volcanoes lecture
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Volcanoes
Chapter 18
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Magma• minerals, dissolved Mixture of molten
rock, suspended gases BELOW Earth’s surface• Rocks melt in asthenosphere (really hot)• Why isn’t the whole rest of the mantle
liquid too?• Pressure- pressure increases with depth,
increased pressure raises the melting point of rock
• Water- tiny water droplets trapped in rock raise melting point
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Types of Magma
• Basaltic- like basalt; dark in color; forms when rocks in asthenosphere melt; low viscosity → flows quickly & rises to surface quickly. Small amounts of silica; Quiet eruptions; Hawaii
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Types of Magma Con’t
• Andesitic- found on continental margins (subduction)→ made from oceanic crust that travels through continental crust; 60% silica; high silica → intermediate eruptions; Mount St. Helens
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Types of magma
• Rhyolitic- forms when molten material rises through thick continental crust and mixes with silica and water; high viscosity → violent eruptions; Yellowstone
• Viscosity- hotter temperature → lower viscosity• Higher silica → Higher viscosity
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Volcano Anatomy
• Vent- hole where lava comes out• Crater- cone-like depression at top of
volcano• Magma Chamber- place under volcano
where magma is stored• Neck- Tube where lava travels from
chamber to surface• Caldera- large depression where top of
volcano collapsed- Crater Lake, OR
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Types of Volcanoes
• Shield- broad, gently sloping sides and circular base; form from layers of basaltic lava flows; non-explosive (quiet) eruptions- Hawaii
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Types of Volcanoes
• Cinder Cone- Ash falls back and piles up near vent; steep sides; small in size; high water and silica content; lots of gases → more explosive
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Types of Volcanoes
• Composite Cone- layers of ash alternate with lava; large amounts of silica, water and gases; much larger than cinder cones; violently explosive; Mount St. Helens
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Volcanic Material
• Tephra- Rock fragments thrown into air by eruption- Classified by size (ash, blocks, bombs)
• Pyroclastic Flows- Clouds of gas, ash, tephra traveling at fast speeds (90mph) and at high temps (370 degrees)
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Volcanoes on the web
• http://www.avo.alaska.edu/• http://vulcan.wr.usgs.gov/• http://hvo.wr.usgs.gov/• http://lvo.wr.usgs.gov/• http://volcanoes.usgs.gov/yvo/
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Intrusive Activity
• Plutons- very large collections of cooled magma• Intrusive Igneous Rock Formations- (“Inside
lava rock”)• Cut across older rocks
• Batholiths- largest pluton, millions of years to form, common in interiors of mountain chains; in N America usually granite; up to 680 miles long
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Intrusive Activity
• Stocks- smaller irregularly shaped pluton
• Laccolith- magma forced up into parallel rock layers close to surface → push some rocks upward → mushroom shaped pluton; small in size (7 miles); Black Hills, SD
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Intrusive Activity
• Sills- parallel to rock layers → like window sill
• Dike- pluton that cuts across the grain• Volcanic Neck- Volcano eroded away
leaving the hardened neck exposed (Devils Tower, WY)
• Plutons are important in mountain building• CA, West Coast, Adirondacks• Yosemite National Park- exposed granite
batholith
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Hot Spots
• Unusually hot regions of Earth’s crust where plumes of magma rise up• Plumes move vertically, not horizontal• Hawaii- Kilauea is over hot spot now,
but Loihi is forming off Hawaii as Pacific Plate moves NW
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