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    andesite

    andesite lava flow
    Close view of andesite lava flow Brokeoff Volcano, California.
    Credit: U.S. Geological Survey
    A dark-colored, fine-grained extrusive igneous rock with about 52 to 63 percent by weight of silica (SiO2). Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine.

    Andesite magma commonly erupts from stratovolcanoes as thick lava flows, some reaching several km in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesites erupt at temperatures between 900° and 1100° C.

    The word "andesite" is derived from the Andes Mountains, located along the western edge of South America, where andesite rock is common. Andesites occur in composite volcanic cones associated with convergent plate margins.


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       • GEOLOGY AND PLANETARY SCIENCE


    Source: U.S. Geological Survey



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