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. This thickening occurs by conductive cooling which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age. In fact oceanic lithosphere is thermal boundary layer for the convection in the mantle. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity. The Moho marks the transition in composition between the rocky outer crust and the more plastic mantle of Earth. The oldest parts of continental lithosphere underlie cratons and the mantle lithosphere there is thicker and less dense than typical. The relatively low density of such mantle roots of cratons helps to stabilize these regions. Craton is a large stable block of crust that forms the nucleus of a continent. A subduction zone is the place where two lithospheric plates come together riding over one another. Because of its relatively low density continental lithosphere that arrives at a subduction zone cannot be pushed much further than about hundred km before resurfacing.