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F13 quiz 1 condensed notes

A pluton is a large body of coarsegrained intrusive

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Unformatted text preview: ture remains constant. A third way to make magma is to add H2O, which is effective in breaking chemical bonds. To say it another way, adding H2O lowers the melting point, encouraging melting. Even if the extrusive and intrusive rock compositions are identical, the resultant textures (sizes, shapes, and arrangements of the crystals) are profoundly different. It takes time for mineral crystals to grow, so igneous rock texture is directly related to cooling rate. Rapidly cooled extrusive rocks are typically a mass of tiny crystals; they are fine ­grained. Obsidian is a type of felsic volcanic glass that cooled so quickly that no 3 ­D crystal framework formed. Very slowly cooled intrusive rocks are typically coarse ­ grained, consisting of large, visible crystals. Igneous rocks are classified by composition as well as texture. Mafic rocks are rich in dark ­ colored, dense, ferromagnesian minerals (ex. pyroxene and amphibole) and are typical of oceanic crust. Mafic magma has relatively low viscosity (it is runny), it is liquid at high temperatures, and contains fairly low concentrations of dissolved volatiles. Basalt is the most common mafic igneous rock. In comparison, felsic rocks are rich in non ­ferromagnesian, light ­ colored, low ­density minerals such as orthoclase feldspar (K ­spar) and quartz. Felsic rocks comprise the bulk of the continents and granite is the most common type. Felsic lava has a low melting temperature, it is viscous, and contains high concentrations of dissolved gases. Thus we may have fine ­grained felsic (rhyolite), fine ­grained mafic (basalt), coarse ­grained felsic (granite), and coarse ­grained mafic (gabbro). As magma cools, crystals grow larger and more numerous at the expense of remaining liquid. In a magma chamber, mafic minerals (ex. olivine, pyroxene, and amphibole) begin to crystallize first, at high temperatures. The magma becomes progressively depleted in iron and magnesium, and the most felsic materials, such as quartz, cool last. Bowen’s reaction series describes this process of magmatic differentiation. Magma composition and cooling location are the main factors that determine the types of igneous rock structures geologists observe on Earth. A pluton is a large body of coarse ­grained intrusive igneous rock that formed when a mass of magma crystallized deep with in the crust. A batholith is a very large, regional ­scale group of linked plutons left after cooling of a group of magma chambers that perhaps fed a volcanic chain (that was later removed by erosion). The granite of the Sierra Nevada on display in Yosemite V...
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