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Download Introduction to volcanoes, volcanic eruptions, and volcanic
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Introduction to volcanoes, volcanic eruptions, and volcanic landforms What mainly controls eruptive style? • Gas content of magma • Viscosity of magma Viscosity in magma 2. Eruptive style: Explosive or effusive? • High viscosity magma= high silica content=more explosive • Low viscosity magma= low silica content=less explosive (effusive) Why are viscosity and gas important? How would they affect volcanic hazards? Effusive eruption: advancing lava flow Felsic (high SiO2) = high viscosity and more explosive (e.g. dacite) Mafic (low SiO2) = low viscosity and effusive (e.g. basalt) Explosive eruption: pyroclastic flow Unzen Volcano, Japan, 1991 1 Where do volcanoes erupt—and how does this relate to plate tectonics? Types of volcanoes and volcanic landforms Shield volcano ex: Mauna Loa; Larch Mtn, OR Composite volcano (stratovolcano) ex: Vesuvius, Mount Rainier, Popo, Mount Fuji Domes ex. Goat Mountain, Black Buttes & Mt. Shasta (CA) Cinder cones and spatter cones ex: Paracutin, Puu O (Kilauea), Sunset Crater (AZ) Maar volcano Caldera ex: Battleground Lake, Ubehebe (CA) ex: Yellowstone, Toba, Crater Lake Flood basalt ex: CRBs 17–15 Ma; Deccan Traps--India 65 Ma; Siberian Traps ~250 Ma) Lower viscosity basaltic lava (mafic) is ~45% to 54% silica Shield Volcano (typically basaltic!) Shield Volcanoes Effusive eruptions (less explosive) Gentle relief + cinder cones. Volcano = chiefly lava flows. Below: aerial view of Hawaii Kupaianaha lava pond, Kilauea: photo by Pat Pringle, July 1987 Humongous landslides from Hawaiian volcanoes (dark gray = islands). The landslide deposits (stippled area) are found on the sea floor (note scale in km!) 2007 gps velocities ~ 7 cm/yr So what? There is more to see below the surface of the ocean! Pacific Plate moves over the Hawaiian hot spot; ages of the Hawaiian Islands + geologic factors reflect plate movement— older to the NW 2 Basaltic volcanism in the Cascade Range Larch Mtn shield, OR <= Tumac Mtn shield, NE of White Pass; view to East Clayton (1983) estimated the age of Tumac Mtn at 30 to 20 ka. ka = kiloannum Underwood Mtn shield, WA Mount Adams composite volcano is constructed on top of a broad shield Composite volcanoes (aka stratovolcanoes) • • • • • • • Where are Composite volcanoes? ~55 – 65 % silica (intermediate SiO2) andesite, dacite Intermediate viscosity Moderately violent explosions ~50/50 lavas/ fragmental deposits Thick lavas (to 20 or even 100 m) Steep cones; can have domes Lahars! (volcanic debris flows) Photo by Pat Pringle Mount Rainier from Glacier View Wilderness • Ring of Fire (Circumpacific belt) • Mediterranean belt Glacial erosion increases relief and supplies groundwater => hydrothermal alteration and weakening 3 Composite volcano Below: isopach map of Vesuvius’ AD 79 tephra Mount Hood Left: tectonic setting of Italian volcanoes; right above: thickness of deposits from AD 79 Vesuvius eruption Composite Volcano Mount Baker aka stratovolcano May 18, 1980 Plinian eruption column Looking NNW Photo by Bob Krimmel, USGS Mount St. Helens => changed the way we think about composite volcanoes! Vesuvius, 1944 Monte Somma – remnant of pre79 AD edifice Crater walls of Mount St. Helens: note abundant fragmental debris! 4 Martha Sabel, USGS, 1982 Incised canyon on the north flank of Mount St. Helens near north end of 1980 Crater floor; note great percentage of fragmental debris! View looks NNE; photo by Pat Pringle, September, 1982 Summit dome dacite of Kalama age (post AD 1479) Basalt of Castle Creek age (~1900 yr B.P.) basaltic dike Crater walls of Mount St. Helens; photos by Pat Pringle; above from south crater rim; right, from top of Lava Dome.) Dacite dome rock of Pine Creek age (~2500 yr B.P.) 5