The layers of the Earth are formed by a process known as planetary differentiation, which consists of the separation of its distinct components based on its chemical characteristics. In this process, the denser material descends to the center, while the lighter material accrues at the surface, forming a core and a mantle. The Earth has differentiated into a solid dense iron and nickel metallic core, a less dense magnesium and silicate rich mantle, and a relatively thin crust composed principally of silicates: aluminum, calcium, sodium and potassium. Planetary differentiation has shaped the planets, asteroids, and satellites of the solar system, including the Moon.
The layers of the Earth are divided into six distinct regions: A solid inner core; a liquid outer core; a viscous lower and upper mantle; transition region, and the oceanic and continental crust. Most of the mass in the inner core is composed of iron and nickel and trace quantities of sulfur, while the mantle and crust are made of lighter elements. Scientists study the Earth's interior by analyzing seismic waves produced by earthquakes. Geologist study the different patterns formed by seismic waves as they pass through different materials. The changes observed give them clues about the composition of the Earth's interior. The magnetic field, which is created by circulations of molten material, give clues about the core of the planet, as well.
The Earth's inner core has a radius of about 1,220 km (757 miles). The inner core is composed principally of 80% iron with trace quantities of nickel and other elements. The temperature at the inner core may reach more than 7000 K (13,000 F). The pressure in the inner core is so tremendous that metals are compressed together and, unlike liquids which are able to move, only produce vibration. During the Earth's formation, 4.5 years ago, melting would have caused denser material to sink towards the center on the Earth, while less dense elements would have remained in the Earth's crust.
The outer core extends out from the inner core to a radius of about 3,400 km (2,110 miles). The liquid outer core surrounds the inner core and is thought to be composed of iron, nickel, and trace quantities of lighter elements. It´s composed of super-heated liquid conductive molten lava in which convection processes occur. The convection processes in the outer core in combination with the rotation of the Earth create electric currents which give rise to the Earth's magnetic field. The field produces a subtle erratic motion in the Earth's rotation.
The Earth's lower mantle extends to a depth of approximately 2,890 km (1,792 miles). The mantle is composed primarily of silicate rocks rich is iron, silicon, oxygen, and magnesium. The mantle, although solid, creates high temperatures which cause silicates to become sufficiently ductile to flow through very long timescales. The upper mantle extends from 10 to 400 km (6-250 miles). This is the portion of the Earth along with the crust forms the asthenosphere which form part of the Earth's tectonic plates. The temperature in the mantle varies from 4000 F (2285 C) near the bottom to 1600 F (870 C) at the top.
The transition region is situated between the lower mantle and the upper mantle at a depth of 410-660 km (255-378 miles). The transition region consists principally of peridotite and ultramafic igneous rock. Geologists divided the mantle into the lower mantle, transition region, and upper mantle, based on discontinued seismic velocities at distances of 410-660 km into the Earth's mantle. This is believed to occur by the rearrangement of atoms resulting from an increase of pressure with increased depth as seen by wave movement during earthquakes.
The outermost layer, known as the crust, is divided into the oceanic crust and the continental crust. The crust is 5-10 km (3-6 miles) thick under the oceanic crust, and 32 km (20 miles) on the continental crust. The oceanic crust lies on the ocean basins, and is composed primarily of basalt which is denser and heavier than granite rock which composes the continental plates. The temperature in the crust increases with depth, attaining values ranging from 200 °C (392 °F) to 400 °C (752 °F) at the boundary with the upper mantle. The crust along with a portion of the upper mantle comprises the lithosphere.