Skip to main content
Geosciences LibreTexts

6: Metamorphic Rocks

  • Page ID
    6880
  • 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

    Metamorphic rocks, 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.

    • 6.1: Prelude to Metamorphic Rocks
    • 6.2: 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.
    • 6.3: 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.
    • 6.4: 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.
    • 6.5: 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.

    Summary

    Metamorphism is the process that changes existing rocks (called protoliths) into new rocks with new minerals and new textures. Increases in temperature and pressure are the main causes of metamorphism, with fluids adding important mobilization of materials. The primary way metamorphic rocks are identified is with texture. Foliated textures come from platy minerals forming planes in a rock, while non-foliated metamorphic rocks have no internal fabric. Grade describes the amount of metamorphism in a rock, and facies are a set of minerals that can help guide an observer to an interpretation of the metamorphic history of a rock. Different tectonic or geologic environments cause metamorphism, including collisions, subduction, faulting, and even impacts from space.

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