The structure of the Earth is layered, and commonly divided into four parts: the silicate crust, the viscous mantle, the liquid iron-nickel outer core, and the solid iron-nickel inner core. Occasionally, the mantle is further subdivided into the inner and outer mantle, based on differences in rock type, temperature, density, and viscosity. The crust, which is the only part of the Earth that is both relatively cool and completely solid, makes up less than 1% of its total volume.
The first part of the structure of the Earth, the crust, consists of cooled rock floating on top of the viscous mantle. The thickness of the crust varies mainly depending on whether it is oceanic (5 km (3 mi) to 10 km (6 mi) thick) or continental (30 km (20 mi) to 50 km (30 mi) thick). The oceanic crust is composed of dense rocks such as gabbro, basalt, and diabase, while the continental crust is made up of somewhat lighter rocks such as granite. The deepest hole ever dug by humans into the crust is 11.26 km (7.62 mi), about a third of the way to the mantle.
Below the crust is the mantle, the shallowest part of which is made up of rocks like olivine, pyroxenes, spinel, and garnet, while the deeper parts are made of high-pressure polymorph minerals with similar elemental composition as the rock above. The mantle is a plastic solid which flows slowly over the course of millennia, creating convection currents similar to those seen when pasta is put into boiling water, just at a much slower speed. These convection currents can create volcanic hotspots and cause continental drift. The mantle is the thickest part of the structure of the Earth, about ~2,890 km thick (1,800 miles), and constituting 70% of the Earth's volume. Scientists have learned much about the mantle by studying the way that it effects seismic waves which pass through it.
The centermost parts of the structure of the Earth are the outer and inner mantle. The outer core is made of molten iron and nickel. At this depth, temperature is sufficient to melt iron and nickel, but pressure is not enough to force it to become solid. The outer core contains the vast majority of the planet's iron and nickel, which sank to the core when the Earth was in the process of forming about 4.6 billion years ago. Eddy currents in the outer core are believed to generate the Earth's magnetic field. The inner core has a similar makeup to the inner core, but the pressure there is sufficient to make it a solid. Temperatures in the inner core may exceed those on the surface of the Sun.