Volcanoes and Volcanism
Volcanoes and Volcanism GEOL 101 001
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GEOL 101 001
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This 3 page Class Notes was uploaded by Madeline Wilson on Sunday March 20, 2016. The Class Notes belongs to GEOL 101 001 at University of South Carolina taught by Dr. Knapp in Spring 2016. Since its upload, it has received 30 views. For similar materials see Introduction to the Earth in Geology at University of South Carolina.
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Date Created: 03/20/16
Volcanoes and Volcanism Molten rock beneath the Earth’s surface is called magma Volcanism Process by which magma (usually from the mantle) rises through the crust, emerges onto the surface as lava, and cools into hard volcanic rock forming a volano Types of Lavas Basaltic Lavas: low-viscosity mafic lavas, typically erupted at 1000 degrees C; cool to form basalt Rhyolitic Lavas: high-viscosity felsic lavas, typically erupted at 8000 degrees to 12000 degrees C; cool to form rhyolite Andesitic Lavas: intermediate in composition and viscosity between mafic and felsic magmas; cool to form andesite Types of Basalts Flood Basalts: thick, widespread accumulations of basalt, typically fed by fissures Pahoehoe: a very low viscosity basaltic lava characterized by ropy texture Aa: a realtively low viscosity basaltic lava characterized by a jagged, blocky texture Pillow Basalts: a basaltic lava extruded beneath the water, with a glassy texture Vesicular Basalts: lava is typically charged with gas Basaltic Lavas Gentle sides: ~2-10 degrees; large, up to 120 km wide. Long duration of activity: 10,000’s of years; non-violent. Magma can erupt on the flanks of a volcano as well as from the central vent. Viscous felsic lavas; Steep-sided and small: ~100’s m wide; Grow slowly, and pile up over the vent in a long time. Basaltic pyroclastic material; Steep sides: ~30 deg; Relatively small: ~ 1km wide; Short-lived - typically a single event. Alternating pyroclastic deposits and andesitic lava flows; Slopes are intermediate in steepness; Relatively large: ~10-15 km wide; Intermittent eruptions over long time span: 1,000’s of yrs; Eruptions often highly explosive Caldera: large depression (~ several km wide) formed by collapse of a volcano into a partially drained magma chamber. Crater: at the summits of most volcanoes. After an eruption, lava often sinks back into the vent and solidifies, to be blasted out by a later eruption. Pyroclastic Deposits (Arenal Volcano, Costa Rica) Fragmentary volcanic rocks ejected high into the air during violent eruptions o Volcanic ash o Volcanic tuffs o Volcanic breccias o Volcanic bombs Pyroclastic Flows A density flow consisting of a hot (up to 800oC), poisonous mixture of gas and pyroclastic material moving downslope at speed in excess of 200 km/hr! Phreatic Explosion caused by magma mixing with water (eruption of superheated steam) Fissure Eruptions A volcanic eruption originating along an elongate fissure rather than a central vent Volcanoes along the Laki Fissure (Iceland) formed in 1783, resulting in the largest lava flow in recorded history Lahars (volcanic Mudflows) Formed by the sudden mixing of large volumes of pyroclasts with water (e.g. heavy rain, draining of crater lake, melting of glacier) Lahars can move 100+ km/hr, can cover large areas, and can kill large numbers of people Geosystem interactions Volcanism and the hydrosphere o fumaroles and geysers Volcanism and the atmosphere o aerosols and ash Hydrothermal Activity The circulation of water through hot volcanic rocks and magma: o Provides the source for Geothermal Energy o Forms fumaroles – volcanic vents emitting gases, some charged with dissolved minerals o Forms geysers – heated hydrothermal waters under pressure, that intermittently erupt Mantle Plumes: originate at the core-mantle boundary Volcanic Hazards More than 600 active volcanoes ~ 10% of world’s population lives near active volcano Life threatening situation Water and air pollution Forest fire Global climate change Mitigation o Avoid area… zoning o Structural/strategic defense Continental arcs will produce the most felsic lava
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