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Volcanoes & Tectonics Seamounts and Guyots Lava Viscosity (silica, Temp.) & explosiveness • Basaltic lava - less silica and higher temperature, low viscosity, gasses escape at surface low pressures, not explosive e.g. Shield volcanoes, Rifts, MOR, cinder cones • Intermediate silica, explosive, composite = stratovolcanoes, Andes, Cascades, Aleutians Japan, Philippines, often pyroclastic clouds = nuée ardente • Rhyolitic lava - more silica and lower temperatures, high viscosity, gasses cannot escape, explosive. Plate Tectonics and Volcanism Volcanism on a tectonic plate moving over a hot spot Pacific Basin 90%Volcanic Output Seamounts and MOR => ocean floor Great Basalt Plateaus Volcanic geomorphic provinces Plumes under Continents yield more silica rich, explosive eruptions A size comparison of the three main types of volcanoes Seamounts are these largest Plume below ocean Above subduction zone Divergent Zone Cinder Cones Volcanic Activities and their products Upper surface pitted Scoria • Exhalative (gas: H2O, CO2, SO2) • Effusive (lava) • Explosive (tephra) Jagged A'a' away from vent Pahoehoe Near vent Hawaii, low silica, low viscosity, notice no tephra in view http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html Basaltic Pillow Lavas The reason there is so much water carried down into a subduction zone Volcanic Activities and their products • Explosive (tephra) nuée ardente = pyroclastic cloud Intermediate to high silica - Andesitic to Rhyolitic Tephra • Coarse = close • Thick = close Lahars - volcanic mudflows Melting collapse of ice embedded cone Copyright © Stephane Veyrat-Charvillon 2002 Characteristic Eruptive Types Special case: Phreatic eruptions occur when magma hits water. And the water flashes to steam explosively Rift Valley Flows Volcanic Landforms 1: Basaltic Basaltic lava flows • Layered – Lava/sediment • Jointed – Columnar Copyright © Richard Kesel 2002 • Columbia Plateau • Deccan Traps • Very large in scale, worldwide effects Basalt Plateaus - Flood Basalts Basaltic Cinder and Spatter Cones • Projectiles – Ballistic – Wind-borne • Divergent Margin + Volcanic Landforms, Basaltic continued Shield Volcanoes Mauna Kea from Mauna Loa •Form above Plumes, huge • also Basaltic, gentle eruptions •As plate moves old volcanoes become inactive, new volcano forms over the plume Volcanic Landforms 2: Andesitic Volcanoes Andesitic - Composite cones "Stratovolcano" Mt. Fuji in summer Form above Subduction zones The Cascades, Washington State Composite cones - section Lava and tephra alternate Andesitic Eruptions Extensive Welded Tuff, Pumice Lavas, and Lahars Cleveland Volcano, Aleutians Plug Domes Rhyolite Eruptions • Rare, extremely explosive • Size of ash cover = violence • Toba and our near-extinction Volcanism on a tectonic plate moving over a hot spot Calderas Caldera – Crater Lake Copyright © Ann Dittmer 2002 Caldera topography – Crater Lake Maars A maar is a broad, low-relief volcanic crater that is caused by a phreatomagmatic eruption, an explosion caused by groundwater coming into contact with hot lava or magma Ukinrek Maars Mono Co, CA Rhyolite, very high silica, very viscous, can’t get rid of gas bubbles, explodes Panum Crater is a good example of both a tuff ring and a Rhyolite dome Rising magma hit groundwater, explosion left crater, then fountain of hot cinders made ring, then remaining magma rose as dome Iceland • • • • • • Is on the mid-Atlantic Ridge, 13 volcanoes have erupted since AD 874. most active & volatile is Grímsvötn. Over the past 500 years, Iceland's volcanoes have erupted a third of the total global lava output. One quarter of Iceland ‘s population died due to eruption of the craters of Laki in 1783-84. The craters are a part of a larger volcanic system with the subglacial Grímsvötn as a central volcano. Because most of Grímsvötn’s eruptions have been subglacial, the interaction of magma and meltwater from the ice causes phreatomagmatic explosions Tephra forms Tuff Ignimbrite or Welded Tuff Source: Gerald & Buff Corsi/Visuals Unlimited, Inc. Erosional Remnants • Plugs • Dikes Ship Rock, NM Intrusive terrains - Laccoliths First studied in 1875-1876 by Grove Karl Gilbert. The term laccolith) is used to describe the characteristic shapes of some of the igneous intrusions that core the mountains. • Laccoliths - Henry Mts. • Colorado Plateau Intrusive terrains - Sierra Nevada Batholith
