Bedrock

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Bedrock

Bedrock (also termed Bed rock ) is a layer of undisturbed rock usually located beneath a surface layer of soil or other material. In areas of high erosion , bedrock may become exposed to the surface. Bedrock can be of igneous, sedimentary, or metamorphic origin and forms the upper surface of the rocky foundation that composes the earth's crust .

A surface exposure of bedrock is called an outcrop. Bedrock is only rarely exposed, or crops out, where sediment accumulates rapidly, for example, in the bottom of stream valleys and at the base of hills or mountains. Outcrops are common where erosion is rapid, for example, along the sides of steep stream channels and on steep hill or mountain slopes. Deserts and mountain tops above the treeline also host good bedrock exposures due to the scarcity of vegetation, and resulting rapid erosion. Man-made outcrops are common where roadways cut through mountains or hilltops, in quarries, and in mines

Generally, the more rock resists erosion, the more likely it is to crop out. Granite and sandstone commonly form well-exposed outcrops. Natural exposures of shale and claystone, both soft, fine-grained rocks, are rareespecially in humid climates.

In addition to the occasional mineral crystal or fossils , all outcrops contain through-going fractures called joints. These form during the application of stresses to bedrock on a regional scale, for example, during mountain building. Even greater stresses may cause faulting movement of the rock on the sides of a fracture. An example is the large-scale bedrock movement that occurs along the San Andreas Fault in California. When stresses cause plastic rather than brittle deformation of bedrock, it folds rather than faulting.

Bedrock is distributed in a predictable pattern. Generally in the central area of a continent, geologists find very ancient (one billion years or more) mountain chains , consisting of igneous and metamorphic rock , eroded to an almost flat surface. This area, called a continental shield, typically contains the oldest continental bedrock. Shields have experienced multiple episodes of deformation so they are intensely folded and faulted. These ancient igneous and metamorphic rocks, called basement rocks, compose much of the continental crust. However, on the shield margins, thick sequences of relatively undeformed, sedimentary rocks cover the basement rocks. These deposits, called the continental platform, commonly exceed 1 mi (1.6 km) in thickness and 100 million years in age.

Together, the shield and platform make up the bedrock area known as the continental craton . The craton is considered more or less stable, that is, it is not currently experiencing significant deformation. On the margins of the craton, there may be areas of geologically active bedrock, called orogens, from the Greek word for mountain. Orogens are relatively young mountain belts where uplift, folding, faulting, or volcanism occurs. The bedrock here varies in age from lava flows that may be only days old to igneous, sedimentary, and metamorphic rock that are hundreds of millions of years old. All bedrock belongs to the continental shield, platform, or the orogens.

See also Earth, interior structure; Faults and fractures; Pluton and plutonic bodies; Soil and soil horizons; Weathering and weathering series

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Bedrock

Bedrock exposures

Outcrop features

Bedrock distribution

Bedrock is hard rock exposed at Earths surface or buried beneath loose sediment sometimes referred to as regolith. It can be of igneous, sedimentary or meta-morphic origin.

Bedrock exposures

A surface exposure of bedrock is called an outcrop. Bedrock is generally not exposed in areas where sediment accumulates rapidly, for example in the bottoms of stream valleys and at the bases of hills. Outcrops are common, however, where erosion is rapid, for example, along the sides of steep stream channels and on steep hillsides. Deserts and mountain-tops above the treeline commonly contain bedrock exposures because vegetation is scarce and erosion is rapid. Manmade outcrops are common where roads cut through mountains or hills as well as in quarries and mines.

Generally, the more resistant bedrock is erosion, the more likely it is to be exposed. Granite and sandstone commonly form well-exposed outcrops. Natural exposures of shale, a soft sedimentary rock, can be uncommon, especially in wet climates.

Outcrop features

In addition to the mineral crystals and fossils, virtually all outcrops contain breaks that are classified according to the movement of rock on either side of the break. If the rocks on each side move away from each other and perpendicular to the break, even imperceptibly, the break is known as a joint. If the rocks slide past each other parallel to the break, the break is known as a fault. Joints and faults can serve as conduits for groundwater and may give rise to springs. Large blocks of rock can also break away from an outcrop along joints and faults to create rockslides or rock falls. Layered rocks, most commonly sedimentary rocks or some kinds of metamorphic rocks, may also be flexed or folded in response to tectonic forces such as those involved in mountain building. In some cases entire folds are visible in single outcrops.

Bedrock distribution

Bedrock is distributed in a fairly predictable pattern. The central portions of continents generally contain metamorphic rock representing the remains of ancient (one billion years or more) mountain ranges that have been eroded to create a flat surface. Such areas are known as continental shields (for example, the Canadian shield). The rocks in shields, which are sometimes referred to as basement rocks, have experienced multiple episodes of deformation so they are intensely folded and faulted. Along the edges of shields, however, thick sequences of relatively unde-formed sedimentary rocks cover the basement rocks and form features known as continental platforms.

Together, the shield and platform make up the continental craton. The craton is considered to be stable, meaning that it is not currently experiencing significant deformation. On the margins of the craton, there may be actively growing mountain belts known as orogens. Orogens are relatively young mountain belts in which uplift, folding, faulting, or volcanism are occurring. The bedrock in orogens varies in age from lava flows that may be only days old to igneous, sedimentary, and metamorphic rock that are hundreds of millions of years old.

Clay Harris

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Bedrock

Bedrock is the solid rock that is exposed at the earth's surface, or buried beneath one or more layers of loose sediment. It is of igneous, sedimentary or metamorphic origin and forms the upper surface of the rocky foundation that composes the earth's crust.

Bedrock exposures

A surface exposure of bedrock is called an outcrop . Bedrock is only rarely exposed, or crops out, where sediment accumulates rapidly, for example, in the bottom of stream valleys and at the base of hills or mountains . Outcrops are common where erosion is rapid, for example, along the sides of steep stream channels and on steep hill or mountain slopes. Deserts and mountain tops above the treeline also host good bedrock exposures due to the scarcity of vegetation, and resulting rapid erosion. Man-made outcrops are common where roadways cut through mountains or hilltops, in quarries, and in mines.

Generally, the more rock resists erosion, the more likely it is to crop out. Granite and sandstone commonly form well-exposed outcrops. Natural exposures of shale and claystone, both soft, fine-grained rocks , are rare—especially in humid climates.


Bedrock features

In addition to the occasional mineral crystal or fossil that attracts rockhounds, all outcrops contain through-going fractures called joints. These form during the application of stresses to bedrock on a regional scale, for example during mountain-building. Even greater stresses may cause faulting movement of the rock on the sides of a fracture. An example is the large-scale bedrock movement that occurs along the San Andreas Fault in California. When stresses cause plastic rather than brittle deformation of bedrock, it folds rather than faulting.


Bedrock distribution

Bedrock is distributed in a fairly predictable pattern. Generally in the central area of a continent you will find very ancient (one billion years or more) mountain chains, consisting of igneous and metamorphic rock , eroded to an almost flat surface. This area, called a continental shield, typically contains the oldest continental bedrock. Shields have experienced multiple episodes of deformation so they are intensely folded and faulted. These ancient igneous and metamorphic rocks, called basement rocks, compose much of the continental crust. However, on the shield margins, thick sequences of relatively undeformed, sedimentary rocks cover the basement rocks. These deposits, called the continental platform, commonly exceed 1 mi (1.6 km) in thickness and 100 million years in age.

Together, the shield and platform make up the bedrock area known as the continental craton. The craton is considered more or less stable, that is, it is not currently experiencing significant deformation. On the margins of the craton, there may be areas of geologically-active bedrock, called orogens, from the Greek word for mountain. Orogens are relatively young mountain belts where uplift , folding, faulting, or volcanism are occuring. The bedrock here varies in age from lava flows that may be only days old to igneous, sedimentary, and metamorphic rock that are hundreds of millions of years old. All bedrock belongs to either the continental shield, platform, or the orogens.

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bed·rock / ˈbedˌräk/ • n. solid rock underlying loose deposits such as soil or alluvium. ∎ fig. the fundamental principles on which something is based: honesty is the bedrock of a good relationship.