Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form" (from the Greek prefix meta, "after", and the noun morphe, "form"). The protolith is subjected to heat (greater than 150 degrees Celsius) and extreme pressure causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock.
Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assembly (metamorphic facies). They are formed deep beneath the Earth's surface by great stresses from rocks above and high pressures and temperatures. They are also formed by the intrusion of molten rock, called magma, into solid rock and form particularly at the place of contact between the magma and solid rock where the temperatures are high.
The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides us with valuable information about the temperatures and pressures that occur at great depths within the Earth's crust.
Some examples of metamorphic rocks are slate, gneiss and schist.
Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals include kyanite, staurolite, sillimanite, andalusite, and some garnets.
Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz, may be found in metamorphic rocks, but are not necessarily the result of the process of metamorphism. These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are stable only within certain limits, and the presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they were formed.
The change in the particle size of the rock during the process
The layering within metamorphic rocks is called foliation (derived from the Latin word folia, meaning "leaves"), and it occurs when a strong compressive force is applied from one direction to a recrystallizing rock. This causes the platy or elongated crystals of minerals, such as mica and chlorite, to grow with their long axes perpendicular to the direction of the force. This results in a banded, or foliated, rock, with the bands showing the colours of the minerals that formed them.
Textures are separated into foliated and non-foliated categories. Foliated rock is a product of differential stress that deforms the rock in one plane, sometimes creating a plane of cleavage: for example, slate is a foliated metamorphic rock, originating from shale. Non-foliated rock does not have planar patterns of stress.
Rocks that were subjected to uniform pressure from all sides, or those which lack minerals with distinctive growth habits, will not be foliated. Slate is an example of a very fine-grained, foliated metamorphic rock, while phyllite is coarse, schist coarser, and gneiss very coarse-grained. Marble is generally not foliated, which allows its use as a material for sculpture and architecture.
Another important mechanism of metamorphism is that of chemical reactions that occur between minerals without them melting. In the process atoms are exchanged between the minerals, and thus new minerals are formed. Many complex high-temperature reactions may take place, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism.
Metasomatism is the drastic change in the bulk chemical composition of a rock that often occurs during the process of metamorphism. It is due to the introduction of chemicals from other surrounding rocks. Water may transport these chemicals rapidly over great distances. Because of the role played by water, metamorphic rocks generally contain many elements that were absent from the original rock, and lack some which were originally present. Still, the introduction of new chemicals is not necessary for recrystallization to occur.
Types of metamorphism
Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock (country rock). The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole. Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away. The formation of important ore minerals may occur by the process of metasomatism at or near the contact zone.
Regional metamorphism is the name given to changes in great masses of rock over a wide area, often within orogenic belts. The high temperatures and pressures in the depths of the Earth are the cause of the changes, and if the metamorphosed rocks are uplifted and exposed by erosion, they may occur over vast areas at the surface. The process of metamorphism may have destroyed the original features that could have revealed the rock's previous history. Recrystallization of the rock will destroy the textures and fossils present in sedimentary rocks. Metasomatism will change the original composition.
Metamorphic rock textures
The five basic metamorphic textures with typical rock types are:
Slaty: slate and phyllite; the foliation is called 'slaty cleavage'
Schistose: schist; the foliation is called 'schistocity'
Gneissose: gneiss; the foliation is called 'gneisocity'
Grandoblastic: granulite, some marbles and quartzite
Hornfelsic: hornfels and skarn