Many pieces of evidence are brought together to support the theory that the formation of continental crust must have started with the subduction of hot oceanic crust. This subducting slab melted, and the rising magma gravitationally differentiated into a more granitic composition, characteristic of continental crust. The result of testing and putting together information has led scientists to believe that crustal growth began approximately 3.8 billion years ago. and continued building slowly, in a sequence of comparatively brief episodes. Then crust quickly stabilized about 2.975 billion years ago. The schematic image above right shows two possible flat subduction mechanisms that could create the tonalite plutons seen in the greenstone belt. However, this theory of crustal formation has been questioned by many scientists.