Craton (from Greek kratos, "strength") is ancient bedrock that forms an old, stable part of the continental lithosphere, in contrast to more unstable, geologically active regions on a continent. The lithosphere is a rigid layer on the Earth's surface, including the crust and upper mantle. This outer layer is underlain by the asthenosphere, the upper mantle's weaker, hotter, and deeper part. This layer of the lithosphere mantle can be 2 to 3 times thicker than the crust above. 

Over time, the more rigid lithosphere breaks into tectonic plates, while the underlying asthenosphere is more viscous and plastic. This results in the movement of the tectonic plates around the surface of the Earth, a process now understood as continental drift, first formulated by Alfred Wegener in 1912. The concept of the lithosphere itself was introduced by Joseph Barrell (1914) and developed by R.A. Daly (1940). The two essential ideas of a rigid lithosphere over a plastic underlayer and of continental drift were synthesized in the theory of plate tectonics which emerged in the 1960s and 70s. 

Cratons have deep lithospheric roots, extending up to several hundred km into the Earth's mantle. Cratons are usually found in the interiors of tectonic plates after having survived various cycles of the merging and splitting of continents. 

Cratons are composed of ancient crystalline basement rock, which is either outcrops as a shield or covered by platforms of younger sedimentary rock. Every continent has cratonic areas, notably the core of North America, Scandinavia, Siberia, India, and most of Australia. A thin layer of sediment may cover them, but often they expose 'basement' rocks such as gneiss. 

The first large cratonic landmasses formed during the Archean eon (4.5-2.0 billion years ago). Much of the original rock is now "Archean reworked" during the Proterozoic eon (2.5 billion- 540 million years ago). Only 5 to 40 percent of the present continental crust remains as fragments from the Archean eon. Examples of cratons are the Slave Craton in Canada, the Kaapvaal Craton in South Africa, and the Pilbara Craton in South Australia. 

Cratons are areas where the earliest fossils of Precambrian microbes have been found, often in the form of stromatolites or mat-like layers of the residues of bacterial colonies.