9.1 How and Where Volcanoes Form

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9.1 How and Where Volcanoes Form
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Learning Objectives

• In this lesson we
• Learn how magma forms as a result of plate
  tectonics motion and interaction.
• Understand and explain why plate boundaries are
  the sites of most volcanic activity.
• Locate the volcanic areas on the world map.

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Magma Formation

• A volcanic eruption occurs when ,magma- molten
  rock that has formed deep within Earth rises to
  the surface.
• Although very hot, the astenosphere is not quite
  liquid, because the melting pressure exerted by
  the lithosphere above it rises the melting point
  of the rocks.

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Magma Formation

• The following Three conditions allow magma to
  form
• A decrease in pressure will lower the melting
  temperature of the rocks. This happens at the mid
  ocean ridge, where the crust is thin and exerts
  less pressure.
• An increase in temperature can cause materials in
  the astenosphere to melt. This happens at hot
  spots.
• An increase in the amount of water in the
  astenosphere can lower the melting temperature of
  the rocks it happens at subduction boundaries.

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Illustrated Magma Formation
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Where Magma Forms

• At subduction boundaries volcanoes always form on
  the overriding plate.
• If an oceanic plate is subducted beneath a
  continental plate, the volcanoes will form on the
  continental plate.
• At the convergent boundaries of two oceanic plate
  an arc of volcanic islands will rise above the
  water.

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Where Magma Forms

• At divergent boundaries
• Below a rift, mantle material rises from
  deeper, hotter regions within Earth. Along the
  rift the pressure is lower than it is elsewhere
  in the mantle. This decrease in pressure lowers
  melting temperatures. The magma rises to the
  surface, because, and because of its density is
  lower than the surrounding rocks.

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Divergent boundaries volcano

• Island, the largest volcanic island in the world,
  formed on the Mid-Atlantic Ridge

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Where Magma Forms

• Over hot spots

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Hot Spots

• Hot spots are pockets of molten rock beneath the
  crust of the Earth. The hot spot remains in the
  same place as a plate moves above it. Hawaii is
  the newest island over a hot spot. The other
  islands formed in the past, a s the plate moved
  above it.

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The Hawaiian Archipelago

• The islands formed one at a time, as the Pacific
  Plate moved NW over the fixed hot spot.