Encyclopedia of Earth


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Geologic map of United States decribing bedrock types and age of bedrock formations. Age of bedrock, from youngest to oldest, is indicated by color change: yellow, green, blue, red. (Image Source: USGS).

Bedrock is generally defined as relatively hard, solid rock that commonly underlies softer rock, sediment, or soil.

Bedrock can also refer to a specific subset of the basement rock that is, harder and usually older igneous and metamorphic rocks that underlie the main sedimentary rock sequences (softer and usually younger) of a region and extend downward to the base of the Earth's crust.

Bedrock geology is a term used for (the study of) the main mass of rocks forming the Earth and present everywhere, whether exposed at the surface in outcrops or concealed beneath superficial deposits or water. The bedrock has formed over vast lengths of geologic time ranging from ancient and highly altered rocks of the Proterozoic, some 2500 million years ago, or older, up to the relatively young Pliocene, 2.6 million years ago.

The bedrock geology includes many lithologies, often classified into three types based on origin: igneous, metamorphic and sedimentary.

  • Igneous rocks are derived from molten magma in the Earth's crust. They may, for example, be extruded at the surface by volcanic activity, to form lavas and tuffs (ash); or intruded into other rocks to form large masses of granite and gabbro at depth or minor crosscutting basalt dykes near the surface.
  • Metamorphic rocks such as schist and gneiss are those that have been changed from one rock type to another in the solid state by the recrystallisation of minerals, often at high temperatures and pressures when buried deep in the Earth's crust.
  • Sedimentary rocks are formed when grains and fragments of existing rocks are eroded away by ice, water and wind action, transported elsewhere and redeposited as a sediment. These sediments are often laid down in layers or strata of loose particles of gravel, sand, silt and clay. Over time they may be buried by later sediments and consolidated or cemented to form stratified or bedded rocks such as conglomerate, sandstone, siltstone and claystone. Other sedimentary rocks such as ironstone and limestone are created by chemical or biogenic (life) action.

The geological sequence of rocks preserved varies from place to place but packages of strata with similar characteristics may be recognisable over considerable distances. Such study has developed into the science of stratigraphy, of which lithostratigraphy is but one type. (Exerpted from British Geological Society)

Bedrock geologic units are units of consolidated (solid) rock that underlie soils or other unconsolidated materials.

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Survey, U. (2013). Bedrock. Retrieved from http://www.eoearth.org/view/article/51cbf0ed7896bb431f6a3924