What is a craton? How many are there? How do they relate? And how did they form?

What is a craton? A craton is a large, coherent domain of Earth's continental crust that has attained and maintained long-term stability, having undergone little internal deformation, except perhaps near its margins due to interaction with neighbouring terranes. Stable continental crust is an end product of intense magmatic, tectonic, and metamorphic reworking; hence, cratons consist of polydeformed and metamorphosed crystalline and metamorphic rocks (e.g., typically "granite-greenstone terrains" in the most ancient cratons). Reworked crust only becomes a craton once the cumulative tectonic, magmatic, and metamorphic reprocessing has self-organized the crust and underlying lithosphere into a stable density, compositional, and thermal profile. Major late-stage "granite bloom" events play a critical role in attaining such stable lithospheric profiles. Once above average stability has been reached, deformation will be concentrated in adjacent domains with weaker strength profiles. Significant rifting events, assisted by mantle plume activity and mafic dyke swarms, are then needed to break up cratonic lithosphere. Where cratons are exposed, they form "shields" dominated by crystalline and metamorphic rocks; where younger, weakly deformed cover overlies cratonic basement, these areas are referred to as "platforms". Shields and platforms are physiographic terms rather than tectonic entities. Another concept, related but not identical to cratons is that of "structural provinces" and the two are commonly confused. Perhaps there is a slight bias for Archean cratons with buoyant mantle keels to form relatively high-standing areas, thus forming shields. However, large parts of Archean cratons are buried underneath platformal cover. There is no strict age connotation to the term "craton", and implied age depends on context. In a context of mantle keels, diamonds and kimberlites, there often is an implicit tendency to equate cratons with stable crust of Archean age. Elsewhere, however, e.g. in the context of younger continental reconstructions such as that of Rodinia or Pangaea, cratons are typically large crustal fragments (e.g., Laurentia) that were only amalgamated and attained stability during the Proterozoic. There are ca. 35 large crustal fragments of Archean age around the globe,the Archean cratons (s.s.). These originated from break-up of larger, transient, late Archean landmasses, which we refer to as "supercratons". Hence, although commonly neglected, the evolution of Archean cratons (and their lithospheric keels) should always be considered in the context of these ancestral landmasses. Given good correlation between some well-known cratons but not others, the ensemble of ca. 35 remaining Archean cratons probably originated from more than one supercraton rather than a single late Archean supercontinent. The fact that the mean age of continental crust is ca. 2 Ga, in conjunction with Archean cratons only representing ca.7-10 percent of the exposed continental crust, must mean that the 35 remaining cratons represent a biased sample of Archean crust and lithosphere, their preservation guided by the Darwinian principle of "survival of the fittest".