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Earth Materials and Structure

Occurrence of metamorphism

Metamorphism occurs under a variety of different conditions that controls the geographic distribution of metamorphic rocks and their significance to earth surface features. When magma intrudes into host rock, localized contact metamorphism  occurs along the contact between the pre-existing rock mass and the cooling pluton. The heat introduced by the intruding magma controls the degree of metamorphism. Contact metamorphism occurs under low to moderate pressure and low to high temperature conditions. Temperatures of metamorphism vary widely from 400-1000°C. The amount of metamorphism is governed by a variety of factors, among which are the differences between the temperature of the pluton and the country rock, the heat capacity and conductivity of both magma and country rock. Hydrothermal fluids circulating through the surrounding rock are also important in metamorphosing the rock as they transport heat. Fluids are particularly important in the metamorphism of carbonate rocks.

Typically, contact metamorphism occurs at shallower levels of the crust, where the pressure is relatively low. At those shallow depths, the stresses characteristic of orogenic belts are generally small or absent thus producing metamorphic rocks that lack foliation. Contact metamorphism commonly produces fine-grained rocks. The metamorphosed rock surrounding the body of magma along the zone of contact is called an aureole.  Many profitable mines are situated in metal-rich aureoles formed by contact metamorphism.

Regional metamorphism occurs over broad areas of the crust. There are two basic kinds of  regional metamorphism, dynamothermal metamorphism and burial metamorphism. Dynamothermal metamorphism occurs in areas that have undergone deformation during mountain building that have since been eroded to expose the metamorphic rocks. It is caused by the differential stress resulting from plate subduction or collision along plate boundaries. Regional metamorphism occurs in a linear belt in the plate overriding the subducting one due to increasing temperature and pressure as a result of compression, thrusting, folding, and intrusion of magmas from below. 

Burial metamorphism occurs in deep basins where sediments or sedimentary rocks have accumulated. At a depth of about 10 kilometers, the confining pressure of the overlying material combined with geothermal heat is great enough to metamorphose rocks. Because the compression does not impose a directed pressure, metamorphic rocks formed from burial are unfoliated.

Assess your basic understanding of the preceding material by "Looking Back at Minerals and Rocks" or continue reading.

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