Chapter 10. Metamorphism and Metamorphic Rocks

Introduction

Learning Objectives

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Summarize the factors that influence the nature of metamorphic rocks and explain why each one is important
  • Describe the mechanisms for the formation of foliation in metamorphic rocks
  • Classify metamorphic rocks on the basis of their texture and mineral content, and explain the origins of these differences
  • Describe the various settings in which metamorphic rocks are formed and explain the links between plate tectonics and metamorphism
  • Summarize the important processes of regional metamorphism, and explain how rocks that were metamorphosed at depths of 10 km or 20 km can now be found on Earth’s surface
  • Summarize the important processes of contact metamorphism and metasomatism, and explain the key role hydrothermal fluids

Metamorphism Is Between Lithification And Melting

Metamorphism is the change that takes place within a body of rock as a result of it being subjected to high pressures and/or  high temperatures.  These are pressures and temperatures which are greater than those experienced by sedimentary rocks during lithification, but temperatures less than those which cause igneous rocks to melt.  Given that pressure and water content affect the temperatures at which rocks melt, this means metamorphism can happen at higher temperatures under some conditions than under others.

Metamorphic rocks can have very different mineral assemblages and textures than their parent rocks (Figure 10.1), but their over-all chemical composition will be more or less the same.

Metamorphism can change the mineral assemblage and texture of a rock without altering its chemical composition substantially. [KP, photos by R. Weller/ Cochise College]

Figure 10.1 Metamorphism can change the mineral assemblage and texture of a rock without altering its chemical composition substantially. [Karla Panchuk CC-BY-NC 4.0, photos by R. Weller/ Cochise College by permission for non-commercial educational use]

 Most metamorphism results from the burial of igneous, sedimentary, or pre-existing metamorphic to the point where they experience different pressures and temperatures than those at which they formed (Figure 10.2). Metamorphism can also take place if cold rock near the surface is intruded and heated by a hot igneous body. Although most metamorphism involves temperatures above 150°C, some metamorphism takes place at temperatures lower than those at which the parent rock formed.

The rock cycle. The processes related to metamorphic rocks are at the bottom of the cycle. [SE ]

Figure 10.2 The rock cycle. The processes related to metamorphic rocks are at the bottom of the cycle. [SE ]

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