There are two primary types of metamorphism:
Regional Metamorphism occurs on a large scale, typically involving hundreds of square kilometers of surface area. It is the most widespread of the metamorphic types and is typical of the major fold-mountain belts such as the Appalachians, Himalayas, Alps or Rocky Mountains. The metamorphism results from deep burial typically associated with the crustal thickening that results from thrust faulting and folding associated with mountain building processes. As such it is a process that is usually associated with convergent plate tectonic boundaries. In this type of environment, folding and differential stress are common; consequently foliation is a common feature of regionally metamorphosed rocks.
Contact Metamorphism is a local type of metamorphism that develops at the "contact" of hot igneous intrusions and the surrounding rocks into which they intrude. The contact metamorphism is driven entirely by addition of heat into the surrounding rocks. Consequently this type of metamorphism is also referred to as thermal metamorphism. Metamorphism is typically restricted to a thin "aureole" between 1 - 2 km wide adjacent to the pluton or batholith. Since this environment is not associated with strong deformation, the metamorphic rocks in contact aureoles usually do not exhibit foliation.
The single most distinctive feature of regional metamorphic rocks that separates them from both igneous and sedimentary rocks is the presence of a preferred orientation of the individual minerals that comprise the rock. The term used to describe a preferred orientation is foliation. Foliation results from the 1) growth, 2) bending, or 3) rotation of minerals into a parallel orientation. The minerals that are most likely to produce foliation are those such as the micas (sheet-like) or elongated minerals such as amphibole that are physically more stable in a particular orientation relative to stress being placed on the rock. The figure to the left illustrated that micas (green in this drawing) will tend to grow with their flat sheets perpendicular to the maximum compressional stress (arrows). This growth results in all the micas being oriented parallel to each other, which produces the foliation. The lower image is a photomicrograph (taken through a microscope) of a medium-grade metamorphosed shale showing the parallel orientation of micas (blue and green in this image). The yellow mineral is staurolite (image dimensions are approximately 2 x 3 mm).
Another characteristic texture that develops only at the very highest grades of metamorphism is knows as segregation. The segregation typically involves the physical and/or chemical movement of minerals into layers that concentrate like minerals. This typically results in light and dark colored minerals being concentrated in alternating layers. Rocks exhibiting metamorphic segregation are called gneiss. The two images shown to the left illustrate segregation typical of gneisses (digital images courtesy of Martin Miller). The upper image, from the Skagit Gneiss from northern Washington, shows the banding developed by segregation of light- and dark-colored minerals. The lower image shows a typical gneiss with folding of the individual segregations. Such extreme folding is typical of regional metamorphic rocks in the major fold mountain belts.
Classification of Metamorphic Rocks
Metamorphic rocks are named primarily on the basis of their textures and grain size. It turns out that both texture and grain size are functions of the degree, or grade of metamorphism. The grade of metamorphism is primarily controlled by temperature. As temperature increases, the grain size of the minerals in a metamorphic rock increases.
The most common names are derived from the metamorphism of shale. At low grades of metamorphism (typically around 300o C), one of the first chemical reactions to take place place converts clay minerals that formed during weathering, into micas such as muscovite (white mica) and chlorite (green mica). If these mica minerals grow during deformation, they will be aligned with their sheets perpendicular to the maximum compressive stress, thus imparting a foliation to the rock. At these low temperatures, however, the micas do not grow large enough to be seen with the naked eye. You can tell they are there because the rock brakes along the foliation planes. Such a low-grade metamorphic rock exhibiting a foliation, but with minerals too small to be seen with the naked eye is called slate. At higher grades of metamorphism, typically in the temperature range of 400 to 600o C, the micas grow large enough to be clearly visible to the naked eye, and consequently the foliation becomes obvious. These rocks are know as schist. At the very highest grades of metamorphism, typically with temperatures near or above 700o C, metamorphic segregation occurs, and produces rocks known as gneiss (see images above). Adjectives may be applied to these general rock names to indicate either major minerals present or parent material. Some common examples include the following: biotite schist, garnet-staurolite schist, sillimanite schist, granitic gneiss, mafic gneiss, etc.
The minerals that develop in a metamorphic rock depend upon:
|chlorite --> biotite-->||garnet --> staurolite --> kyanite-->||sillimanite|
Many different minerals are found in metamorphosed shales because they are so chemically reactive and undergo a variety of chemical reactions with changing temperature.
In contrast, the metamorphism of quartz sandstone is not very exciting
because the parent material contains no other minerals which might react
with quartz to produce new metamorphic minerals. The table below
gives the minerals and rock name for some other common metamorphic parent
|Sandstone||quartz +/- feldspar||quartzite|
|Limestone||calcite +/- dolomite||marble|
|Basalt||amphibole + plagioclase||(see below)|
|Amphibole Type||Stability Range||Metamorphic Rock Name|
|green amphibole||low temperature and low pressure||Greenschist|
|black amphibole||medium temperature and pressure||Blackschist or "amphibolite"|
|blue amphibole||low temperature and high pressure||Blueschist|
The Transition from Metamorphism to Magmatism
One of the most
important types of chemical reactions that takes place during metamorphism
is a so-called dehydration reaction. These reactions involve
hydrated (OH-bearing) minerals that react to form anhydrous minerals and
release H2O. One of the most important of these is a reaction
that only occurs at the very highest grades of metamorphism (typically
in the range of 700o C). It involves the breakdown of
white mica (muscovite) in the presence of quartz to produce K-feldspar,
sillimanite and H2O. Recall from our earlier discussion,
the normal continental geothermal gradient is not high enough to produce
melting of dry continental crust. However, if H2O is present,
the temperature required for melting is drastically lowered. The
diagram to the left shows the reaction boundary for the muscovite + quartz
breakdown, the "dry" melting boundary and the melting curve for "wet" melting.
Note that the metamorphic reaction intersects the wet melting curve at
temperatures of about 700o C and pressures of about 5000 atms.
Dry crust will not begin to melt at these conditions. However, if
metamorphic rocks undergo this reaction, H2O is released, fluxes
the crust and produces melting. This mechanism is responsible for
partial melting in the deep levels of the continental crust to produce
The Rock Cycle
Over the past several weeks, we have seen that Earth material can start out as igneous rock, become weathered, eroded, transported and deposited as sediment. This sediment can become lithified into sedimentary rocks which can subsequently be metamorphosed. If metamorphism is of sufficient intensity the rock can be melted and once again become an igneous rock. The interrelationships of the various rock types and the processes which relate the rock types to each other are shown in the following diagram, known as the rock cycle, or rock recycle (yes, Mother Nature recycles). The cycle is driven by both internal processes (heat and convection) and external processes (weathering, transportation, etc.), and will continue until the Earth cools sufficiently to no longer have a dynamic interior.