Matrix (geology)

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Orthoclase phenocrysts within a finer-grained matrix of a granite porphyry FeldsparsGranite.JPG
Orthoclase phenocrysts within a finer-grained matrix of a granite porphyry

The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.

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The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, called phenocrysts, are embedded. This porphyritic texture is indicative of multi-stage cooling of magma. For example, porphyritic andesite will have large phenocrysts of plagioclase in a fine-grained matrix. Also in South Africa, diamonds are often mined from a matrix of weathered clay-like rock (kimberlite) called "yellow ground".

The matrix of sedimentary rocks is finer-grained sedimentary material, such as clay or silt, in which larger grains or clasts are embedded. It is also used to describe the rock material in which a fossil is embedded.

Cementation

All sediments are at first in an incoherent condition (e.g. sands, clays and gravels, beds of shells), and they may remain in this state for an indefinite period. Millions of years have elapsed since some of the early Tertiary strata gathered on the ocean floor, yet they are quite friable (e.g. the London Clay) and differ little from many recent accumulations. There are few exceptions to the rule that with increasing age sedimentary rocks become more and more indurated. Generally, the older the strata the more likely it is that they will have the firm consistency generally implied in the term "rock". [1]

The pressure of newer sediments on underlying masses is apparently one cause of this hardening, though not in itself a very powerful one. More efficiency is generally ascribed to the action of percolating water, which takes up water-soluble materials and then redeposits them in pores and cavities. This operation is probably accelerated by the increased pressure produced by superincumbent masses, and to some extent also by the rise of temperature which inevitably takes place in rocks buried to some depth beneath the surface. The rise of temperature, however, may be only a tiny increase; we know more than one instance of sedimentary deposits which have been buried beneath four or five miles of similar strata (e.g. parts of the Old Red Sandstone), and yet no perceptible difference is apparent between beds of similar composition at the top of the series and near its base. Should the deposits have been truly "baked", that is, subject to a large increase in temperature, then differences would be evident. [1]

The redeposited cementing material is most commonly calcareous or siliceous. Limestones, which were originally a loose accumulation of shells, corals, etc., become compacted into firm rock in this manner; and the process often takes place with surprising ease, as for example, in the deeper parts of coral reefs, or even in wind-blown masses of shelly sand exposed merely to the action of rain. The cementing substance may be regularly deposited in crystalline continuity on the original grains, where these were crystalline, and even in sandstones (such as Kentish rag), a crystalline matrix of calcite often envelops the sand grains. The change of aragonite to calcite and of calcite to dolomite, by forming new crystalline masses in the interior of the rock, usually also accelerates consolidations. Silica is less easily soluble in ordinary waters, but even this ingredient of rocks is dissolved and redeposited with great frequency. Many sandstones are held together by an infinitesimal amount of colloid or cryptocrystalline silica; when freshly dug from the quarry they are soft and easily trimmed, but after exposure to the air for some time they become much harder, as their siliceous cement sets and passes into a rigid condition. Others contain fine scales of kaolin or of mica. Argillaceous materials may be compacted by mere pressure, like graphite and other scaly minerals. [1]

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<span class="mw-page-title-main">Limestone</span> Type of sedimentary rock

Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.

<span class="mw-page-title-main">Sandstone</span> Type of sedimentary rock

Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and cementation of particles

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

<span class="mw-page-title-main">Chert</span> Hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

<span class="mw-page-title-main">Volcanic rock</span> Rock formed from lava erupted from a volcano

Volcanic rocks are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "pyroclastics," and these are also technically sedimentary rocks.

<span class="mw-page-title-main">Lithology</span> Description of its physical characteristics of a rock unit

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.

<span class="mw-page-title-main">Conglomerate (geology)</span> Sedimentary rock composed of smaller rock fragments

Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.

<span class="mw-page-title-main">Greywacke</span> Sandstone with angular grains in a clay-fine matrix

Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume.

<span class="mw-page-title-main">Quartz-porphyry</span> Type of volcanic rock containing large porphyritic crystals of quartz

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.

The Folk classification, in geology, is a technical descriptive classification of sedimentary rocks devised by Robert L. Folk, an influential sedimentary petrologist and Professor Emeritus at the University of Texas.

<span class="mw-page-title-main">Clastic rock</span> Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.

<span class="mw-page-title-main">Texture (geology)</span>

In geology, texture or rock microstructure refers to the relationship between the materials of which a rock is composed. The broadest textural classes are crystalline, fragmental, aphanitic, and glassy. The geometric aspects and relations amongst the component particles or crystals are referred to as the crystallographic texture or preferred orientation. Textures can be quantified in many ways. The most common parameter is the crystal size distribution. This creates the physical appearance or character of a rock, such as grain size, shape, arrangement, and other properties, at both the visible and microscopic scale.

<span class="mw-page-title-main">Igneous textures</span> Property of igneous rocks

Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin of igneous rocks and are used in rock classification. The six main types of textures are phaneritic, aphanitic, porphyritic, glassy, pyroclastic, and pegmatitic.

The Mount Pleasant Caldera is a large eroded Late Devonian volcanic caldera complex, located in the northern Appalachian Mountains of southwestern New Brunswick, Canada. It is one of few noticeable pre-Cenozoic calderas, and its formation is associated to a period of crustal thinning that followed the Acadian orogeny in the northern Appalachian Mountains. It sits relatively near to the coastline.

<span class="mw-page-title-main">Case hardening of rocks</span>

Case hardening is a weathering phenomenon of rock surface induration. It is observed commonly in: felsic alkaline rocks, such as nepheline syenite, phonolite and trachyte; pyroclastic rocks, as pyroclastic flow deposit, fine air-fall deposits and vent-filling pyroclastic deposits; sedimentary rocks, as sandstone and mudstone.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

<span class="mw-page-title-main">Cementation (geology)</span> Process of chemical precipitation bonding sedimentary grains

Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms "bridges" between original sediment grains, thereby binding them together. In this way, sand becomes sandstone, and gravel becomes conglomerate or breccia. Cementation occurs as part of the diagenesis or lithification of sediments. Cementation occurs primarily below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process. Common mineral cements include calcite, quartz, and silica phases like cristobalite, iron oxides, and clay minerals; other mineral cements also occur.

<span class="mw-page-title-main">Geology of Cape Town</span> Geological formations and their history in the vicinity of Cape Town

Cape Town lies at the south-western corner of the continent of Africa. It is bounded to the south and west by the Atlantic Ocean, and to the north and east by various other municipalities in the Western Cape province of South Africa.

<span class="mw-page-title-main">Vasquez Formation</span> Late Oligocene to Early Miocene sedimentary formation in the Sierra Pelona of California

The Vasquez Formation (Tvz) is a geologic formation cropping out at the eponymous Vasquez Rocks in southern California. The formation dates to the Late Oligocene to Early Miocene.

References

  1. 1 2 3 Wikisource-logo.svg One or more of the preceding sentences incorporates text from a publication now in the public domain : Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica . Vol. 21 (11th ed.). Cambridge University Press. p. 332.