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. As a result continental lithosphere is not recycled at subduction zones the way oceanic lithosphere is recycled. Instead continental lithosphere is a nearly permanent feature of the Earth. The concept of the lithosphere as strong outer layer of Earth was described by A.E.H. Love and further developed by Joseph Barrell who introduced the term lithosphere. The concept of lithosphere was based on the presence of significant gravity anomalies over continental crust from which he inferred that there must exist a strong solid upper layer above a weaker layer which could flow. They have been broadly accepted by geologists and geophysicists.