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VOLCANIC ACTIVITY

Explore the fascinating world of volcanic activity, from the deep molten rock inside the Earth to the types of magma that create different volcanic formations. Learn about the characteristics of basaltic, andesitic, and rhyolitic magma, including viscosity and eruption styles. Discover intrusive igneous rock formations, such as batholiths and sills, and how they relate to tectonic movements. Delve into the anatomy of volcanoes, including vent structures, craters, and calderas, as well as the occurrence of volcanoes at plate boundaries and hotspots.

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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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