Delbridge, Brent G. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts.
Ishii, Miaki Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts.
Last reviewed:November 2019
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The dense kernel of Earth comprised primarily of iron and nickel that contains nearly one-third of the mass of the planet. Three distinct compositional spheres make up the Earth’s interior: the core, mantle, and crust (Fig. 1). A model of the Earth’s interior, constructed using seismic data and theoretical consideration of mineral physics, as well as the Earth’s mass and moment of inertia, shows the core–mantle boundary at about 2900 km below the surface of the Earth, at 3400-km radius (Fig. 1). The metallic core lies beneath the rocky shells of the crust and mantle and is composed largely of iron and nickel, accompanied by some lighter elements (such as silicon, oxygen, sulfur, hydrogen, and carbon). Because of the sensitivity of the melting temperatures of iron–nickel alloys on pressure, namely, that the melting point of iron increases more rapidly with pressure than with temperature, the core is further divided into two distinct regions: a solid inner core where the temperature is below the melting point and a liquid outer core where the temperature exceeds the melting point (Fig. 2). Pressure within the core is nearly hydrostatic, increasing from about 135 gigapascals (GPa) at the core–mantle boundary to 365 GPa at the center of the Earth. See also: Hydrostatics; Earth's interior
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