Metamorphism

Scientists recognize several types of metamorphism: regional metamorphism, contact metamorphism, dynamic metamorphism, and hydrothermal metamorphism. These occur between the low-temperature process of diagenesis (temperature above 392°F [200°C] and pressure greater than 1,000 bars) and the high temperatures at which rocks melt and later cool to form igneous rocks (approximately 1,112–1,472°F [600–800°C] in temperature and more than 10,000 bars pressure).

Regional metamorphism, the wide-scale alteration of rocks during major tectonic events, can produce spectacular textures and structures in rocks, including folds of layers of rocks, folds of individual minerals (mica, for example), and rotated garnet crystals. Examples of regional metamorphism abound, from Acadia National Park in Maine and the Appalachian Mountains in the eastern part of the United States, to the Llano Uplift of central Texas, and the Precambrian rocks of the Grand Canyon. The Alps of Europe and the Himalayas of Asia also show effects of regional metamorphism.

Contact metamorphism, or thermal metamorphism, occurs when heat from igneous intrusions, melted rocks that move upward, come in contact with cooler rocks above. The cooler rocks do not melt, but recrystallize as a result of heating. The Palisades sill, an igneous intrusion, produced contact metamorphism in the rocks into which it intruded, and is well exposed beneath the George Washington Bridge near New York City.

Dynamic metamorphism occurs along faults that have zones of intense pressure. Rocks along faults grind past each other during faulting. The finely ground rock in the fault can recrystallize under pressure, especially if friction along the fault produces heat or if hot fluids move through the fault.

Hydrothermal metamorphism requires the presence of hot fluids derived from igneous rock nearby. The fluids react with minerals in the surrounding rock to produce different minerals. The metamorphic mineral serpentine forms when dense igneous rocks, such as those in the oceanic crust, metamorphose in the presence of hot fluids to form less dense metamorphic rock called serpentinite.