Chapter 10. Metamorphism and Metamorphic Rocks

Adapted by Karla Panchuk from Physical Geology by Steven Earle

Introduction

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.

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.

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