Volcanoes and Plate Tectonics

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Chapter 3 - 1

• Volcanoes and Plate Tectonics

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What is a Volcano?

• A weak spot in the crust where molten material,
  or magma, comes to the surface
• Magma molten mixture of rock-forming
  substances, gases, and water from the mantle
• Lava magma that reaches the surface

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Location of Volcanoes

• Ring of Fire volcanic belt formed by many
  volcanoes that rim the Pacific Ocean
• About 600 active volcanoes on land, but more lie
  beneath the sea

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Where?

• Most volcanoes occur along diverging plate
  boundaries, such as the mid-ocean ridge, or in
  subduction zones around the edges of oceans
• Some at hot spots far from boundaries or
  continental plates

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Volcanoes at Divergent Plate Boundaries

• Along mid-ocean ridge (long, underwater rift
  valley)
• Lava pours out of cracks in the ocean floor
• Only in a few places (Iceland, Azores Islands) do
  they rise above the oceans surface

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Volcanoes at Convergent Boundaries

• Where oceanic crust returns to the mantle
• Subduction causes slabs of oceanic crust to sink
  back into mantle
• Crust melts and forms magma
• Magma erupts from melted crust as lava
• Where oceanic crust collides
• String of islands island arc (mimics curve of
  trench)
• Can also occur where oceanic crust collides with
  continental crust

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Hot Spot Volcanoes

• An area where magma from deep within the mantle
  melts through the crust like a blow torch
• Often lie in the middle of continental or oceanic
  plates
• Do not result from subduction
• Gradually forms a series of volcanoes (Hawaii)
• Can form under continents Yellowstone National
  Park in Wyoming (75,000 years ago was last
  eruption)