Magma generation

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Magma generation Plate Boundaries
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What explains the location of 95 of the worlds
volcanoes?
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Volcanism

• Magma Liquid rock underground
• Lava Liquid rock on the surface of the Earth
• Vent Opening in a volcano where lava erupts
• Magma chamber Large store of magma

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If the asthenosphere is a solid how is magma
(liquid) generated at each place?

• Divergent plate boundary
• Convergent subduction plate boundary-
• Hot spots (not at plate boundary)-

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Hot Spots

• Hot spots in the mantle can cause melting of the
  crust within a tectonic plate.
• Hot spots remain stationary in the mantle, but
  the lithospheric plate moves across it.
• The volcano on the surface is eventually carried
  away from the hot spot and becomes inactive.
• A new volcano then forms where new crust has
  moved over the hot spot.

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Mafic Hawaiian Islands and Emperor Seamounts

• Hotspot under moving oceanic crust

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Felsic Yellowstone Hot Spot
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Magma types
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Magma Types
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Mafic Shield volcanoes

• On ocean floor or oceanic islands
• Made from mafic lava
• Because gases can easily escape from mafic lava,
  eruptions are relatively quiet.
• Form broad, gently sloping shield volcanoes
• Built from many lava flows
• Ex Mauna Loa

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Mafic Pahoehoe lava flow
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Mafic Aa Lava Flow
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Mafic Lava Tubes
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Mafic Pillow Lava

• Ocean floor near mid-ocean ridges
• pillow lava video

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Intermediate Composite Volcanoes /
Stratovolcanoes

• In a quiet eruption, lava flows build up cone
• In an explosive eruption, large amounts of
  pyroclastic material are deposited on the cone
• The resulting cone is formed of alternating
  layers of hardened lava flows and pyroclastic
  material
• Largest explosions destroy part of volcano
• Often tall
• Moderately steep due to stickier lava and
  pyroclastic material
• Ex.-Mount St. Helens, Mt. Fuji, Mt. Hood, Mt.
  Rainier

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Intermediate Mount St. Helens before 1980
eruption
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Intermediate Mt. St. Helens following the 1980
eruption
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Intermediate Mt. Rainier today
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Intermediate/Felsic Pyroclastic flows

• Pyroclastic material flows downhill after an
  eruption
• Common on stratovolcanoes

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Intermediate/Felsic Lahars

• Pyroclastic material mixes with water to form a
  mudflow
• Common on tall stratovolcanoes that have deep
  snowpack or glaciers

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Lahar hazard in Seattle
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Cinder Cones

• Made from any magma type
• Caused by small explosive eruptions
• Made from solid fragments ejected from the
  volcano
• Very steep slopes
• Very small, only a few hundred meters high

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Size comparison of the three types of volcanoes
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Cinder cone with crater

• Ex Parícutin, Mexico

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Intermediate/Felsic Calderas

• When the magma chamber below a volcano is
  emptied, the ground collapses
• This leaves a large basin-shaped depression
  called a caldera

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Felsic Long Valley Caldera, Mammoth, CA
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Predicting Volcanic Eruptions

• Use seismographs to monitor small earthquakes
• Indicates growing pressure on the surrounding
  rocks as magma works its way upward
• Use surveying with lasers and GPS to detect a
  slight bulging of the surface of a volcano
• Changes in the gases given off by a volcano
• Knowledge of previous eruptions

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Igneous Intrusions

• Batholith largest, at least 100 km2, means deep
  rock, form the cores of many major mountain
  chains.
• Sills sheet of magma flows between the layers of
  rock and hardens parallel to the rock layers
• Dikes magma goes through vertical fractures and
  hardens, cut across rock layers

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• Igneous Extrusions - Igneous rock masses that
  form on the surface
• Volcanic necks - are formed when the vent of an
  extinct volcano solidifies and the flanks are
  eroded away
• Lava Plateaus - develop from lava that flows out
  of long cracks in the earths surface. The lava
  then spreads over a vast area, filling in valleys
  and covering hills.