 Earth 's interior , like that of the other terrestrial planets , is divided into layers by their chemical or physical ( rheological ) properties , but unlike the other terrestrial planets , it has a distinct outer and inner core . The outer layer is a chemically distinct silicate solid crust , which is underlain by a highly viscous solid mantle . The crust is separated from the mantle by the Mohorovičić discontinuity , and the thickness of the crust varies : averaging 6 km ( kilometers ) under the oceans and 30 – 50 km on the continents . The crust and the cold , rigid , top of the upper mantle are collectively known as the lithosphere , and it is of the lithosphere that the tectonic plates are composed . Beneath the lithosphere is the asthenosphere , a relatively low @-@ viscosity layer on which the lithosphere rides . Important changes in crystal structure within the mantle occur at 410 and 660 km below the surface , spanning a transition zone that separates the upper and lower mantle . Beneath the mantle , an extremely low viscosity liquid outer core lies above a solid inner core . The inner core may rotate at a slightly higher angular velocity than the remainder of the planet , advancing by 0 @.@ 1 – 0 @.@ 5 ° per year . The radius of the inner core is about one fifth of that of Earth . 
