contrast in composition and nature of the oceanic and continental plates, which together form the overall crust.The difference between continental and Oceanic crust. Timing Oceanic and continental crusts are, at the present day, produced and maintained through plate tectonic processes. However, the same mechanisms are unlikely to have produced the crustal dichotomy of the early lithosphere. This is thought to be true on the basis that sections of the thin, low density continental lithosphere thought to have originally covered the planet could not have been subducted under each other. Consequently, a proposed relative timing for crustal dichotomy has been put forward stating that the dichotomy began before the commencement of global plate tectonics. This is so a difference in crustal density could be established to facilitate plate subduction. Formation Impact cratering Large and numerous impact craters can be recognised on planetary bodies across the Solar System. These craters are thought to date back to a period where there was an increased frequency and intensity of asteroid impacts with terrestrial planets, known as the Late Heavy Bombardment, which terminated approximately 4 billion years ago. This proposal goes on to claim the Earth would have also sustained the same relative intensity of cratering as other planetesimals in the Solar System. It is therefore only due to Earth's high erosional rates and constant plate tectonics that the craters are not visible today. By scaling up the number and size of impact craters seen on the Moon to fit the size of Earth, it is predicted that at least 50% of the Earth's initial crust was covered in impact basins. This estimate provides a lower limit of the effect impact cratering had on the Earth's surface. Effects The main effects of impact cratering on the early lithosphere were: Formation of large craters. Isostatic rebound would adjust the depth of the craters making them relatively shallow in comparison to their diameter; some reaching 4 km deep and 1000 km in diameter. Topographic division between the low-lying impact basins and the now elevated surface. Release in pressure at the surface from the removal of overburden. This produced a greater increase in temperature with depth below the surface. Increased surface temperatures caused the partial melting of mantle which erupted and deposited within the surface basins. The pyrolite mantle would have produced basaltic partial melts, compositionally contrasting to the existing sialic crust. The magnitude of these impacts is