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Volcanoes Lecture



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  • 1. Volcanoes Chapter 18
  • 2. 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
  • 3. 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
  • 4. 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
  • 5. 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
  • 6. 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
  • 7. Types of Volcanoes
    • Shield- broad, gently sloping sides and circular base; form from layers of basaltic lava flows; non-explosive (quiet) eruptions- Hawaii
  • 8. 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
  • 9. 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
  • 10. 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)
  • 11.  
  • 12. Volcanoes on the web
    • http:// /
  • 13. 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
  • 14. 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
  • 15. 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
  • 16.  
  • 17. 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
  • 18.  
  • 19.