Ch. 7.2 Volcanic Eruptions

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Ch. 7.2 Volcanic Eruptions
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• Two general types of lava.
• Mafic lavadark when hardened rich in iron and
  magnesium usually thin and flowing commonly
  forms ocean floor (basalt).
• Felsic lavarich in silicaless iron and
  magnesium forms continental crust (granite)
  lighter colored when hardened.

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Kinds of Eruptions

• Lava flow eruptionsusually quiet produce
  red-hot rivers of flowing lava. Thin mafic lava.
  Gases escape easily.
• Explosive eruptionsthicker felsic lava with lots
  of trapped gases, produce ash, and various sizes
  of molten and solid particles shooting into the
  air.

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Volcanic Rock Fragments

• Pyroclastic material results from exploded felsic
  lava. The type of pyroclastic material depends
  on particle size.
• Volcanic ashparticles less than 2mm in diameter.
• Volcanic dustparticles less than .25mm.
• Lapillilarger particles less than 64mm.

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• Volcanic bombslarge red-hot lava blobs thrown
  out of the volcano.
• Volcanic blockssolid rock blasted away from the
  volcano. May be as big as houses.

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Volcanic Features

• Volcanic conethe buildup of lava and pyroclastic
  materials around the vent. Three main types

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Shield Volcanoes

• Broad at the base with gently sloping sides.
• Result from eruptions of mafic lava.

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Cinder Cones

• Only a few hundred meters high at most very
  steep sides.
• Result from explosive eruptions of solid
  fragments.

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Stratovolcanoes (Composite Volcanoes)

• Formed when a volcano experiences both quiet lava
  flows and explosive eruptions.
• Alternating layers of different materials build
  up into very high volcanic mountains.

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• Craterthe funnel-shaped pit at the top of a
  volcanic vent.
• Calderalarge basin-shaped depression formed when
  a volcanic cone collapses because the magma
  chamber below empties (Ex Yellowstone Park).

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Predicting Eruptions

• Small earthquakes often increase in frequency and
  strength just before eruptions.
• There may be slight bulging of the volcanos
  surface prior to an eruption.
• There may be changes in gases coming out of the
  volcano.
• Past history is important.