A craton is an old and stable part of continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are composed of ancient crystalline basement rocks covered by younger sedimentary rocks. They have a thick crust and deep lithospheric roots extending several hundred kilometres into Earth's mantle. In simpler terms, craton is a layer of the earths crust on that has been almost unchanged through all of history. This crust consists of shields and platforms. Shields are made of basement rock and are visible, while platforms are the portion not visible made up of sedimentary rock.