Volcanoes

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Volcanoes
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Lets Talk Magma

• Felsic Magma
• High silica content
• Thick
• Light colored
• Slow moving
• Mafic Magma
• Low silica content
• Thinner
• Dark colored
• Easy flow

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Magmas Journey

• Magma rises from the asthenosphere to cracks in
  the lithosphere
• On its way to the surface, decreasing pressure
  lets dissolved gases expand
• Once on the surface, magma becomes LAVA (because
  previously dissolved gases have escaped to the
  atmosphere and new materials have been added, the
  chemical composition has changed)

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Lava Fragments

• Tephra solid fragments of lava
• Ash smallest pieces of tephra
• Blocks large pieces of tephra erupted as solids
• Bombs large pieces of tephra erupted as liquid
  (they harden as they fall)
• Tephra Gases Superheated cloud

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Types of Volcanoes

• Cinder Cones
• Shield Volcanoes
• Composite Volcanoes
• Volcanic Domes

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Shield Volcanoes

• The common product of hotspots, but they can also
  be found along subduction-related volcanic arcs
  or all by themselves
• Almost exclusively BASALT (which is very fluid
  when erupted)
• Slow, steady, non-explosive flow gives it a
  shield-like shape
• Magma mixes with only oceanic crust on the way to
  the surface
• Lava has a LOW silica content (50), so it has a
  LOW viscosity (resistance to flow)

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Composite Volcanoes

• Occur from the build-up of multiple eruptions
  that can be hundreds of years apart
• Almost exclusively ANDESITE
• Volcanologists call these "strato-" volcanoes
  because they consist of layers of solid lava
  flows mixed with layers of sand- or gravel-like
  volcanic rock called cinders or volcanic ash.
• Magma mixes with oceanic (basalt) and continental
  (rhyolite) crust on the way to the surface
• Lava has an intermediate silica content (60),
  soit moves at a medium pace

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Volcanic Domes

• Highly explosive!
• Mainly RHYOLITE
• Magma is typically too viscous to move far from
  the vent before cooling and crystallizing.
  Sothey are very steep sided
• Magma mixes only with continental crust on way to
  surface
• Lava has a high silica content (70) and moves
  slowly!