Volcanoes and Plate Tectonics

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Volcanoes and Plate Tectonics

• S E C T I O N 3 - 1

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Objectives

• Where are Earths volcanic regions found, and why
  are they found there?

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• A volcano is a weak spot in the crust where
  molten material from the mantle comes to the
  surface.

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• The molten mixture of rock-forming materials,
  gases, and water is called magma.

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• Liquid magma that reaches the surface is called
  lava.

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• When lava cools, it forms solid rock.

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• In this way, volcanic activity adds new rock to
  existing land and forms new islands.

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• Volcanoes occur in belts that extend across
  continents and oceans.

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• One major belt is the Ring of Fire, formed by the
  many volcanoes that rim the Pacific Ocean.

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• Volcanic belts form along the boundaries where
  Earths plates are diverging (pulling apart) or
  converging (pushing together).

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• Most volcanoes occur along diverging plate
  boundaries, such as the mid-ocean ridge, or in
  subduction zones around the edges of oceans.

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• Some volcanoes form in the middle of plates.

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• The mid-ocean ridge marks a diverging plate
  boundary.

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• There, volcanoes form when lava pours out of
  cracks in the ocean floor.

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• In a few places, these volcanoes rise above the
  oceans surface to form islands.

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• Many volcanoes form near converging plate
  boundaries where subduction occurs.

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• Subduction causes slabs of oceanic crust to sink
  beneath a deep-ocean trench into the mantle.

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• The crust melts and forms magma, which then rises
  back toward the surface.

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• Volcanoes form when the magma erupts as lava.

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• At the boundary where two oceanic plates collide,
  volcanoes can create a string of islands called
  an island arc.

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• Volcanoes also occur at the boundary where an
  oceanic plate is subducted beneath a continental
  plate.

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• Some volcanoes occur in the middle of a plate far
  from plate boundaries.

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• At these places, called hot spots, magma from
  deep within the mantle melts through the crust
  above it.

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• Over millions of years, a hot spot may form a
  series of volcanic mountains.

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• The Hawaiian Islands were formed as the Pacific
  plate drifted over a hot spot.

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• Hot spots also occur under continents.

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• Yellowstone National Park in Wyoming marks a
  major hot spot under the North American plate.

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End