Metamorphic rock
The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C (300 to 400 °F) and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes.
During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition.
They may be formed simply by being deeply buried beneath the Earth's surface, where they are subject to high temperatures and the great pressure of the rock layers above.
The importance of heating in the formation of metamorphic rock was first noted by the pioneering Scottish naturalist, James Hutton, who is often described as the father of modern geology.
This hypothesis was tested by his friend, James Hall, who sealed chalk into a makeshift pressure vessel constructed from a cannon barrel and heated it in an iron foundry furnace.
French geologists subsequently added metasomatism, the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism.
[18] Many kinds of metamorphic rocks show a distinctive layering called foliation (derived from the Latin word folia, meaning "leaves").
This causes crystals of platy minerals, such as mica and chlorite, to become rotated such that their short axes are parallel to the direction of shortening.
[21] Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated.
A slate is a fine-grained metamorphic rock that easily splits into thin plates but shows no obvious compositional layering.
[23] Special classifications exist for metamorphic rocks with a volcaniclastic protolith or formed along a fault or through hydrothermal circulation.
[23] Special names may also be applied more generally to rocks dominated by a single mineral, or with a distinctive composition or mode or origin.
Special names still in wide use include amphibolite, greenschist, phyllite, marble, serpentinite, eclogite, migmatite, skarn, granulite, mylonite, and slate.
Eskola drew upon the zonal schemes, based on index minerals, that were pioneered by the British geologist, George Barrow.
[27] Metamorphic rock is extensively exposed in orogenic belts produced by the collision of tectonic plates at convergent boundaries.
[32] Eclogites are occasionally found at sites of continental collision, where the subducted rock is rapidly brought back to the surface, before it can be converted to the granulite facies in the hot upper mantle.
They are characterized by low-angle faulting that exposes domes of middle or lower crust metamorphic rock.
These were first recognized and studied in the Basin and Range Province of southwestern North America,[36] but are also found in southern Aegean Sea, in the D'Entrecasteaux Islands, and in other areas of extension.
The rock exposed in the oldest regions of shields, which is of Archean age (over 2500 million years old), mostly belong to granite-greenstone belts.
The greenstone belts contain metavolcanic and metasedimentary rock that has undergone a relatively mild grade of metamorphism, at temperatures of 350–500 °C (662–932 °F) and pressures of 200–500 MPa (2,000–5,000 bar).
[38] The greenstone belts are surrounded by high-grade gneiss terrains showing highly deformed low-pressure, high-temperature (over 500 °C (932 °F)) metamorphism to the amphibolite or granulite facies.
[38] The granite-greenstone belts are intruded by a distinctive group of granitic rocks called the tonalite-trondhjemite-granodiorite or TTG suite.
[30] 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 aureole.
Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away.
[41] The term hornfels is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism.
[42] The contact aureole typically shows little deformation, and so hornfels is usually devoid of schistosity and forms a tough, equigranular rock.
[46] Coesite is rarely found in eclogite brought to the surface in kimberlite pipes, but the presence of stishovite is unique to impact structures.
On August 17, 1959, a magnitude 7.2 earthquake destabilized a mountain slope near Hebgen Lake, Montana, composed of schist.
[49] Metamorphosed ultramafic rock contains serpentine group minerals, which includes varieties of asbestos that pose a hazard to human health.
