Chapter 15- Volcanoes

  1. Volcanoes and Plate Tectonics-mountains that form when layers of lava and volcanic ash erupt and buildup.
  1. What causes volcanoes?
    1. Magma is less dense than the rock around it.
    2. Magma uses vents to flow to the Earth's surface.
    3. This flowing magma cools and forms a crater.
  1. Where do volcanoes occur?
    1. Divergent Boundaries- When the plates separate they form rifts- magma flows from the rifts as lava cooling instantly and builds up sometimes rising above sea level forming islands.

      2. Ex. Iceland.

    2. Convergent Boundaries- Magma created in the subduction zone works its way to the surface. Ex. Pacific Ring of Fire-edge of the pacific plate where volcanoes are common.
    3. Hotspots- Areas of the mantle that are hotter than normal. Magma rises to the surface. Ex. Hawaiian Islands. Figure 15.4

  1. Geothermal Energy from Volcanoes

A. Electricity from G.E. - Heat from magma can be used to heat water and produce steam. The steam is pressurized and spins generators that make electricity. Figure 15.5

B. Problems with G.E.-
    1. Clearing of rain forest to build G.E. power plants.
    2. Release of harmful gases from the magma.
  1. Eruptions and Forms of Volcanoes
    1. Types of Eruptions
    1. Explosive- caused by trapped gas and water vapor and magma that is Granitic, thick and has a high water content.

2. Quiet- The gas escapes before it reaches the surface and the magma is Basaltic and very fluid.
    1. Forms of Volcanoes
    1. Shield volcanoes- quiet eruptions spread out Basaltic lava in flat layers.
    2. Cinder-cone Volcanoes

      3. Explosive eruptions throw lava high in the air. When this lava cools it is called Tephra Cinder-cone volcanoes are created when Tephra falls to earth and piles up. Figure 15.9

       

    3. Composite Volcanoes- eruptions that vary between quiet and explosive. Layers are made of lava and Tephra repeating.

  1. Volcanic Features
    1. Intrusive Features
    1. Batholiths- huge bodies of magma that cools underground.
    2. Dikes- magma squeezed into a vertical crack and cooled.
    3. Sill- magma squeezed into a horizontal crack and cooled.
    4. Loccolith- magma that form a sill continues to push the rock upward forming a dome.
    1. Other Features
    1. Volcanic Neck- The solid magma core of a volcano
    2. Caldera- large depression formed by the collapse of a volcano. Figure 15.19