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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 l.jpg
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 l.jpg
Environments of Magma Formation

  • Spreading centers associated with divergent boundaries

  • Subduction zones associated with convergent boundaries

  • Mantle plumes






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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.



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    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







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    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 l.jpg
    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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