activity * lava: the molten rock issued from a volcano; the term used for magma once it has erupted onto the Earth’s surface * may be two types of material: mafic or felsic * mafic or basaltic lava: high in Mg & Fe, < 50% silica thus less viscous which means it flows readily, and thus less gas is trapped within leading to a less explosive eruption * felsic: richer in silica thus more viscous meaning it flows slower: it is also more acidic, and more gases are trapped leading to more explosive eruptions * pyroclastics or tephra: fragments expelled explosively from a volcano, quite common with felsic magma explosive eruptions) * bombs, pumice, scoria, cinders, lapilli, ash, & dust (Based on size; bombs largest, dust/ash smallest.) ** 2 Types of Eruptions Effusive: * relatively gentle, non-violent eruptions because gases readily escape, thus less explosive * the material issued is primarily mafic magma * if eruption is through a vent, it may form a shield volcano EX: islands of Hawai’i, the Galápagos Islands * if eruption is through a fissure, then it forms a plateau basalt or flood basalt EX: Columbia Plateau in the state of Washington (on land), from mid-oceanic ridges under the water to form sea floor Explosive: * relatively violent eruptions because conduit often gets plugged with cooled/solidified magma, and thus a greater amount of gases are trapped in the magma, increasing pressure * most commonly found along subduction zones * the material issued is primarily felsic magma with a much greater content of pyroclastics * if the eruption is through a vent, this often forms a composite volcano or stratovolcano EX: Mt. St. Helens, Mt. Ranier, Mt. Pinatubo, Mt. Vesuvius ** Other events that may accompany an explosive eruption: * lahar: a flood of ash, mud & water created by melting of snowcap during an eruption or volcanic event EX: one created by the 1985 eruption of Nevado del Ruiz in Colombia killed ~ 23,000 of 25,000 people in the town ofArmero, by covering the town in over 20 ft of mud * nuée ardentes or pyroclatic flow (glowing cloud): a cloud of hot volcanic gas & ash that moves down the flanks of the volcano EX: a flow created during the 1902 eruption of Mont Pelée on the island of Martinique, West Indies annihilated the town of St. Pierre, killing over 30,000 people ** Phreatic eruptions: