8.1.2: Contact Metamorphism

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8.6.jpg — Figure 8.6: Contact metamorphism around a pluton

Although regional metamorphism, which accounts for most metamorphism, generally occurs at relatively deep levels within Earth, metamorphism can also occur at shallow levels or even at Earth’s surface. This occurs when magma that intrudes the crust rises close to, or all the way to, the surface. In such cases, heat from the magma can cause contact metamorphism that affects shallow or surface rocks. The effects of contact metamorphism may be profound because of the high temperature contrast between magma temperatures and upper crustal rock temperatures.

As seen in Figure 8.6, contact metamorphism leads to the development of metamorphic zones called contact aureoles, or skarns, that wrap around an intrusion. Aureoles may be anywhere from a few centimeters to many kilometers thick. The formation of contact aureoles frequently involves metasomatism, a change in rock composition due to flowing metamorphic fluids. The width of an aureole mainly depends on the size of the intrusion and how much fluid (mostly H₂O and CO₂) it gives off. Aureoles often develop concentric zones or layers, each containing distinct metamorphic minerals and mineral assemblages that reflect the maximum metamorphic temperature attained and the amount of metasomatism. These zones, too, vary from being quite thin to being kilometers thick.