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  1. Volcanoes Volcanoes

  2. Magma formation • Magma formation is influenced by depth, pressure, temperature, and the presence or absence of water (which influences melting point). • Three major igneous rock types, basalt, andesite, and rhyolite, form three corresponding types of magma. • Different magmas have different viscosities, which influences their behavior

  3. Peanut butter vs. ketchup

  4. Basaltic magma • Forms when rocks in the upper mantle melt • Low viscosity • Rises quickly to surface and reacts little with crustal rocks • Contains small amounts of dissolved gases and silica, so erupts quietly

  5. Hawaii’s volcanoes are made with basaltic magma

  6. Basaltic magma

  7. A mild basaltic magma eruption

  8. Andesitic magma • Found along continental margins, where oceanic crust is subducted into Earth’s mantle • Source for magma can be either oceanic crust or oceanic sediments • Contains about 60% silica (vs. 50% for basaltic magma), so is more explosive and more viscous

  9. Andesitic magma formation

  10. Rhyolitic magma • Forms when molten material rises and mixes with overlying silica- and water-rich continental crust • High viscosity inhibits its movement • Resistance to flow + lots of gas trapped inside it make it very explosive

  11. Mt. St. Helens

  12. Explosivity and viscosity are determined by gas and silica content

  13. Magma that cools within the Earth is called a PLUTON • Batholiths: the largest plutons, cover ≥ 100km2. Found in the interiors of major mountain chains. Mostly made of granite. • Stocks: irregularly-sized plutons that are smaller than batholiths • Laccoliths: mushroom-shaped plutons (rock bows upward as a result of intense heat and pressure of magma body) • Sills: plutons that form when magma intrudes parallel to layers of rock • Dikes: plutons that cut across preexisting rock

  14. Types of Volcanoes • A shield volcano is circular, with gently sloping sides. Layers of lava accumulate during non-explosive eruptions. Thus, formed of basaltic lava • A cinder cone volcano is made of more viscous magma. Explosions rocket material high into the air. It piles up around the vent. • A composite volcano forms when layers of volcanic pieces alternate with lava. Larger than cinder cone volcanoes; potentially dangerous.

  15. Shield cone volcano

  16. Which kind of volcano?

  17. Which kind of volcano?

  18. Which kind of volcano?

  19. Which kind of volcano?

  20. Where do volcanoes occur?

  21. Where do volcanoes occur? • Divergent boundaries

  22. Where do volcanoes occur? • Hot spots, either on land or at sea (more common)

  23. What about Yellowstone? • http://www.dailymail.co.uk/sciencetech/article-1350123/Worlds-largest-volcano-Yellowstone-National-Park-wipe-thirds-US.html • Is it likely to erupt? • What kind of volcano is it? • Why would there be a volcanic eruption in the continental interior?