11.1: Introduction

Overview

In any introductory textbook on physical geology, the reader will find the discussion on metamorphic rocks located after the chapters on igneous and sedimentary rocks, and for very good reason. Metamorphic rocks form by the physical and sometimes chemical alteration of a pre-existing rock, whether it is igneous or sedimentary. In some cases, even metamorphic rocks can be altered into a completely different metamorphic rock. With igneous rocks forming from the melt produced by any rock type and a sedimentary rock-forming from the weathered product of any rock type, the alteration of any rock to produce a metamorphic one completes the components of what is known as the rock cycle. Basically, the rocks we encounter today that we classify as either igneous, metamorphic, or sedimentary, could have belonged to a different rock classification in the past, as rocks are recycled throughout geologic time, driven by the motion of the tectonic plates (see Chapter 4). It is easy to see that increasing the temperature of rocks can produce magma and that rocks on the surface of the earth can break up into sediments that can ultimately lithify into sedimentary rocks. But how can we alter a solid rock into a new rock, without melting it or making it become sediment?

 

All rocks are formed at certain temperatures and pressures on or more commonly, beneath the earth’s surface, and these rocks are the most stable at the conditions under which they form. Therefore, changing the temperature and/or pressure conditions may lead to a different rock, one that changed in order to be stable under new external conditions. This new rock that forms in response to changes in its physical and chemical environment is called a metamorphic rock; the word metamorphism means to change form and, for rocks, this means the recrystallization of minerals (crystals) under subsolidus (temperatures too low for melt production) conditions. A metamorphic change can also occur if the rock’s composition is altered by hot, chemically reactive fluids, causing a change in the mineral content of the rock. To distinguish between the pre-existing rock and the new metamorphic one, the term protolith or parent rock is used to describe the pre-existing rock, and all metamorphic rocks have at least one protolith that has altered during metamorphism. In this chapter you will learn that all metamorphic rocks are identified by the mineral content and texture of the rock; for metamorphic rocks, texture refers to the orientation of the minerals in the rock, although crystal size does convey important information regarding the temperature conditions during metamorphism.

 

To summarize, metamorphism is the process by which a pre-existing rock (the protolith) is altered by a change in temperature, pressure, or by contact with chemically reactive fluids, or by any combination of these three parameters. An alteration process is a recrystallization event, where the initial rock’s minerals (crystals) have changed size, shape, and/or composition in response to these new external conditions. The end result is a new (metamorphic) rock that has an altered appearance, sometimes strikingly different from what it used to look like before metamorphism occurred. What metamorphic rock you end up with is strongly dependent on what rock you started with before the metamorphic event. Of secondary importance is the agent of metamorphic change: was recrystallization due to increased temperature, pressure, or both, or were chemically reactive fluids involved? Also, one must consider how high the temperatures and pressures were, as metamorphism can occur in a range of temperature and pressure conditions. However, as mentioned in the preceding paragraph, metamorphism occurs under subsolidus conditions, meaning that elevated temperatures that promote the recrystallization of rocks are not high enough to cause melting.

 

Key Terms

• Contact metamorphism
• Differential pressure
• Dynamic (fault zone) metamorphism
• Foliation
• Gneissic banding
• Hydrothermal metamorphism
• Index mineral
• Lineation
• Lithostatic pressure
• Metamorphism
• Mylonite
• Protolith
• Recrystallization
• Regional metamorphism
• Schistose foliation
• Shock (impact) metamorphism
• Slaty cleavage
• Tektite
• Thermodynamic (dynamothermal) metamorphism