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Geosciences LibreTexts

7.0: 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 the change that takes place within a body of rock as a result of it being subjected to conditions that are different from those in which it formed. In most cases, but not all, this involves the rock being deeply buried beneath other rocks, where it is subjected to higher temperatures and pressures than those under which it formed. Metamorphic rocks typically have different mineral assemblages and different textures from their parent rocks (Figure 7.0.1) but they may have the same overall composition. 

Figure 7.0.1 Metamorphic rock (gneiss) of the Okanagan Metamorphic and Igneous Complex at Skaha Lake, B.C. The dark bands are amphibole-rich, the light bands are feldspar-rich. [SE photo]

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 7.0.2). 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.

Figure 7.0.2 The rock cycle. The processes related to metamorphic rocks are at the bottom of the cycle. [SE ]