The outer layers of the Earth's structure, varying between 6 and 48 km in thickness, and comprising all the material above the Mohorovicic Discontinuity (a seismic discontinuity occurring between the crust of the earth and the underlying mantle; the discontinuity occurs at an average depth of 35 km below the continents and at about 10 km below the oceans). The earlier idea of a cool solid skin overlaying a hot molten interior has now been replaced by a concept of a crust composed of two shells: an inner basic unit composed of sima (oceanic crust) and an outer granitic unit composed of sial (continental crust).