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Transcript
Dr. James Wittke [email protected] Explosiveness mainly controlled by magma viscosity Viscosity measures a material’s resistance to flow ◦ Low viscosity magma flows easily ◦ High viscosity magma is very “sticky” Magma viscosity controlled by… ◦ Composition (mainly SiO2 content) ◦ Temperature ◦ Dissolved gases Composition ◦ Higher silica higher viscosity ◦ Lower silica lower viscosity Temperature ◦ Cooler higher viscosity ◦ Hotter lower viscosity Gas held in magma by pressure Gas expands as magma rises (decreasing pressure) ◦ Bubbles form and decrease magma density ◦ Lower density increases rate of rise Eruption violence related to how easily gas can escape from magma Hot, low viscosity, gas-poor magma (basalt) quiet eruptions Cooler, high viscosity, gas-rich magmas (rhyolite or andesite) explosive eruptions Pahoehoe has twisted or ropey texture A’a’ has rough, jagged blocky texture Lava transported in tubes through partially cooled flows; erupted at edges of flow Pillow lavas form when lava contacts water Examples of lava flows in Hawaii Thick, steep-sided, obsidian flows Thurston Lava Tube, Hawaii Gas mainly water vapor and carbon dioxide High gas content pyroclastic rocks ◦ 1-6 % of magma (by weight) Pumice: porous rock (“frothy” lava) Ash and dust: fine, glassy fragments Lapilli: walnut-sized material Particles larger than lapilli ◦ Blocks: hardened or cooled lava ◦ Bombs: ejected as hot lava Ash Ash Fall Lapilli Block Bomb A place where magma reaches the surface. Vent – connected to magma chamber by a pipe or conduit Crater – steepwalled depression at volcano top Caldera – very large depression formed by collapse after massive eruption Parasitic Cone – small vent on side of volcano (fed by branch off main conduit) Fumarole – vent on side that emits only gasses Volcanic landforms ◦ Shield volcanoes ◦ Composite volcanoes ◦ Cinder cones Volcanic pipes Fissure eruptions Large caldera Broad, slightly domed-shaped Primarily basaltic lava Mild eruptions of large volumes of lava (generally cover large areas) Example: Mauna Loa (Hawaii) Curtain of Fire Fissure eruptions associated with Pu’u O’o’ Summit caldera Built from ejected lava (mainly lapilli-sized) fragments Steep slope angle Rather small size Occur as parasitic vents on shield volcanoes or in groups Also called scoria cones SP Crater, located north of Flagstaff Sunset Crater SP Crater Paracutin Pu’u O’o’ Large, cone-shape volcano (1000s ft. high & miles wide at base) Most next to Pacific Ocean in “Ring of Fire” (e.g., Fujiyama, Mt. St. Helens) Alternating lava flows and layers of pyroclastic debris Most violent type of activity (e.g., Mt. Vesuvius) Produced by composite volcanoes ◦ Associated with felsic & intermediate magma ◦ Consist of hot gases (up to 800 C), ash, pumice, and other debris ◦ Flow down sides of a volcano at speeds up to 200 km/hr ◦ Also called nueé ardente May 8, 1902 eruption killed 29,000 people and destroyed St. Pierre Volcanic mud flow Very often due to reactivation of volcanic deposits by heavy rains or snow melt Rapid movement (up to 100 kph) Results of lahars associated with Mount St. Helens Large roughly circular depressions with diameters >1 km Form by three processes ◦ Collapse of summit of large composite volcano (Crater Lake) ◦ Gradual subsidence at top of shield volcano due to magma withdrawal (Kilauea) ◦ Collapse of large area due to eruption of large amounts of silica-rich magma along ring fractures (Yellowstone, Long Valley) Fluid basaltic lava erupts from fractures in Earth’s surface (fissures) Lavas cover very large areas flood basalt plateaus ◦ Columbia River Plateau (northwest USA ◦ Deccan Traps (central India) Laki Bulbous mass of congealed silicarich lava Form after main explosive eruptions, blocking vent Solidified magma conduit Resistant rock left after erosion has removed the overlying volcano Examples, Devil’s Tower (WY), Shiprock (NM) Aglatha Peak, Navajo Nation dikes Form when magma hits groundwater Steam explosion forms circular pit Examples, Coliseum (AZ), Kilbourne Hole (NM) Coliseum, Hopi Buttes, Navajo Nation Weinfelder Maar, Eifel Volcanic Field, Germany Deep volcanic conduits into mantle Gas-rich magma brings up pieces of mantle very rapidly In kimberlite pipes, magma may carry diamonds A maar forms at surface Kimberley, South Africa Mirny, Russia, East Siberia Global distribution of volcanism is not random ◦ Most volcanoes within or near ocean basins Basalt common in both oceans and continents Granite/rhyolite rarely found in oceans Greatest volume produced at oceanic ridges ◦ Lithosphere pulls apart (mid-ocean ridges, continental rifts) ◦ Decompression melting of mantle ◦ Large quantities of basalt Places of subduction marked by deep oceanic trenches Descending plate adds water to mantle hydration melting Rising magma can form either… ◦ Island arc (in ocean) ◦ Volcanic arc (continental margin) Activity inside plate (not at edge) Localized volcanic region inside plate called hot spot Columns of hot rising material in mantle (plumes) decompression melting ◦ Basaltic magma in oceanic crust (e.g., Hawaii, Iceland) ◦ Flood basalts ◦ Basalt intrusion of continental crust granitic melts (e.g., Yellowstone Park) Pyroclastic flows Lahars Collapse of flank or dome of volcano Ash (hazard to aircraft) Lava flows