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|Upper Mantle||Lithospheric Mantle|
|Deeper Upper Mantle|
|Topics: | | | | | | ||
In , a crust is the outermost layer of a , part of its . Planetary crusts are generally composed of a less material than that of its deeper layers. The crust of the is composed mainly of and . It is cooler and more rigid than the deeper layers of the and .
On stratified planets, such as Earth, the lithosphere is on interior layers. Because of in the , although non-molten, upper mantle and , the lithosphere is broken into that move. Oceanic crust is different from that of the continents. The () is 5 to 10 km thick and is composed primarily of a dark, dense rock called basalt. The () is 20-70 km deep and is composed of a variety of less dense rocks. The crust's temperature ranges from the air temperature to about 900°C near the upper mantle.
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The Earth is considered to have differentiated from an aggregate of into its , and crust within ~100 million years of the formation of the planet, at 4.4 billion years ago. The primordial crust was very thin, and was likely recycled by much more vigorous and destroyed by significant impacts, which were much more common in the early stages of the solar system. Of particular note is a theory that the was formed by one such very large impact.
The Earth has likely always had some form of basaltic oceanic crust, but there is evidence that it has also had continental style crust for as long as 3.8 to 3.9 billion years. The oldest crust on Earth is the in at 3.9, and certain parts of the and the are also of this age.
The majority of the current Earth's continental crust was formed primarily between 4.6 billion years and 3.9 billion years before present, in the . The vast majority of rocks of this age are located in where the crust is up to 70km thick. The lower density of the continental crust as compared to the oceanic crust prevents it being destroyed by . Crust formation is linked to periods of intense or mountain building; these periods coincide with the formation of the such as , and . The crust forms not so much by accumulation of and fold belts, but by depletion of the to form buoyant lithospheric mantle.