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VOLCANIC ACTIVITY

18.1 Magma. VOLCANIC ACTIVITY. Magma. Molten rock, mineral grains, and dissolved gasses deep inside Earth Rocks begin to melt at 800 o -1200 o C Depth, pressure and melting point have a direct relationship However, wet rock melts at a lower temperature (p.472). Types of Magma.

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VOLCANIC ACTIVITY

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  1. 18.1 Magma VOLCANIC ACTIVITY

  2. Magma • Molten rock, mineral grains, and dissolved gasses deep inside Earth • Rocks begin to melt at 800o-1200oC • Depth, pressure and melting point have a direct relationship • However, wet rock melts at a lower temperature (p.472)

  3. Types of Magma • Basaltic- Hawaiian Islands • Andesitic- Mount St. Helens • Rhyolitic- Yellowstone National Park

  4. Magma Composition • Viscosity- resistance to flow • Basaltic- low viscosity, low gas, low silica, quiet eruptions • Andesitic- found near subduction zones, intermediate viscosity, 60% silica, intermediate eruptions • Rhyolitic- high silica, high gas, very explosive!

  5. Viscosity • Hotter = less viscous • Basaltic lava temperatures = 1000o- 1250oC • Rhyolitic lava temperatures = 700o- 900oC • More silica = higher viscosity

  6. 18.2 Intrusive Activity VOLCANIC ACTIVITY

  7. Plutons • Bodies of intrusive igneous rock • Exposed on the surface by erosion

  8. Batholiths- large formations that spread over at least 100 km2 • Stocks- similar to batholiths but cover less than 100 km2 at the surface.

  9. Laccolith- mushroom-shaped; form when magma flows between rock layers and pushes up the overlying rock layers.

  10. Sill- parallel to surrounding rock layers. • Dike- cuts across layers of rock. • Sills and dikes vary in thickness from a few centimeters to hundreds of meters.

  11. Plutons and Tectonics • Plutons are caused by mountain-building processes along convergent plate boundaries.

  12. 18.3 Volcanoes VOLCANIC ACTIVITY

  13. Anatomy of a Volcano • Lava erupts through openings in the crust called vents. • Bowl shaped areas atop vents are called craters.

  14. Volcanoes can collapse and form larger depressions called calderas.

  15. Types of Volcanoes • Shield- broad, gentle slope; non-explosive means of formation • Cinder-cone- small, steep sides, explosive; formed by piled debris • Composite- violent; composed of fragments and solidified lava

  16. Volcanic Material • Tephra- rock fragments thrown into the air by an eruption • Classified by size: dust, ash, lapilli, volcanic blocks and volcanic bombs • Blocks are angular; bombs are rounded

  17. Pyroclastic Flow • Rapidly moving cloud of volcanic material • Speeds can exceed 100 mph - 450 mph • Temperatures can reach 1800oF

  18. Where Do Volcanoes Occur? • Convergent boundaries- subduction zones where magma is forced upward • Divergent boundaries- magma is forced upward at ridges, rifts, fractures and faults

  19. Where Do Volcanoes Occur? • Hotspots- hot stationary plumes of magma far from plate boundaries

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