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22-529: The mountains are made of schist , part of the Nason terrane . The original rock in the Nason terrane was laid down 210 Myr ago. The terrane then smashed into the western coastline of the continent, and the Mount Stuart batholith intruded into it in the late Cretaceous . The intrusion of the granitic rock metamorphosed the rock into schist. In July 2014, the lightning-caused Chiwaukum Creek Fire burned

44-429: 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. A hazard may exist even in undisturbed terrain. On August 17, 1959, a magnitude 7.2 earthquake destabilized a mountain slope near Hebgen Lake , Montana, composed of schist. This caused a massive landslide that killed 26 people camping in

66-553: A schistose metasandstone (if the protolith is known to have been a sandstone ). If all that is known is that the protolith was a sedimentary rock, the schist will be described as a paraschist , while if the protolith was an igneous rock, the schist will be described as an orthoschist . Mineral qualifiers are important when naming a schist. For example, a quartz-feldspar-biotite schist is a schist of uncertain protolith that contains biotite mica, feldspar , and quartz in order of apparent decreasing abundance. Lineated schist has

88-403: A porphyroblast grows over a developing microfold while maintaining a stable position in the external reference frame. Repetition of this process can then produce complex spiral-shaped patterns. Although many researchers continue to adopt the classic rotational model, most researchers who have published research testing both models by measuring the orientations of porphyroblasts have come to support

110-494: A preferred orientation. Schists make up one of the three divisions of metamorphic rock by texture , with the other two divisions being gneiss , which has poorly developed schistosity and thicker layering, and granofels , which has no discernible schistosity. Schists are defined by their texture without reference to their composition, and while most are a result of medium-grade metamorphism, they can vary greatly in mineral makeup. However, schistosity normally develops only when

132-412: A strong linear fabric in a rock which otherwise has well-developed schistosity. Schistosity is developed at elevated temperature when the rock is more strongly compressed in one direction than in other directions ( nonhydrostatic stress ). Nonhydrostatic stress is characteristic of regional metamorphism where mountain building is taking place (an orogenic belt ). The schistosity develops perpendicular to

154-408: A very large class of metamorphic rock, geologists will formally describe a rock as a schist only when the original type of the rock prior to metamorphism (the protolith ) is unknown and its mineral content is not yet determined. Otherwise, the modifier schistose will be applied to a more precise type name, such as schistose semipelite (when the rock is known to contain moderate amounts of mica) or

176-406: Is a thin layering of the rock produced by metamorphism (a foliation ) that permits the rock to easily be split into flakes or slabs less than 5 to 10 millimeters (0.2 to 0.4 in) thick. The mineral grains in a schist are typically from 0.25 to 2 millimeters (0.01 to 0.08 in) in size and so are easily seen with a 10× hand lens . Typically, over half the mineral grains in a schist show

198-490: Is derived ultimately from the Greek word σχίζειν ( schízein ), meaning "to split", which refers to the ease with which schists can be split along the plane in which the platy minerals lie. Before the mid-19th century, the terms slate , shale and schist were not sharply differentiated by those involved with mining. Geologists define schist as medium-grained metamorphic rock that shows well-developed schistosity. Schistosity

220-402: Is described as having a porphyroblastic texture . As porphyroblasts grow, the foliation may be preserved as oriented inclusions trapped by the porphyroblast as it overgrows them, and this is helpful for tracking changing deformation planes. In metamorphic rocks that experience deformation during metamorphism, porphyroblasts may grow before, during, or after the phase of deformation recorded by

242-494: Is particularly common and is often very rich in mica (a mica schist ). Where the type of the original rock (the protolith ) is discernible, the schist is usually given a name reflecting its protolith, such as schistose metasandstone . Otherwise, the names of the constituent minerals will be included in the rock name, such as quartz-felspar-biotite schist . Schist bedrock can pose a challenge for civil engineering because of its pronounced planes of weakness . The word schist

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264-482: Is the orientation of grains in the surrounding medium-grained rock. The composition of the rock must permit formation of abundant platy minerals. For example, the clay minerals in mudstone are metamorphosed to mica, producing a mica schist. Early stages of metamorphism convert mudstone to a very fine-grained metamorphic rock called slate , which with further metamorphism becomes fine-grained phyllite . Further recrystallization produces medium-grained mica schist. If

286-418: Is uncommon but can form from metamorphosis of sedimentary beds containing abundant organic carbon . This may be of algal origin. Graphite schist is known to have experienced greenschist facies metamorphism , for example in the northern Andes . Metamorphosis of felsic volcanic rock , such as tuff, can produce quartz- muscovite schist. In geotechnical engineering a schistosity plane often forms

308-479: The Earth's surface during metamorphism and deformation has long been the subject of debate. The question focused on so-called "spiral garnets" , also known as "snowball garnets" , whose inclusion trails define spiral patterns. These microstructures are interpreted classically as having formed by shearing induced rotation of a growing garnet crystal. Later research, however, led to an alternative formation model in which

330-468: The area. Porphyroblast A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix . Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyroblasts in metapelites (metamorphosed mudstones and siltstones ) are garnets and staurolites , which stand out in well- foliated metapelites (such as schists ) against

352-456: The direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers. While platy or elongated minerals are most obviously reoriented, even quartz or calcite may take up preferred orientations. At the microscopic level, schistosity is divided into internal schistosity , in which inclusions within porphyroblasts take a preferred orientation, and external schistosity , which

374-471: The east flank of the Chiwaukum Mountains. Download coordinates as: Partial list of peaks: Nearby peaks: Schist Schist ( / ˈ ʃ ɪ s t / SHIST ) is a medium-grained metamorphic rock showing pronounced schistosity (named for the rock). This means that the rock is composed of mineral grains easily seen with a low-power hand lens , oriented in such a way that

396-475: The matrix minerals. The relationship of porphyroblast growth to deformation is typically evaluated by comparing the shape orientation of trails of mineral inclusions in the porphyroblast to the matrix fabric. Some garnet porphyroblasts contain curving trails of quartz and other mineral inclusions that record rotation of the crystals relative to their surroundings. However, the question of how much porphyroblasts actually rotate in an external reference frame fixed to

418-527: The metamorphism proceeds further, the mica schist experiences dehydration reactions that convert platy minerals to granular minerals such as feldspars, decreasing schistosity and turning the rock into a gneiss. Other platy minerals found in schists include chlorite, talc, and graphite. Chlorite schist is typically formed by metamorphism of ultramafic igneous rocks, as is talc schist. Talc schist also forms from metamorphosis of talc-bearing carbonate rocks formed by hydrothermal alteration . Graphite schist

440-485: The platy mica matrix. A similar type of crystal is a phenocryst , a large crystal in an igneous rock . Porphyroblasts are often confused with porphyroclasts , which can also be large outstanding crystals, but which are older than the matrix of the rock . If a porphyroblastic mineral has small inclusions of minerals within it, the mineral is described as poikiloblastic . This observation can help interpret deformation history. A rock which has many porphyroblasts

462-539: The rock contains abundant platy minerals, such as mica or chlorite . Grains of these minerals are strongly oriented in a preferred direction in schist, often also forming very thin parallel layers. The ease with which the rock splits along the aligned grains accounts for the schistosity. Though not a defining characteristic, schists very often contain porphyroblasts (individual crystals of unusual size) of distinctive minerals, such as garnet , staurolite , kyanite , sillimanite , or cordierite . Because schists are

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484-593: The rock is easily split into thin flakes or plates. This texture reflects a high content of platy minerals, such as mica , talc , chlorite , or graphite . These are often interleaved with more granular minerals, such as feldspar or quartz . Schist typically forms during regional metamorphism accompanying the process of mountain building ( orogeny ) and usually reflects a medium grade of metamorphism. Schist can form from many different kinds of rocks, including sedimentary rocks such as mudstones and igneous rocks such as tuffs . Schist metamorphosed from mudstone

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