is a common, widespread type of metamorphism typically associated with mountain building, where large segments of Earth’s crust are intensely deformed by the collision of two continental crust blocks (Figure).
Figure – Regional metamorphism is often associated with a continental collision where rocks are squeezed between two converging plates, resulting in mountain building.
Whereas denser oceanic crust subducts under more buoyant continental crust, with the collision of continental crust blocks, two landmasses instead collide and deform. Sediments and crustal rocks that form the margins of the colliding continents are folded and faulted and, as a result, shorten and thicken like a rumpled carpet. Continental collisions may also cause crystalline basement rocks lying under sedimentary layers, as well as slices of oceanic crust that once floored the intervening ocean basin, to be uplifted and deformed.
The general thickening of the crust that occurs during mountain building results in buoyant lifting, in which deformed rocks are elevated high above sea level. Crustal thickening also results in the deep burial of large quantities of rock as one crustal block is thrust beneath another. Deep in the roots of mountains, elevated temperatures caused by deep burial are responsible for the most intense metamorphic activity within a mountain belt.
In some settings, deeply buried rocks become heated beyond their melting points, producing magma.
When these magma bodies grow large enough to buoyantly rise, they intrude the overlying metamorphic and sedimentary rocks (see Figure). Consequently, the cores of many mountain belts consist of folded and faulted metamorphic rocks, often intertwined with igneous bodies. Over time, these deformed rock masses are uplifted, and erosion removes the overlying material to expose the igneous and metamorphic core of the mountain range.
Regional metamorphism produces some of the most common metamorphic rocks.
In this process, shale is metamorphosed to produce the sequences of slate, phyllite, schist, and gneiss. In addition, quartz sandstone and limestone are metamorphosed into quartzite and marble.