13.10: What Are the Different Metamorphic Environments?
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 Links to an external site. 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 Links to an external site. occurs when sedimentary rocks are buried to depths of several kilometers (more than a mile), and temperatures greater than 300oC (570°F). Burial metamorphism overlaps, to some extent, with diagenesis Links to an external site., 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 Links to an external site. typically are formed in this environment.
Hydrothermal Metamorphism: A variety of regional metamorphism, hydrothermal metamorphism Links to an external site. 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.
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 Links to an external site.; CC-BY-SA 3.0; Jillcurie via Wikipedia Links to an external site.)
Small-Scale Metamorphic Environments
Contact Metamorphism: Contact metamorphism Links to an external site. 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 Links to an external site..
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 Links to an external site. takes place in a matter of seconds. This type of metamorphism occurs at the site of impact craters Links to an external site.. 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 Links to an external site.. Shock metamorphism is characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as coesite Links to an external site. and stishovite Links to an external site.) and textures are characteristic of these conditions.
Fault Zone Metamorphism: Fault zone metamorphism Links to an external site. affects rocks along geologic faults Links to an external site. 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 Links to an external site., or, if the breccia has been chemically altered, there may be a fine clay powder called incohesive fault gouge Links to an external site. (Figure 13.22). Faults at depth, between 4-10 km (2.5-6 mi) will form cohesive fault breccia (cataclasite Links to an external site.). Faults deeper than 10 km (>6 mi) will be affected by ductile deformation and mylonite Links to an external site. is formed. The minerals in mylonitic rocks are deformed due to shear stresses associated with movement along the fault.
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 Links to an external site.; CC-BY 2.0; James St. John via Flickr Links to an external site.)