Contact metamorphism, or thermal metamorphism, occurs in Earth’s upper crust (low pressure), when rocks immediately surrounding a molten igneous body are “baked” (high temperature). Because contact metamorphism does not involve directional stress, the resulting metamorphic rocks are not foliated.
Contact metamorphism alters rocks in a discrete zone adjacent to the heat source, called an aureole (Figure 1).
Small intrusions such as dikes and sills typically form aureoles only a few centimeters thick. By contrast, large molten bodies that eventually cool to form batholiths can produce aureoles that extend outward for several kilometers. These large aureoles often consist of distinct zones of metamorphism.
Close to the magma body, high-temperature minerals such as garnet may form, whereas farther away, lowgrade minerals such as chlorite are produced.
Depending mainly on the composition of the parent rock, a variety of metamorphic rocks can form in the same setting (Figure 8.20).
For example, during contact metamorphism of mudstones and shales, the clay minerals are baked, much like clay is baked in a kiln to make pottery. The result is a very hard, finegrained metamorphic rock called hornfels (see Figure 2).
Hornfels can also form from a variety of other materials, including volcanic ash and basaltic rocks. Other metamorphic rocks that are produced by contact metamorphism are marble and quartzite (see Figure 8.20).
Recall that limestone is the parent of marble and that the metamorphism of quartz sandstone produces quartzite.
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