The interior of Earth, similar to the other terrestrial planets, is chemically divided into layers. 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 2,886 kilometres (1,793 mi).The mantle makes up about 84% of Earth's volume.It is predominantly solid but in geological time it behaves as a very viscous fluid.he inner core is solid, the outer core is liquid, and the mantle solid/plastic. This is because of the relative melting points of the different layers (nickelâiron core, silicate crust and mantle) and the increase in temperature and pressure as depth increases.