Unlike the mineral-rich crust and mantle, the core is made almost entirely of metal-specifically, iron (Fe) and nickel (Ni). The primary contributors to heat in the core are the decay of radioactive elements, leftover heat from planetary formation, and heat released as the liquid outer core solidifies near its boundary with the inner core. The geothermal gradient is about 25° Celsius per kilometer of depth (1° Fahrenheit per 70 feet). The geothermal gradient measures the increase of heat and pressure in Earth’s interior. This important process is called planetary differentiation.Įarth’s core is the furnace of the geothermal gradient. Droplets of iron, nickel, and other heavy metals gravitated to the center of Earth, becoming the early core. These materials became the early mantle and crust. Relatively buoyant material, such as silicates, water, and even air, stayed close to the planet’s exterior. The iron catastrophe allowed greater, more rapid movement of Earth’s molten, rocky material. This pivotal moment in Earth’s history is called the iron catastrophe. Eventually, after about 500 million years, our young planet’s temperature heated to the melting point of iron-about 1,538° Celsius (2,800° Fahrenheit). Radioactive decay and leftover heat from planetary formation (the collision, accretion, and compression of space rocks) caused the ball to get even hotter. When Earth was formed about 4.5 billion years ago, it was a uniform ball of hot rock. The core is found about 2,900 kilometers (1,802 miles) below Earth’s surface, and has a radius of about 3,485 kilometers (2,165 miles). The ball-shaped core lies beneath the cool, brittle crust and the mostly solid mantle. Earth’s core is the very hot, very dense center of our planet.
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