Structure of the Earth. Cutaway computer illustration showing the internal structure of the Earth. From the centre outwards, the four layers shown are: inner core, outer core, mantle, and crust. The core is around 15% of the volume of the Earth, and the mantle is around 80%. The core is composed of heavy iron and nickel metal. The inner core is solid, whilst the outer core is liquid and generates the Earth's magnetic field. The mantle is composed molten rock (magma), whilst the crust is a thin (5-70 kilometres thick) layer of rock covering the outer surface of the Earth.