The Interior of the Earth The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The crust is on top of the mantle. The lithosphere is the solid outer section of Earth, which includes Earth's, as well as the underlying cool, dense, and rigid upper part of the upper mantle. The lithosphere extends from the surface of Earth to a depth of about 70–100 km. The asthenosphere is the highly viscous, mechanically weak region of the upper mantle of the Earth. It lies below the lithosphere, at depths between approximately 80 and 200 km below the surface. The mantle is a layer between the crust and the outer core. Earth's mantle is a silicate rocky shell with an average thickness of 2,900 kilometres. The mantle makes up about 84% of Earth's volume. The Earth's inner core is the Earth's innermost part and according to seismological studies, it is primarily a solid ball with a radius of about 1220 kilometers. The outer core of the Earth is a liquid layer about 2,300 km thick and composed of iron and nickel that lies above Earth's solid inner core and below its mantle. Its outer boundary lies 2,900 km beneath Earth's surface.