Chapter Goals

Complete this chapter so you can:

• Summarize the factors that influence the nature of metamorphic rocks.
• Explain how foliation forms in metamorphic rocks.
• Classify metamorphic rocks based on their texture and mineral content, and explain the origins of both.
• Describe settings where metamorphic rocks form, and explain the links to plate tectonics.
• Describe the different types of metamorphism, including burial metamorphism, regional metamorphism, seafloor metamorphism, subduction zone metamorphism, contact metamorphism, shock metamorphism, and dynamic metamorphism.
• Explain how metamorphic facies and index minerals are used to characterize metamorphism in a region.
• Explain why fluids are important for metamorphism, and describe what happens during metasomatism.

Metamorphism Occurs Between Diagenesis And Melting

Metamorphism is the change that takes place within a body of rock as a result of it being subjected to high pressure and/or high temperature.  The parent rock or protolith is the rock that exists before metamorphism starts. New metamorphic rocks can form from old ones as pressure and temperature progressively increase. This means that the term parent rock is sometimes applied to an initial unmetamorphosed rock, but it could also be applied to a metamorphic rock before it undergoes even further metamorphic change.  We don’t always know whether metamorphism occurred in an uninterrupted sequence or whether metamorphism stopped and started again for different reasons at different times.

Metamorphic rocks form under pressures and temperatures that are higher than those experienced by sediments and sedimentary rocks during diagenesis (a blanket term for a range of low-temperature and low-pressure physical and chemical changes that happen to buried sediments and sedimentary rocks, including interactions with groundwater), but at temperatures too low to allow melting.  Given that pressure and water content affect a rock’s melting point, metamorphism can still be ongoing at higher temperatures for some kinds of rocks, whereas other rocks will begin to melt under these same conditions.

Metamorphic rocks can have very different mineral assemblages and textures than their parent rocks (Figure 10.2), but their over-all chemical composition usually doesn’t change very much.

Most metamorphism results from the burial of igneous, sedimentary, or pre-existing metamorphic rocks, to the point where they experience different pressures and temperatures than those at which they originally formed. Metamorphism can also take place if cold rock near the surface is intruded and heated by a hot igneous body. Metamorphism usually involves temperatures above 150 °C, but some types of metamorphism do occur at temperatures lower than those at which the parent rock formed.