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6: Metamorphic Rocks

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    32346

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    Learning Objectives
    • Describe the temperature and pressure conditions of the metamorphic environment.
    • Identify and describe the three principal metamorphic agents.
    • Describe what recrystallization is and how it affects mineral crystals.
    • Explain what foliation is and how it results from directed pressure and recrystallization.
    • Explain the relationships among slate, phyllite, schist, and gneiss in terms of metamorphic grade.
    • Define index mineral.
    • Explain how metamorphic facies relate to plate tectonic processes.
    • Describe what a contact aureole is and how contact metamorphism affects surrounding rock.
    • Describe the role of hydrothermal metamorphism in forming mineral deposits and ore bodies.
    The wall is a large cliff of dark gray rock with marbled white veins running from upper left to lower right.
    Figure \(\PageIndex{1}\): Painted Wall of Black Canyon of the Gunnison National Park, Colorado, made of 1.7 billion-year-old gneiss intruded by younger pegmatites.

    Metamorphic rock, meta- meaning change and –morphos meaning form, is one of the three rock categories in the rock cycle. Metamorphic rock material has been changed by temperature, pressure, and/or fluids. The rock cycle shows that both igneous and sedimentary rocks can become metamorphic rocks. And metamorphic rocks themselves can be re-metamorphosed. Because metamorphism is caused by plate tectonic motion, metamorphic rocks provide geologists with a history book of how past tectonic processes shaped our planet [1].

    Magma crystallizes to an igneous rock. Igneous rocks weather into sediment. Sediment is transported and deposited, buried and lithified into sedimentary rocks. Sedimentary rocks undergo heat and pressure, resulting in textural and/or chemical changes and turning into metamorphic rocks. Metamorphic rocks can melt into magma, closing the cycle. Igneous rocks can also undergo heat and pressure and turn into metamorphic rocks. Metamorphic and sedimentary rocks can be exhumed to Earth's surface and weather into sediment (and continue to turn into sedimentary rocks). Metamorphic rocks can also turn into other metamorphic rocks.
    Figure \(\PageIndex{2}\): Rock cycle showing the five materials (such as igneous rocks and sediment) and the processes by which one changes into another (such as weathering). (Source: Peter Davis)

    • 6.1: Metamorphic Processes
      Metamorphism occurs when solid rock changes in composition and/or texture without the mineral crystals melting, which is how igneous rock is generated. Metamorphic source rocks, the rocks that experience the metamorphism, are called the parent rock or protolith, from proto– meaning first, and lithos- meaning rock. Most metamorphic processes take place deep underground, inside the earth’s crust.
    • 6.2: Metamorphic Textures
      Foliation is a term used that describes minerals lined up in planes. Certain minerals, most notably the mica group, are mostly thin and planar by default. Foliated rocks typically appear as if the minerals are stacked like pages of a book, thus the use of the term ‘folia’, like a leaf. Other minerals, with hornblende being a good example, are longer in one direction, linear like a pencil or a needle, rather than a planar-shaped book.
    • 6.3: Metamorphic Grade
      Metamorphic grade refers to the range of metamorphic change a rock undergoes, progressing from low (little metamorphic change) grade to high (significant metamorphic change) grade. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions. The sequence slate → phyllite → schist → gneiss illustrates an increasing metamorphic grade.
    • 6.4: Metamorphic Environments
      As with igneous processes, metamorphic rocks form at different zones of pressure (depth) and temperature as shown on the pressure-temperature (P-T) diagram. The term facies is an objective description of a rock. In metamorphic rocks, facies are groups of minerals called mineral assemblages. The names of metamorphic facies on the pressure-temperature diagram reflect minerals and mineral assemblages that are stable at these pressures and temperatures.
    • 6.S: Summary

    Thumbnail: Folded foliation in a metamorphic rock from near Geirangerfjord, Norway. (CC-SA-BY 3.0; Siim);

    This page titled 6: Metamorphic Rocks is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher (OpenGeology) via source content that was edited to the style and standards of the LibreTexts platform.