What Are the Different Metamorphic Environments?

Metamorphic environments are areas or regions where metamorphic rock forms. These environments experience varying conditions – amounts of heat and pressure – which can transform a protolith in to a metamorphic rock.

Large-Scale Metamorphic Environments

Regional Metamorphism: Most regional metamorphism takes place within the continental crust and is commonly associated with convergent plate boundaries and the production of mountains. Regional metamorphism is common during continental-continental convergence, like that which is forming the Himalayan Mountains in Asia, and at subduction zones, like in the Pacific Northwest, and also characterized what-would-become-California during the Mesozoic Era as the Sierra Nevada Mountains were forming (Figure 13.21, left). Commonly, regional metamorphism will produce foliated metamorphic rocks because there is directional pressure; for example, a clay-rich protolith will respond to the pressure by converting the clay to mica, which aligns to produce foliation. Increased temperature will also increase grain size and encourage the development of new index minerals.

Burial Metamorphism: Burial metamorphism occurs when sedimentary rocks are buried to depths of several kilometers (more than a mile), and temperatures greater than 300^oC (570°F). Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase. To the unaided eye, metamorphic changes may not be apparent at all. Rocks like anthracite and metaconglomerate typically are formed in this environment.

Hydrothermal Metamorphism: A variety of regional metamorphism, hydrothermal metamorphism occurs in oceanic crust near divergent boundaries. Here, the rocks are affected by hot, chemically reactive fluids. Typically, soapstone and serpentinite are produced as a result of hydrothermal metamorphism.

Left, cross-section of metamorphic environments. Right, Cross-section of an igneous intrusion. — **Figure 13.21:** *Left,* environments of metamorphism in the context of plate tectonics: (A) regional metamorphism related to mountain building at a continent-continent convergent boundary, (B) regional metamorphism of oceanic crust in the area on either side of a spreading ridge, (C) regional metamorphism of oceanic crustal rocks within a subduction zone, (D) contact metamorphism adjacent to a magma body at a high level in the crust, and (E) regional metamorphism related to mountain building at a convergent boundary. *Right,* contact metamorphism. (CC-BY 4.0; Steven Earle via OpenTextBC; CC-BY-SA 3.0; Jillcurie via Wikipedia)

Small-Scale Metamorphic Environments

Contact Metamorphism: Contact metamorphism occurs as the result of a temperature increase due to nearby magma body or lava flow. Recrystallization due to the increased temperature results in the formation of larger minerals, or sometimes in the formation of new minerals. The rocks closer in contact to the magma will form larger crystals due to the higher heat and may form high temperature index minerals such as garnet, staurolite, kyanite and sillimanite. The area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole (Figure 13.21, right). Since differential pressures are not involved, contact metamorphism typically results in the formation of non-foliated rocks, like marble, quartzite, or hornfels.

Shock Metamorphism: While regional and contact metamorphism require significant time to pass for metamorphism to occur (up to several hundred thousand years), shock (impact) metamorphism takes place in a matter of seconds. This type of metamorphism occurs at the site of impact craters. Upon impact, the rocks in the vicinity are subjected to extreme stresses, and sometimes portions of these surface rocks become tiny blobs of melt that cool to form a type of glass called a tektite. Shock metamorphism is characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as coesite and stishovite) and textures are characteristic of these conditions.

Fault Zone Metamorphism: Fault zone metamorphism affects rocks along geologic faults which are deformed due to pressures associated with shearing, compression, or extensional stresses, with minor changes due to heat. Shallow faults, those close to the earth’s surface and affected primarily by brittle deformation (<4 km or <2.5 mi), may grind the nearby rocks into smaller, angular fragments called incohesive fault breccia, or, if the breccia has been chemically altered, there may be a fine clay powder called incohesive fault gouge (Figure 13.22). Faults at depth, between 4-10 km (2.5-6 mi) will form cohesive fault breccia (cataclasite). Faults deeper than 10 km (>6 mi) will be affected by ductile deformation and mylonite is formed. The minerals in mylonitic rocks are deformed due to shear stresses associated with movement along the fault.

Left, road cut of the San Andreas Fault. Right, a close-up of a red, shiny rock. — **Figure 13.22:** *Left,* San Andreas Fault Zone gouge, in southern California. *Right,* fault gouge sample from the San Andreas Fault Zone in California. (CC-BY 2.0; Michael R. Perry via Flickr; CC-BY 2.0; James St. John via Flickr)