Core (earth science), innermost part of Earth. The core of Earth extends from beneath the mantle to the very center of the planet. The radius of the core is approximately 3,500 km (2,200 mi). This radius is larger than the radius of the planet Mars. The core forms approximately one-third of Earth’s total mass and about one-sixth of its volume. At the earliest stages of its formation, Earth was basically a mixture of gas and dust. Over millions of years the planet took shape and solidified. At some point, as more material accumulated, the whole planet reached extremely high temperatures and changed into a liquid. Geologists believe that at this time the different elements in the planet separated. In the process of separation, the heavier elements (such as iron and nickel) sank to the center, and the lighter elements (such silicon and aluminum) rose to the surface.
Earth’s core experiences pressures that are millions of times greater and temperatures that are thousands of degrees higher than those at the surface. Core temperatures range from 4000° to 5000° C (7200° to 9000° F). Scientists cannot retrieve samples of material from the core because the temperatures and pressures are so high. However, scientists believe that the core consists primarily of heavy elements, such as iron and nickel. This composition of the core is believed to be similar to that of stony meteorites, which geologists think may have provided the material for Earth’s core. Seismic studies indicate that the core consists of two parts: a solid inner core and a molten outer core. Scientists estimate that the core is extremely dense—about 13.5 times as dense as water. A high concentration of iron in the core is believed to account for the high density.