Transcript PPT
Volcanoes and volcanism Goals To examine the relationship between magma composition, the kinds of volcanoes and volcanic processes that occur, and plate-tectonic setting. Magma viscosity Viscosity: Measure of resistance to flow. The greater the viscosity, the harder it is to get a material to flow. Honey more viscous Olive oil less viscous Magma viscosity Viscosity of magma is controlled by two things: temperature and composition – Higher T = lower viscosity – Higher silica content = higher viscosity. Felsic magmas don’t like to flow. Magma gas content • All magmas contain 1–9% dissolved gases— mostly CO2 and H2O. • These gases behave like the carbonation in a soft-drink or a beer. Eruptive materials: What comes out of volcanoes Lava flows: Liquid or Pyroclastic materials: semi-liquid magma flowing on Earth’s surface Material blasted out of volcano during an explosive eruption Lava flows “Ropy” pahoehoe-type lavas: Lowest viscosity, always mafic. “Blocky” a’ a’-type lavas: Low viscosity, colder mafic lavas and hot intermediate lavas Pyroclastic materials Tephra: Magma blasted into the air to form very hot (500° C) solid particles Volcanic dust: Tephra particles less than 1/1000 mm in diameter. Volcanic ash: Tephra particles between 1/1000 mm to 2mm in diameter. Volcanic bombs: Tephra chunks >64mm in diameter Volcanic dust can stay suspended in the upper atmosphere for years • Large eruptions can alter global climate • Really awesome sunsets Volcanic ash falls like snow for 100’s of miles down-wind from eruption Pyroclastic materials Pyroclastic flows a.k.a. Nuee Ardente: Billowing clouds of super-heated gas and ash that flow down the slope of the volcano Pyroclastic flows travel at more than 50 mph Pyroclastic materials Volcanic mudflows a.k.a. Lahars: Eruption melts snow, glacial ice, and/or frozen soil • Mixture of water and debris flow rapidly down the slope of a volcano Types of Volcanoes—Dictated by magma type Most volcanoes have two basic elements: A volcanic cone and a central crater Crater Mafic magmas Shield volcanoes: Enormous, gently sloping volcanic mounds • Form over oceanic hotspots: localized zone of hot mantle upwelling • Largest topographic features on Earth Shield volcano—Big Island of Hawaii Hot Spot Volcano Tracks Hawaiian Islands are hot-spot volcanoes on the Pacific plate Which way is the pacific plate moving? ? ? Hawaii Mafic magmas Fissure eruptions: Low-viscosity magma flows up along km-long cracks in the crust • Often form in continental rifts • Form Flood basalts: Lava spreads out over large areas Small fissure eruption 15-m.y. old flood basalts in the western United States Mafic magmas Cinder cones: Small volcanic cones produced by fountain eruptions • Often found on the flanks of larger volcanoes or in continental rifts Cinder cone in Arizona Fountain eruption Intermediate magmas Composite volcanoes: Contain alternating layers of pyroclastic deposits and lava flows • Explosive eruptions producing ash clouds and pyroclastic flows • Found almost exclusively in volcanic arcs related to subduction zones Mt. Vesuvius, Italy Mt. Fuji, Japan Felsic magmas Volcanic domes: Mounds formed when lava is too viscous to flow away from the vent Lava Dome Felsic magmas Caldera eruptions: Large magma chamber collapses, causing enormous, explosive eruptions • Form over continental hotspots and in continental rifts • Form Very large, steep-walled depressions called calderas Steps in the formation of a caldera Yellowstone National Park is a giant caldera Caldera The Yellowstone volcanic system has had 3 catastrophic eruptions in the past 2.1 m.y. • 2.1 m.y. eruption released 600 cubic miles of tephra • 640 k.y. eruption released 240 cubic miles of tephra • 1.3 m.y. eruption released 67 cubic miles of tephra Tectonic Settings and Volcanic Activity