Volcanoes

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Volcanoes
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Magma Reaches the Earths Surface

• KEY CONCEPTWhen a Volcano erupts, the force of
  the expanding gases pushes magma from the magma
  chamber through the pipe until it flows or
  explodes out of the vent.
• The volcano has a pocket of magma below the
  surface called a magma chamber. A long tube,
  called a pipe, connects the magma chamber to the
  surface.
• The opening at the top of the pipe is called the
  vent.
• When it leaves the volcano magma becomes lava.

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Kinds of Eruptions

• KEY CONCEPTGeologists classify volcanic
  eruptions as quiet or explosive.
• How a volcano erupts depends on the magma.
• A quiet eruption moves slowly because it is low
  in silica and flows very easily.
• An explosive eruption has magma that is high in
  silica and does not flow easily. Magma builds up
  until it explodes out of the vent.

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Stages of Volcanic Activity

• KEY CONCEPT Geologists often use the term
  active, dormant, or, extinct to describe a
  volcanos stage of activity.
• Active volcanoes are erupting or showing signs
  that it will erupt soon.
• Dormant volcanoes are no longer erupting, but can
  again in the future.
• Extinct volcanoes are not likely to erupt any
  longer.

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Where Volcanoes Form

• KEY CONCEPT Volcanoes form along the boundaries
  of the Earths plates.
• A Volcano is a weak spot in the crust where
  melted material comes to the surface.
• Most Volcanoes happen at plate boundaries.
• Lava can build up to form islands and mountains.
• Some Volcanoes form above a hot spot when magma
  erupts through the crust and reaches the surface.