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Volcanoes Processes that Form Magma in the Crust and Upper Mantle Increase in temperature Pressure-Release Melting: decrease in confining pressure lowers the melting point of rock Addition of water lowers the melting point of rock Fig. 8-2, p.171 Environments of Magma Formation

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processes that form magma in the crust and upper mantle
Processes that Form Magma in the Crust and Upper Mantle
  • Increase in temperature
  • Pressure-Release Melting: decrease in confining pressure lowers the melting point of rock
  • Addition of water lowers the melting point of rock
environments of magma formation
Environments of Magma Formation
  • Spreading centers associated with divergent boundaries
  • Subduction zones associated with convergent boundaries
  • Mantle plumes
partial melting and magma formation
Partial Melting and Magma Formation
  • Formation of Basaltic magmas
      • Most originate from partial melting of ultramafic rock in the mantle
      • Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones
  • Formation of Granitic magmas
      • Basaltic magma pools beneath granitic continental rock and melts it, forming granitic magma
      • Granitic magma often does not reach the surface, but instead forms intrusive rocks at depth.
formation of plutons from granitic magma
Formation of Plutons from Granitic Magma
  • Formation of Granitic magmas
      • Basaltic magma pools beneath granitic continental rock and melts it, forming granitic magma
      • Granitic magma often does not reach the surface, but instead forms intrusive rocks at depth.
  • Pluton – a large mass of intrusive rock
      • Most plutons are granitic in composition
      • Granitic magma forms at base of continental crust and rise up because it is less dense
volcanoes17
Volcanoes
  • Types of Volcanoes
      • Shield volcano
        • Broad, slightly domed-shaped
        • Composed primarily of basaltic lava
        • Generally cover large areas
        • Produced by mild eruptions of large volumes of lava
        • Mauna Loa on Hawaii is a good example
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Volcanoes
  • Types of Volcanoes continued
      • Cinder cone
        • Built from ejected lava (mainly cinder-sized) fragments
        • Steep slope angle
        • Rather small size
        • Frequently occur in groups
sunset crater a cinder cone near flagstaff arizona
Sunset Crater – a cinder cone near Flagstaff, Arizona

Copyright © 2006 Pearson Prentice Hall, Inc.

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Volcanoes
  • Types of volcanoes continued
      • Composite cone (Stratovolcano)
        • Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)
        • Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)
        • Composed of interbedded lava flows and layers of pyroclastic debris
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