Our Earth is a dynamic planet. It continually changes inside and outside. The Earth's interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun. The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometres) in diameter. Although this inner core is white-hot, the pressure is so high the iron cannot melt. The interior of the Earth consists of crust, mantle and core. The Earth’s crust is made up of various types of rocks, and there are three major types of rocks: igneous rocks, sedimentary rocks and metamorphic rocks. Minerals are naturally occurring substances which have specific physical properties and definite chemical composition.