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Volcanoes

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Magma vs. Lava

  • Lava is molten rock on the earth’s surface.
  • Magma is molten rock below the earth’s surface.

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The Formation of Magma �In a Subduction Zone:

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  • Friction raises the temperature
  • Addition of water lowers the melting point
  • Subducted crust melts
  • Some of the molten rock rises towards the surface

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Formation of Magma

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Formation of Cascade Volcanoes

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Types of Volcanoes

  • Type of volcano depends on the tectonic environment.
  • Most occur at plate boundaries.
  • Some occur at HOT SPOTS.

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1. Composite Volcanoes

  • typical “volcanic” look
  • large, cone shaped
  • results from recurrent eruptions which build up ash and lava
  • felsic magma is thick and traps gas meaning eruptions can be explosive
  • occur at subduction zones

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Animations

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2. Shield Volcanoes

  • usually form at HOT SPOTS not at tectonic boundaries
  • Hot Spot: a weak part of a lithospheric plate where molten material can burst through
  • mafic magma is very fluid, so eruptions are not explosive
  • slopes are not steep, but shield volcanoes form the largest volcanoes
  • examples: Hawaiian Islands, Galapagos Islands

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Hot Spot Animation

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3. Rift Eruptions

  • occur at divergent plate boundaries
  • magma comes to the surface through long fissures forming fountains of lava
  • not as violent as other eruptions
  • can release huge amounts of lava
  • example: Columbia River Plateau - stretches from the Canadian border to California - up to 3 km deep, Iceland

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Intrusive Igneous Features

  • Intrusions form when magma cools under ground inside other rock formations.
  • They are sometimes referred to as PLUTONS.

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Dikes

  • Sheets of igneous rock that cut across the rock layers they intrude (pass through).
  • Form when magma is forced into vertical cracks.

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Sills

  • Sheets of igneous rock that are PARALLEL to the layers they intrude.
  • Form when magma is forced between rock layers.
  • Can be very large (100s of meters deep and many km long)

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Volcanic Necks

  • A plug of hardened magma left in a volcanic vent.
  • May erode more slowly than the volcano.

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Batholiths

  • Largest of all intrusions.
  • Form the cores of many mountain ranges.
  • Form deep in the Earth’s crust.
  • Eventually exposed by erosion.

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Volcanic Ejecta

  • What can come out of a volcano?
  • ASH: gritty, sand sized particles of rock
  • PYROCLASTIC MATERIAL: “fire fragments” - materials produced from erupting volcanoes
  • LAVA: molten rock that reaches the Earth’s surface

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Environmental Impacts

  • ASH FALLS - have the most far reaching effects
  • reach very high altitudes and can affect the world’s weather/temperature as well as crops/food supply
  • covers the ground killing crops, damaging buildings, machinery and clogging rivers

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Environmental Impacts

  • LAVA FLOWS - reach temperatures over 1000 degrees Celsius and can destroy anything in path (buildings, crops, forests etc)

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Environmental Impacts

  • PYROCHLASTIC FLOW: deadly mix of gas, ash and debris heated to over 1000 degrees Celsius
  • travel at 300 km/hr
  • may melt snow and cause a LAHAR (debris flow of mud, water and ash)

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