Metamorphism can be defined as the mineralogical, chemical and crystallographic changes in a solid-state rock, i.e. without melting, in response to new conditions of pressure and/or temperature, and/or introduction of fluids.
Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.
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2 Kinds of metamorphism |
The temperature lower limit of metamorphism is considered to be between 100-150°C, to exclude diagenetic changes, due to compaction, which result in sedimentary rocks. There is no agreement as for a pressure lower limit. Some workers argue that changes in atmospheric pressures are not metamorphic. But, see below, some types of metamorphism can occur at extremely lower pressures.
The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The temperature interval is between 700-900°C, with pressures that depend on the composition of the rock. Migmatites are rocks formed on this borderline. They present both melting and solid-state features.
See also: metamorphic rock, metamorphic faciesLimits of metamorphism
Kinds of metamorphism
Regional metamorphism
This type of metamorphism occurs over broad areas of the Earth's crust. Regionally metamorphosed rocks are originated in the core of mountain belts, formed during an orogenic event. These mountains are later eroded exposing the metamorphic rocks. Regional metamorphic rocks are usually strongly deformed. Structural geology is largely based in studies of these rocks because they contain useful information. Regional metamorphism can be described metamorphic zones. It can also be separated into Barrovian- or Buchan-type metamorphism, according to the pressure-temperature gradients recorded in the rocks.Contact metamorphism
Contact metamorphism occurs typically around igneous intrusive rocks, as a result of the temperature increased caused by the igneous body. Pressures are usually low because the contrasting temperature effect is more effective at shallow crustal depths. The area surrounding the igneous rock where the contact metamorphism effects are present is called metamorphic aureole. As expected, the contact metamorphism effects are greater in the vicinity of the intrusive rock and fade away to the exterior of the aureole. Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Rocks formed by contact metamorphism do not present signs of strong deformation and are usually fine grained. Contact metamorphic rocks are usually known as hornfels. Skarns are another example of contact metamorphism and can have great economic interest.Hydrothermal metamorphism
Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of distinct composition. The difference in composition between protolith and fluid triggers a set of methamorphic reactions. This kind of metamorphism is responsible for many economic metal deposits. Convection circulation of water in the ocean floor basalts produces extensive hydrothermal metamorphism.Impact metamorphism
This kind of metamorphism occurs when an extraterrestrial object (a meteorite for instance) collides with the Earth's surface, or, during an extremely violent volcanic eruption. Impact metamorphism is, therefore, characterized by ultrahigh pressures conditions and low temperature. The resulting minerals (such as SiO2 polymorphs coesite and stishovite) and textures are characteristic of these conditions.
