Schist

Schist (pronounced /ʃɪst/ SHIST) is a medium-grade metamorphic rock.[1] Schist has medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). It is defined by having more than 50% platy and elongated minerals (such as micas or talc),[2] often finely interleaved with quartz and feldspar.[3] These lamellar (flat, planar) minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite.[4] Geological foliation (metamorphic arrangement in layers) with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.[4]

The names of various schists are derived from their mineral constituents. For example, schists primarily composed of biotite and muscovite are called mica schists.[1][5] Most schists are mica schists, but graphite and chlorite schists are also common. Schists are also named for their prominent or perhaps unusual mineral constituents, as in the case of garnet schist, tourmaline schist, and glaucophane schist.

The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen with the naked eye. Schist is characteristically foliated, meaning that the individual mineral grains split off easily into flakes or slabs. The word schist is derived ultimately from the Greek word σχίζειν (schízein) meaning "to split",[6] which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie.

Most schists are derived from clays and muds that have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists are derived from fine-grained igneous rocks such as basalts and tuffs.

Schist detail
Schist specimen showing the characteristic "scaly" schistose texture, caused by platy micas

Historical mining terminology

Before the mid-18th century, the terms slate, shale and schist were not sharply differentiated by those involved with mining.[7]

Formation

During metamorphism, rocks which were originally sedimentary, igneous or metamorphic are converted into schists and gneisses. If the composition of the rocks was originally similar, they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pink mica-schist. Usually, however, it is possible to distinguish between sedimentary and igneous schists and gneisses. If, for example, the whole district occupied by these rocks has traces of bedding, clastic structure, or unconformability, then it may be a sign that the original rock was sedimentary. In other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. The last appeal is often to the chemistry, for there are certain rock types which occur only as sediments, while others are found only among igneous masses, and however advanced the metamorphism may be, it rarely modifies the chemical composition of the mass very greatly. Such rocks as limestones, dolomites, quartzites, and aluminous shales have very definite chemical characteristics that distinguish them even when completely recrystallized.[8]

The schists are classified principally according to the minerals they consist of and on their chemical composition. For example, many metamorphic limestones, marbles, and calc-schists, with crystalline dolomites, contain silicate minerals such as mica, tremolite, diopside, scapolite, quartz and feldspar. They are derived from calcareous sediments of different degrees of purity. Another group is rich in quartz (quartzites, quartz schists and quartzose gneisses), with variable amounts of white and black mica, garnet, feldspar, zoisite and hornblende. These were once sandstones and arenaceous rocks. The graphitic schists may readily be believed to represent sediments once containing coal or plant remains; there are also schistose ironstones (hematite-schists), but metamorphic beds of salt or gypsum are exceedingly uncommon. Among schists of igneous origin there are the silky calc-schists, the foliated serpentines (once ultramafic masses rich in olivine), and the white mica-schists, porphyroids and banded halleflintas, which have been derived from rhyolites, quartz-porphyries and felsic tuffs. The majority of mica-schists, however, are altered claystones and shales, and pass into the normal sedimentary rocks through various types of phyllite and mica-slates. They are among the most common metamorphic rocks; some of them are graphitic and others calcareous. The diversity in appearance and composition is very great, but they form a well-defined group not difficult to recognize, from the abundance of black and white micas and their thin, foliated, schistose character. A subgroup is the andalusite-, staurolite-, kyanite- and sillimanite-schists which usually make their appearance in the vicinity of gneissose granites, and have presumably been affected by contact metamorphism.[8]

Thin section of garnet-mica-schist

Microscopic view of garnet-mica-schist in thin section under polarized light with a large garnet crystal (black) in a matrix of quartz and feldspar (white and gray grains) and parallel strands of mica (red, purple and brown).

Normal View of Garnet-Mica-Schist

View of cut garnet-mica-schist

Schist

Manhattan schist from southeastern New York State

NY-Central-Park-Rock-7333

Manhattan schist outcropping in New York City's Central Park

Talc-schist - Collezione mineralogica - Università dell’Insubria

Talc-scist from Saint-Marcel, France

Engineering considerations

In geotechnical engineering a schistosity plane often forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example, tunnel, foundation, or slope construction.

See also

References

  1. ^ a b "Schist definition". Dictionary of Geology. Retrieved 2013-07-12.
  2. ^ Jackson J.A., Mehl J.P. & Neuendorf K.K.E. (2005). Glossary of Geology. Springer. p. 577. ISBN 9780922152766.
  3. ^ Bishop A.C., Woolley A.R. & Hamilton W.R. (1999). Cambridge Guide to Minerals, Rocks and Fossils. Cambridge University Press. p. 153. ISBN 9780521778817.
  4. ^ a b Essentials of Geology, 3rd Ed, Stephen Marshak
  5. ^ J., Tarbuck, Edward (2012). Earth science. Lutgens, Frederick K. (13th ed.). Upper Saddle River, N.J.: Prentice Hall/Pearson. ISBN 0321688503. OCLC 693684089.
  6. ^ "Schist". English Dictionary. Oxford University Press. Retrieved 10 February 2014.
  7. ^ R. W. Raymond, Slate, A Glossary of Mining and Metallurigical Terms, American Institute of Mining Engineers, 1881, p. 78.
  8. ^ a b  One or more of the preceding sentences incorporates text from a publication now in the public domainFlett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. 21 (11th ed.). Cambridge University Press. p. 333.

External links

  • An Examination of Mica Schist by Andrea Samuels, Micscape magazine. Photographs of Manhattan schist.
  • [1] by USGS: Idaho, Univ. of Idaho, Moscow, articles cited.
Amphibolite

Amphibolite ( ) is a metamorphic rock that contains amphibole, especially the species hornblende and actinolite, as well as plagioclase.

Amphibolite is a grouping of rocks composed mainly of amphibole and plagioclase feldspar, with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.

Amphibolite need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals entirely based upon the chemistry of the protolith, certain 'dirty marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolite (tremolite-schist, grunerite-schist, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalt creates ortho-amphibolite and other chemically appropriate lithologies create para-amphibolite.

Tremolite, while it is a metamorphic amphibole, is derived most usually from highly metamorphosed ultramafic rocks, and thus tremolite-talc schist is not generally considered as 'amphibolite'. A holocrystalline plutonic igneous rock composed primarily of hornblende amphibole is called a hornblendite, which is usually a crystal cumulate rock. Igneous rocks with >90% amphiboles, which have a feldspar groundmass, may be a lamprophyre.

Antietam Formation

The Antietam Formation or Antietam Sandstone is a geologic formation in Pennsylvania, Maryland and West Virginia. It is largely quartz sandstone with some quartzite and quartz schist. It preserves Skolithos trace fossils dating back to the Cambrian Period.

Argillite

"Argillite" may also refer to Argillite, Kentucky.

Argillite ( ) is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts of silt-sized particles. The argillites grade into shale when the fissile layering typical of shale is developed. Another name for poorly lithified argillites is mudstone. These rocks, although variable in composition, are typically high in aluminium and silica with variable alkali and alkaline earth cations. The term pelitic or pelite is often applied to these sediments and rocks. Metamorphism of argillites produces slate, phyllite, and pelitic schist.

Bennett Park (New York City)

Bennett Park, also known as James Gordon Bennett Park, is a public park in New York City, named for James Gordon Bennett, Sr., the newspaper publisher who launched the New York Herald in 1835. It is located between Pinehurst and Fort Washington Avenues and West 183rd and 185th Streets in the Hudson Heights neighborhood of Washington Heights in northern Manhattan, on land purchased by Bennett in 1871, the year before his death. It sits opposite the northern Fort Washington Avenue entrance to the 181st Street subway station on the IND Eighth Avenue Line, serviced by the A train.

The park, which opened in 1929, was built on the site of Fort Washington, from which the Continental Army delayed the advance of British troops in 1776. The commemorative marble, bronze and granite stele, with sculpture by Charles R. Lamb, is located on the eastern perimeter wall of the park, and was dedicated in 1901. In 1932, in commemoration of the bicentennial of the birth of George Washington, the Washington Heights Honor Grove Association planted an American elm tree, which is indicated with a marker. Other memorials in the park include the Emilio Barbosa Memorial, given in 1996 by Joseph Barbosa to honor his father, who died on the USS Nevada at Okinawa in 1945.On the west side of the park lies an outcropping of Manhattan schist which is the highest natural point in Manhattan – 265 feet (81 m) above sea level – with a square stone marker attesting to the fact. The schist is part of the bedrock foundation of New York City, which allows the construction of skyscrapers where it lies close to the surface.The park's playground was constructed in the 1940s, and service buildings were added in 1964.Bennett Park hosts a variety of events, such as the Revolutionary War Reenactment, which Redcoats and George Washington's army actors converge and fight in the park, reenacting the battle of Fort Washington. An annual Harvest Festival is held in the park's field.

As part of the "Northern Manhattan Parks 2030 Master Plan", devised in 2010-11, the playground and comfort station in Bennett Park will be reconfigured to "improve sight-lines and play value." In addition, the condition of the park's perimeter will be improved.

Blueschist

Blueschist ( ), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (200 to ~500 degrees Celsius), approximately corresponding to a depth of 15 to 30 kilometers.

The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.

Blueschists are typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks.

Bündner schist

The Bündner schist or Bündner slate (German: Bündnerschiefer; French: schistes lustrés) is a collective name for schistose rocks that form a number of geologic formations in the Penninic nappes of the Alps. Bündner schists were originally marine sediments that underwent metamorphism at large depths.

The Bündner schists were deposited in the two small oceanic basins (the Valais Ocean and the Piemont-Liguria Ocean) that were located south of the European continent in the Mesozoic era. They formed a kilometers thick monotonous layer of dark clays, marbles and sandy limestones. These sediments were subducted to great depths during the Alpine orogeny. The resulting metamorphism and deformation turned them into calcareous phyllites and schists, strongly foliated rocks rich in micas.

The Bündner schists can be found throughout the Penninic zone of the Alps, often forming zones of high strain between or large infolded synclines (so called Mulde or Mulden) in the crystalline nappes that are made of more competent gneiss.Bündner schists are often found along ophiolites. The contacts between the two types of rock have always seen many phases of folding and are complex.

Foliation (geology)

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from one direction than in others). The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation (orogenic belts).

More technically, foliation is any penetrative planar fabric present in metamorphic rocks. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and gneiss. The slatey cleavage typical of slate is due to the preferred orientation of microscopic phyllosilicate crystals. In gneiss, the foliation is more typically represented by compositional banding due to segregation of mineral phases. Foliated rock is also known as S-tectonite in sheared rock masses.

Examples include the bands in gneiss (gneissic banding), a preferred orientation of planar large mica flakes in schist (Schistocity), the preferred orientation of small mica flakes in phyllite (with its planes having a silky sheen, called phylitic luster – the Greek word, phyllon, also means "leaf"), the extremely fine grained preferred orientation of clay flakes in slate (called "slaty cleavage"), and the layers of flattened, smeared, pancake-like clasts in metaconglomerate.

Gneiss

Gneiss () is a common and widely distributed type of metamorphic rock. Gneiss is formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Orthogneiss is gneiss derived from igneous rock (such as granite). Paragneiss is gneiss derived from sedimentary rock (such as sandstone). Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct foliation.

Greenschist

Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (14,500–58,000 psi). Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and platy minerals such as muscovite and platy serpentine. The platiness causes the tendency to split, or have schistosity. Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).Greenschist is a general field petrologic term for metamorphic or altered mafic volcanic rock. In Europe, the term prasinite is sometimes used. A greenstone is sometimes a greenschist but can also be rock types without any schistosity, especially metabasalt (spilite or picrite). The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the rock. However, basalts may remain quite black if primary pyroxene does not revert to chlorite or actinolite. To qualify for the name a rock must also exhibit schistosity or some foliation or layering. The rock is derived from basalt, gabbro or similar rocks containing sodium-rich plagioclase feldspar, chlorite, epidote and quartz.

Man-Thing (film)

Man-Thing is a 2005 Australian-American horror film directed by Brett Leonard and featuring the Marvel Comics swamp creature the Man-Thing created by Stan Lee, Roy Thomas, and Gerry Conway. The plot is based on a storyline by Steve Gerber, who wrote the best-known series of Man-Thing comics.

Agents of an oil tycoon vanish while exploring a swamp marked for drilling. The local sheriff investigates and faces a Seminole legend come to life: the Man-Thing, a shambling swamp monster.

The film appeared on the Sci Fi Channel in 2005 under the Sci Fi Pictures label. It stars Matthew Le Nevez, Rachael Taylor, and Jack Thompson. The film was released theatrically in a handful of international markets. The film was a box office bomb, grossing only $1 million, and received generally negative reviews from critics.

Manhattan

Manhattan (), often referred to locally as the City, is the most densely populated of the five boroughs of New York City and its economic and administrative center, cultural identifier, and historical birthplace. The borough is coextensive with New York County, one of the original counties of the U.S. state of New York. The borough consists mostly of Manhattan Island, bounded by the Hudson, East, and Harlem rivers; several small adjacent islands; and Marble Hill, a small neighborhood now on the U.S. mainland, physically connected to the Bronx and separated from the rest of Manhattan by the Harlem River. Manhattan Island is divided into three informally bounded components, each aligned with the borough's long axis: Lower, Midtown, and Upper Manhattan.

Manhattan has been described as the cultural, financial, media, and entertainment capital of the world, and the borough hosts the United Nations Headquarters. Anchored by Wall Street in the Financial District of Lower Manhattan, New York City has been called both the most economically powerful city and the leading financial center of the world, and Manhattan is home to the world's two largest stock exchanges by total market capitalization: the New York Stock Exchange and NASDAQ. Many multinational media conglomerates are based in Manhattan, and the borough has been the setting for numerous books, films, and television shows. Manhattan real estate has since become among the most expensive in the world, with the value of Manhattan Island, including real estate, estimated to exceed US$3 trillion in 2013; median residential property sale prices in Manhattan approximated US$1,600 per square foot ($17,000/m2) as of 2018, with Fifth Avenue in Midtown Manhattan commanding the highest retail rents in the world, at US$3,000 per square foot ($32,000/m2) in 2017.Manhattan traces its origins to a trading post founded by colonists from the Dutch Republic in 1624 on Lower Manhattan; the post was named New Amsterdam in 1626. Manhattan is historically documented to have been purchased by Dutch colonists from Native Americans in 1626 for 60 guilders, which equals roughly $1038 in current terms. The territory and its surroundings came under English control in 1664 and were renamed New York after King Charles II of England granted the lands to his brother, the Duke of York. New York, based in present-day Manhattan, served as the capital of the United States from 1785 until 1790. The Statue of Liberty greeted millions of immigrants as they came to the Americas by ship in the late 19th and early 20th centuries and is a world symbol of the United States and its ideals of liberty and peace. Manhattan became a borough during the consolidation of New York City in 1898.

New York County is the United States' second-smallest county by land area (larger only than Kalawao County, Hawaii), and is also the most densely populated U.S. county. It is also one of the most densely populated areas in the world, with a census-estimated 2017 population of 1,664,727 living in a land area of 22.83 square miles (59.13 km2), or 72,918 residents per square mile (28,154/km2), higher than the density of any individual U.S. city. On business days, the influx of commuters increases this number to over 3.9 million, or more than 170,000 people per square mile (65,600/km2). Manhattan has the third-largest population of New York City's five boroughs, after Brooklyn and Queens, and is the smallest borough in terms of land area.Many districts and landmarks in Manhattan are well known, as New York City received a record 62.8 million tourists in 2017, and Manhattan hosts three of the world's 10 most-visited tourist attractions in 2013: Times Square, Central Park, and Grand Central Terminal. The borough hosts many prominent bridges, such as the Brooklyn Bridge; skyscrapers such as the Empire State Building; and parks, such as Central Park. Chinatown incorporates the highest concentration of Chinese people in the Western Hemisphere, and the Stonewall Inn in Greenwich Village, part of the Stonewall National Monument, is considered the birthplace of the modern gay rights movement. The City of New York was founded at the southern tip of Manhattan, and the borough houses New York City Hall, the seat of the city's government. Numerous colleges and universities are located in Manhattan, including Columbia University, New York University, Cornell Tech, Weill Cornell Medical College, and Rockefeller University, which have been ranked among the top 40 in the world.

Mejillones Metamorphic Complex

The Mejillones Metamorphic Complex is made up of two separate outcrops of metamorphic rocks in the Mejillones Peninsula of northern Chile. Turbidites of low metamorphic grade make up the northern outcrop at Morro Mejillones. The southern outcrop lies at Morro Jorgiño and is made up of schist, gneiss, amphibolite and quartzite. Rocks at Morro Jorgiño are intruded by garnet-bearing leucogranites.

Mencia

Mencía is a Spanish grape variety primarily found in the northwestern part of the country. It is planted on over 9,100 hectares (22,000 acres), and it is primarily found in the Bierzo, Ribeira Sacra and Valdeorras regions.Most wines produced from Mencía have traditionally been light, pale, relatively fragrant red wines for early consumption. This style of wine was the result of post-Phylloxera plantations on fertile plains, which tended to give high yields but diluted wine. In recent years, much more concentrated and complex wines have been produced by a new generation of winemakers, primarily from old vines growing on hillsides, often on schist soils, in combination with careful vineyard management. This has led to a renewed interest in Mencía and the Denominaciones de Origen using it, such as Bierzo, Valdeorras, Ribeira Sacra and the little-known Liébana.

Since the 1990s, the grape is increasing in popularity, and an increasing number of noted Spanish winemakers are now working with it.

Metamorphic rock

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides information about the temperatures and pressures that occur at great depths within the Earth's crust.

Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.

Phyllite

Phyllite is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation. It is primarily composed of quartz, sericite mica, and chlorite.Phyllite has fine-grained mica flakes in a preferred orientation, whereas slate has extremely fine mica flakes that achieve a preferred orientation, and schist has large mica flakes in a preferred orientation. Among foliated metamorphic rocks, it represents a gradation in the degree of metamorphism between slate and schist.The minute crystals of graphite, sericite, or chlorite, or the translucent fine-grained white mica, impart a silky, sometimes golden sheen to the surfaces of cleavage, called "phyllitic luster".The word comes from the Greek phyllon, meaning "leaf".The protolith (or parent rock) for phyllite is shale or pelite, or slate, which in turn came from a shale protolith. Its constituent platy minerals are larger than those in slate but are not visible with the naked eye. Phyllites are said to have a texture called "phyllitic sheen," and are usually classified as having formed through low-grade metamorphic conditions through regional metamorphism metamorphic facies.

Phyllite has good fissility (a tendency to split into sheets). Phyllites are usually black to gray or light greenish gray in color. The foliation is commonly crinkled or wavy in appearance.

Phyllite is commonly found in the Dalradian metasediments of northwest Arran. In north Cornwall, there are Tredorn phyllites and Woolgarden phyllites.

Schist Lake (Manitoba)

Schist Lake is located approximately 3 km southeast of Flin Flon. The lake has a maximun depth of 30m. It is composed of four main channels, the West Arm, Northwest Arm, Inlet Arm and Northeast Arm. It drains into Lake Athapapuskow via Schist Creek and is part of the Nelson River watershed. The Flin Flon/Channing Water Aerodrome is located on the north end of the Northwest Arm.

The name comes from the predominant type of metamorphic rock which surrounds the lake, part of the Canadian Shield. It was originally called Manistikiwan which is Cree for "Devil's Head Lake" This name was later applied to nearby Big Island Lake, Manitoba.

Slate

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.The foliation in slate is called "slaty cleavage". It is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression. When expertly "cut" by striking parallel to the foliation, with a specialized tool in the quarry, many slates will display a property called fissility, forming smooth flat sheets of stone which have long been used for roofing, floor tiles, and other purposes. Slate is frequently grey in color, especially when seen, en masse, covering roofs. However, slate occurs in a variety of colors even from a single locality; for example, slate from North Wales can be found in many shades of grey, from pale to dark, and may also be purple, green or cyan. Slate is not to be confused with shale, from which it may be formed, or schist.

The word "slate" is also used for certain types of object made from slate rock. It may mean a single roofing tile made of slate, or a writing slate. They were traditionally a small, smooth piece of the rock, often framed in wood, used with chalk as a notepad or noticeboard, and especially for recording charges in pubs and inns. The phrases "clean slate" and "blank slate" come from this usage.

Vishnu Basement Rocks

The Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks (metamorphic and igneous) exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limestone of the Unkar Group of the Grand Canyon Supergroup and the Tapeats Sandstone of the Tonto Group. These basement rocks have also been called either the Vishnu Complex or Vishnu Metamorphic Complex. These Early Proterozoic crystalline rocks consist of metamorphic rocks that are collectively known as the Granite Gorge Metamorphic Suite; sections of the Vishnu Basement Rocks contain Early Paleoproterozoic granite, granitic pegmatite, aplite, and granodiorite that have intruded these metamorphic rocks, and also, intrusive Early Paleoproterozoic ultramafic rocks.In the past, the term Zoroaster Plutonic Complex was used for all Paleoproterozoic granitic and grandioritic plutonic rocks in the Grand Canyon. This term has been abandoned and specific names have been assigned to individual plutons and dike swarms because the plutons and swarms differ greatly in their age, origin, and tectonic significance. The oldest of these plutonic complexes, Elves Chasm Gneiss, likely represent a small fragment of basement upon which the metavolcanic rocks that comprise the Granite Gorge Metamorphic Suite accumulated. The remainder of the Early Paleoproterozoic granites, granitic pegmatites, aplites, and granodiorites – are parts of either younger plutons or dike swarms, that have intruded the Granite Gorge Metamorphic Suite, either contemporaneously with, or after they were metamorphosed.It was named after a natural rock structure in the Colorado River valley which was named "Temple of Vishnu" from its appearance.

Wissahickon Formation

The Wissahickon Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and Delaware. It is named for the Wissahickon gorge in Fairmount Park, Philadelphia.

In Maryland formations, the term "Wissahickon" is no longer used. Rocks in this classification have since been divided into several units, such as Lower Pelitic Schist and Prettyboy Schist.

Metamorphic petrology
Types of metamorphism
Metamorphic rocks
Metamorphic processes

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