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Metamorphic Rocks: Types and Facies

© Geoff Habiger

In our previous discussions on metamorphism we have seen how increases in heat and pressure are the contributing factors to altering igneous or sedimentary rocks into metamorphic rocks. These processes do not always happen in equal amounts; sometimes there is more heat than pressure and vice versa. These differences can result in contact metamorphism or regional metamorphism.

Contact metamorphism occurs when temperature is the dominant factor. As the name implies this is metamorphism that happens when one rock type comes in contact with a high temperature source, usually magma. This type of metamorphism is also called thermal metamorphism. As the magma intrudes the existing country rock it heats the rock to very high temperatures. This creates a 'baked' zone of rock around the magma that is altered by the high temperatures to form new metamorphic rocks. This 'baked' zone is called the aureole and is usually very narrow, only 1 to 50 meters wide. Pressure has little affect on contact metamorphism because it usually takes place close to the earth's surface (less than 10 kilometers deep). Because of the shallow depth there is almost no confining pressure and no differential stresses are applied to the rock leaving most rocks formed by contact metamorphism to be non-foliated.

Types of rocks that are formed by contact metamorphism are those that have no foliation like hornfels, marble, and quartzite.

Regional metamorphism is responsible for most of the metamorphic rocks found on earth and is the result of both high temperatures and confining pressure. Rocks formed through regional metamorphism are almost always foliated, the result of differential stresses when the rock recrystallized. Temperature during regional metamorphism can vary widely and depends on how depth of burial. Typical depths for regional metamorphism are usually deeper than 10 kilometers so pressure plays a more important role.

Regional metamorphism is associated with major mountain building events. When mountains are formed there are large changes in temperature and pressure, the result of the tectonic forces that created the mountains. Often it is possible to determine that mountains once covered an area that is not flat by examining the metamorphic rocks that lie beneath the surface.

The type of rock formed by regional metamorphism is very dependent on small changes in temperature and pressure. A single parent rock can form many different types of metamorphic rocks under different metamorphic conditions. Through extensive field study it has been learned that the chemical composition of metamorphic rocks is quite similar to the parent rock from which they formed. This led, in the early 20th century, to a description of metamorphic rocks based on their mineralogy. This descriptive concept is known as the metamorphic facies. The mineralogy of the facies is controlled by the parent rock and the metamorphic conditions (temperature and pressure). This concept allows a geologist to study widely separated metamorphic rocks and draw accurate conclusions about parent material and the metamorphic conditions. The diagram below shows the basic metamorphic facies. It is not important to memorize the different facies or know by heart the mineral assemblage that makes up the facies. Rather, understanding the concept that the diagram shows us, that by knowing the facies we can infer the approximate temperate and depth at which the metamorphic rock formed.