Lithosphere is the rigid outermost shell of a terrestrial type planet or natural satellite. It is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The crust and upper mantle are distinguished on the basis of chemistry and mineralogy. Lithosphere includes the crust and the uppermost mantle which constitutes the hard and rigid outer layer of the Earth. The lithosphere is subdivided into tectonic plates. The lithosphere is underlain by the asthenosphere which is the hotter and deeper part of the upper mantle. The boundary between lithosphere and asthenosphere is defined by a difference in response to stress. The lithosphere remains rigid for long periods of geologic time in which it deforms elastically and through brittle failure while the asthenosphere deforms viscously and accommodates strain through plastic deformation. The thickness of the lithosphere is thus considered to be the depth to the level which is associated with the transition between brittle and viscous behaviour. The lithosphere can be divided into oceanic and continental lithosphere. Oceanic lithosphere is associated with oceanic crust and exists in the ocean basins. Continental lithosphere is associated with continental crust and underlies the continents and continental shelves. Oceanic lithosphere is denser than continental lithosphere and contains igneous rock rich in magnesium and iron. Young oceanic lithosphere found at mid ocean ridges is no thicker than the crust but oceanic lithosphere thickens as it ages and moves away from the mid ocean ridge. The oldest oceanic lithosphere is typically about one forty km thick.