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6.5: Chapter 7 Summary and Key Term Check

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    Chapter 7 Main Ideas

    7.1 Magma and How It Forms

    Magma is molten rock, and in most cases, it forms from partial melting of existing rock. The chemistry of magma depends on the original rock that’s melting, as well as how much partial melting happens. Magma forms by decompression melting, flux-induced melting (fluid-induced melting), and conduction.

    Practice Again

    • Melting types and phase diagrams
    • Melting types and plate tectonic settings

    7.2 Crystallization of Magma

    Magmas range in composition from ultramafic to felsic. Mafic rocks are rich in iron, magnesium, and calcium, and contain approximately 50% silica. Felsic rocks are richer in silica (~70%) and have lower levels of iron, magnesium, and calcium, and higher levels of sodium and potassium than mafic rocks. Bowen’s reaction series allows us to predict the order of crystallization of magma as it cools. Magma can be modified by fractional crystallization (separation of early-forming crystals), by mixing in material from the surrounding rocks by partial melting, and by mixing with magmas of differing chemistry.

    Practice Again

    • Figuring out magma compositions from oxide content
    • Bowen’s reaction series

    Extra!

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    7.3 Classification of Igneous Rocks

    Igneous rocks are classified based on their mineral composition and texture. Felsic igneous rocks have less than 20% dark minerals (ferromagnesian silicates including amphibole and/or biotite) with varying amounts of quartz, both potassium and plagioclase feldspars, and sometimes muscovite. Mafic igneous rocks have more than 50% dark minerals (primarily pyroxene) plus plagioclase feldspar. Most intrusive igneous rocks are phaneritic (individual crystals are visible unmagnified). If there were two stages of cooling (slow then fast), the texture may be porphyritic (large crystals in a matrix of smaller crystals).

    Practice Again

    • Igneous rock names
    • Igneous texture flashcards

    7.4 Intrusive Igneous Bodies

    Magma intrudes into country rock by pushing it aside or melting through it. Intrusive igneous bodies tend to be irregular (stocks and batholiths), tabular (dikes and sills), or pipe-like. Batholiths have areas of 100 km2 or greater, while stocks are smaller. Sills are parallel to existing layering in the country rock, while dikes cut across layering. A pluton that intruded into cold rock is likely to have a chilled margin.

    Practice Your Plutons Again

    Key Term Check

    What key term from Chapter 7 is each card describing? Turn the card to check your answer.

    The original version of this chapter contained H5P content. You may want to remove or replace this element.

    Igneous Rock Name Check

    Which of the igneous rocks from Chapter 7 is each card describing? Turn the card to check your answer.

    The original version of this chapter contained H5P content. You may want to remove or replace this element.

    This page titled 6.5: Chapter 7 Summary and Key Term Check is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Allison Jones via source content that was edited to the style and standards of the LibreTexts platform.