8.1.1: 8.1.1 Regional Metamorphism

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8.3.png — Figure 8.3: Regional metamorphism is more intense at depth

The most significant causes of metamorphism are mountain building processes (tectonism) that bury, while heating and squeezing, rocks. This kind of metamorphism, called regional metamorphism, creates large metamorphic terranes, regions characterized by distinctive metamorphic rocks and intensity of metamorphism that may vary laterally. Regional metamorphism occurs because both pressure and temperature increase with depth in Earth (Figure 8.3). The deeper the rocks, the greater the metamorphism. The photos below show two outcrops of regional metamorphic rocks. The schist outcrop on the left (Figure 8.4) is in Vermont’s Green Mountains; it formed about 450 million years ago during metamorphism associated with the Taconic Orogeny. The gneiss seen in outcrop on the right (Figure 8.5) is much older; it is from Precambrian terrane 750 km northwest of the Green Mountains, near Sudbury, Ontario.

8.4.png — Figure 8.4: Outcrop of schist, Green Mountains, Vermont

8.5.png — Figure 8.5: Outcrop of gneiss near Sudbury, Ontario

Mountain building brings rocks from deep in Earth to the surface. So, many examples of regional metamorphism are found in mountain belts, for example the outcrop in Green Mountains in Figure 8.4, above. Other examples are found in Precambrian shields, relatively flat-lying areas that may be thousands of kilometers across, that are the exposed roots of ancient mountains. The gneiss seen in Figure 8.5 is from the Canadian Shield in central Ontario.