During Earth’s early formation, the planet underwent a period of differentiation that allowed the heaviest elements to sink to the center and lighter ones to rise to the surface. Earth’s internal layering can be defined by this resulting chemical composition. The three main layers of Earth include the crust (1 percent of Earth’s volume), the mantle (84 percent), and the core (inner and outer combined, 15 percent).

Crust

The solid crust is the outermost and thinnest layer of our planet. The crust averages 25 miles (40 kilometers) in thickness and is divided in to fifteen major tectonic plates that are rigid in the center and have geologic activity at the boundaries, such as earthquakes and volcanism.

Mantle

Mantle material is hot (932 to 1,652 degrees Fahrenheit, 500 to 900 degrees Celsius) and dense and moves as semi-solid rock. The mantle is 1,802 miles (2,900 km) thick and is composed of silicate minerals that are similar to ones found in the crust, except with more magnesium and iron and less silicon and aluminum.

Outer Core

The outer core is composed mostly of iron and nickel, with these metals found in liquid form. The outer core reaches between 7,200 and 9,000 degrees Fahrenheit (4,000 and 5,000 degrees Celsius) and is estimated to be 1,430 miles (2,300 km) thick. It is the movement of the liquid within the outer core that generates Earth’s magnetic field.

Inner Core

The inner core is the hottest part of our planet, at temperatures between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius). This solid layer is smaller than our Moon at 750 miles (1,200 km) thick and is composed mostly of iron. The iron is under so much pressure from the overlying planet that it cannot melt and stays in a solid state.