Volcanoes

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Volcanoes
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Volcanoes and Plate tectonics

• Volcano is a mountain formed when layers of lava
  and volcanic ash erupt and build up
• Most are dormant
• About 600 are active

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What cause Volcanoes

• Magma is less dense then solid rock
• It slowly rises to the earths surface
• Flows out through an opening called a vent
• Lava cools quickly and forms igneous rock
• Opening at the top of the vent is called a crater
  
• Pyroclastic flow massive avalanches of hot
  glowing rock and gases (complete destruction)

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Where do Volcanoes Form

• 1. Divergent Boundaries
• Mid Atlantic Ridge
• Forms long deep cracks called rifts where magma
  flows from
• 2. Convergent boundaries
• Cascade Mt. Range in Oregon and Washington
• Mt. Saint Helen
• Pacific Ring of Fire

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• 3. Hot Spots
• Hawaiian Islands
• Far from the edge of any plate
• Some area of the mantle are hotter then others
• These hot spots melt rock which rise to the
  surface to form volcanoes
• Once the volcano rises above the water it becomes
  an island

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What determines how a volcano Erupts

• Composition of the magma plays a big role in
  determining the manner in how a volcano erupts
• Lave more silica (silicon and oxygen) tend to
  be thicker and resists flow
• Lava containing more iron and magnesium and less
  silica tend to flow easier

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• Amount of water vapor in lava influences how lava
  erupts
• More water (steam) the more pressure builds up
  and the more violent the eruption is

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Forms of Volcanoes

• Three basic volcanoes
• Shield volcanoes
• Cinder volcanoes
• Composite volcanoes

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1. Shield

• Quiet eruption
• Broad volcanoes
• Gently sloping sides
• Ex. Hawaiian , rift zones like Iceland

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Cinder Cone

• Explosive Volcano
• Throws lava into the air, lava cools and hardens
  
• Tephra small- cinders , large bombs
• Forms steep sides
• Sunset crater

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Composite Volcano

• Can vary between quiet or violent
• Alternating layers of lava and tephra
• Convergent boundaries above subduction zone
• Mt. St. Helens

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• Fissure Eruption
• Magma that is highly fluid can ooze from cracks
  or fissures in earths surface

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