14.5.4: Metamorphic Rocks
Metamorphic rocks are formed by the alteration of pre-existing rocks from exposure to heat and pressure while remaining in a solid form. Metamorphism occurs by breaking bonds between atoms in a mineral so that the atoms rearrange themselves into new, more stable, mineral forms. Rocks are transformed and remain in a solid state because not all the bonds in the rock's minerals are broken - if they were the rock would melt. Metamorphism occurs in solid rock because only some of the bonds between atoms are broken in an unstable mineral. As a result, the freed atoms and ions can migrate to another location within the mineral, or bond with atoms in a different mineral. The end result is to produce minerals that are more stable under the environmental conditions in which they exist.
Metamorphism involves the transformation of a pre-existing rock to form new minerals and textures. The original mineral content of a rock can change in several ways. Unstable minerals like clays will breakdown and their elements will recombine to form new minerals. More stable minerals like quartz, will stay quartz but change shape and size to form a new configuration. At high temperatures, atoms and ions may move into a new orientation and bond into more stable forms. Hence, the type of minerals and its texture may change but the chemical composition of the rock itself can stay the same.
Rocks buried deep beneath the Earth are exposed to lithostatic pressure , the confining pressure created by the material that sits above a particular location. Lithostatic pressure is equal in all directions and compresses the volume of rock into a denser material. At the contacts between mineral grains, the pressure breaks the bonds between atoms allowing them to migrate toward regions of less pressure where they rebind with other atoms.
Along the boundary of tectonic plates where collision or subduction is occurring, directed pressure is exerted on rock. Under these circumstances, pressure is imposed in a particular direction. Directed pressure flattens and lengthens the rock in the direction of greatest pressure. The pressure is not great enough to affect new mineralization however. Instead, directed pressure affects the shape and arrangement the minerals. Under great pressure, mineral grains may be smeared or partially melted and recrystallized into bands aligned perpendicular to the direction of greatest pressure. This creates foliated metamorphic rocks with minerals in distinct bands. Where the pressure is not directed, nonfoliated metamorphic rocks are formed and lack the banding of minerals typical of foliated rocks.
|
Rock |
Parent Rock |
Key Minerals |
Metamorphic Environment |
|
|
Foliated |
Slate |
Shale |
Clay minerals, micas. chlorite. graphite |
Relatively low temperature and pressure |
|
Foliated |
Schist |
Shale, basalt, graywacke sandstone, impure limestone |
Mica, chlorite, garnet, talc, epdiodte, hornblende, graphite, staurite, kyanite |
Intermediate - to - high temperature and pressure |
|
Foliated |
Gneiss |
Shale, felsic igneous rocks, graywacke, sandstone, granite, impure limestone |
Garnet, mica, augite, hornblende, staurolite, kyanite, sillimanite |
High Temperature and pressure |
|
Nonfoliated |
Marble |
Pure limestone |
calcite, dolomite |
Contact with hot magma, or confining pressure from deep burial |
|
Nonfoliated |
Quartzite |
Pure sandstone, chert |
Quartz |
Contact with hot magma, or confining pressure from deep burial |
Occurrence of metamorphism
Metamorphism occurs under a variety of different conditions that controls the geographic distribution of metamorphic rocks and their significance to earth surface features. When magma intrudes into host rock, localized contact metamorphism occurs along the contact between the pre-existing rock mass and the cooling pluton. The heat introduced by the intruding magma controls the degree of metamorphism. Contact metamorphism occurs under low to moderate pressure and low to high temperature conditions. Temperatures of metamorphism vary widely from 400-1000°C. The amount of metamorphism is governed by a variety of factors, among which are the differences between the temperature of the pluton and the country rock, the heat capacity and conductivity of both magma and country rock. Hydrothermal fluids circulating through the surrounding rock are also important in metamorphosing the rock as they transport heat. Fluids are particularly important in the metamorphism of carbonate rocks.
Typically, contact metamorphism occurs at shallower levels of the crust, where the pressure is relatively low. At those shallow depths, the stresses characteristic of orogenic belts are generally small or absent thus producing metamorphic rocks that lack foliation. Contact metamorphism commonly produces fine-grained rocks. The metamorphosed rock surrounding the body of magma along the zone of contact is called an aureole. Many profitable mines are situated in metal-rich aureoles formed by contact metamorphism.
Regional metamorphism occurs over broad areas of the crust. There are two basic kinds of regional metamorphism, dynamothermal metamorphism and burial metamorphism . Dynamothermal metamorphism occurs in areas that have undergone deformation during mountain building that have since been eroded to expose the metamorphic rocks. It is caused by the differential stress resulting from plate subduction or collision along plate boundaries. Regional metamorphism occurs in a linear belt in the plate overriding the subducting one due to increasing temperature and pressure as a result of compression, thrusting, folding, and intrusion of magmas from below.
Burial metamorphism occurs in deep basins where sediments or sedimentary rocks have accumulated. At a depth of about 10 kilometers, the confining pressure of the overlying material combined with geothermal heat is great enough to metamorphose rocks. Because the compression does not impose a directed pressure, metamorphic rocks formed from burial are unfoliated.