Bedrock

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Bedrock Summary

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 rare—especially 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.