Basement rocks

In, basement and crystalline basement are the rocks below a  or cover, or more generally any rock below s or s that are  or  in origin. In the same way, the s or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover".

Continental crust
Basement rock is the thick foundation of ancient, and oldest, and  that forms the  of s, often in the form of. Basement rock is contrasted to overlying s which are laid down on top of the basement rocks after the continent was formed, such as and. The sedimentary rocks which may be deposited on top of the basement usually form a relatively thin veneer, but can be more than 3 miles thick. The basement rock of the crust can be 20–30 miles thick, or more. The basement rock can be located under layers of sedimentary rock, or be visible at the surface.

Basement rock is visible, for example, at the bottom of the, consisting of 1.7-2 billion year old  and. The Vishnu Schist is believed to be highly metamorphosed igneous rocks and, from ,  and  laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu Schist. An extensive cross section of sedimentary rocks laid down on top of it through the ages is visible as well.

Age
The basement rocks of the continental crust tend to be much older than the oceanic crust. The oceanic crust can be from 0-250 million years in age, and is usually thinner (10 miles or so) and composed of ic rocks. Continental crust is older because continental crust is light and thick enough so it is not subducted, while oceanic crust is periodically subducted and replaced at and  areas.

Complexity
The basement rocks are often highly metamorphosed and complex. They may consist of many different types of rock - volcanic, intrusive igneous and metamorphic. They may also contain fragments of that became wedged between plates when a  was accreted to the edge of the continent. Any of this material may be folded, refolded and metamorphosed. New igneous rock may freshly intrude into the crust from underneath, or may form underplating, where the new igneous rock forms a layer on the underside of the crust. It is said that the majority of continental crust on the planet is around 1-3 billion years old, and it is theorised that there was at least one period of rapid expansion and accretion to the continents during the Precambrian.

Much of the basement rock may have originally been oceanic crust, but it was highly metamorphosed and converted into via a series of events. A typical pattern is as follows. It is possible for oceanic crust to be subducted down into the, at subduction fronts, where oceanic crust is being pushed down into the mantle by an overriding plate of oceanic or continental crust.

Volcanism
When a plate of oceanic crust is subducted beneath an overriding plate of oceanic crust, as the underthrusting crust melts, it can cause upwelling of magma that can cause along the subduction front on the overriding plate. This produces an oceanic chain of es, like. This volcanism causes of, intrusions of  that produce  rocks such as granite, and thickens the crust by depositing additional layers of rock from volcanoes. This tends to make the crust lighter and thicker, as a result making it immune to subduction.

Oceanic crust can be subducted, while continental crust cannot. Eventually, the subduction of the underthrusting oceanic crust can bring the volcano chain close to a continent, and collide with it. When the overriding plate collides with the continent, instead of being subducted, it is to the edge of the continent and becomes a part of that continent. Thin strips or fragments of the underthrusting plate may also remain attached to the edge of the continent causing those fragments of oceanic crust to be wedged and tilted between the converging plates. In this manner, continents can grow over time as new terranes are accreted to their edges, and so continents can be composed of a complex quilt of terranes of varying ages.

As such, the basement rock can become younger going closer to the edge of the continent. There are exceptions of however, such as s. Exotic terranes are pieces or fragments of other continents that have broken off their original parent continent and have become accreted to a different continent.

Cratons
Many continents can consist of several continental s - blocks of crust built around an initial original core of continents - that gradually grew and expanded as additional newly created terranes were added to their edges. For instance, consisted of most of the Earth's continents being accreted into one giant. Most continents, such as Asia, Africa and Europe, include several continental cratons, as they were formed by the accretion of many smaller continents.

Usage
In, the basement generally refers to rocks older than the. On top of this older basement s and  s were deposited. The evaporites formed a zone on which the harder (stronger) limestone cover was able to move over the hard basement, making the distinction between basement and cover even more pronounced.

In geology the basement refers to the,  and early  ( to ) rock units as the basement to the late Mesozoic and  Andean sequences developed following the onset of  along the western margin of the.

When discussing the of  the basement include Proterozoic, Paleozoic and Mesozoic age rocks for the Oaxaquia, the Mixteco and the Guerrero s respectively.

The term basement is used mostly in disciplines of geology like, and  in which the (typically ) crystalline basement is not of interest as it rarely contains  or. The term economic basement is also used to describe the deeper parts of a cover sequence that are of no economic interest.