Bedrock

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.