Earth's outer core is a fluid layer about 2,400 km (1,500 mi) thick and composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. Its outer boundary lies 2,890 km (1,800 mi) beneath Earth's surface. The transition between the inner core and outer core is located approximately 5,150 km (3,200 mi) beneath the Earth's surface. Unlike the inner (or solid) core, the outer core is liquid.
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Estimates for the temperature of the outer core are about 3,000–4,500 K (2,730–4,230 °C; 4,940–7,640 °F) in its outer regions and 4,000–8,000 K (3,730–7,730 °C; 6,740–13,940 °F) near the inner core. Evidence for a fluid outer core includes from seismology which shows that seismic shear-waves are not transmitted through the outer core. Because of its high temperature, modeling work has shown that the outer core is a low-viscosity fluid that convects turbulently. The dynamo theory sees eddy currents in the nickel–iron fluid of the outer core as principal source of the Earth's magnetic field. The average magnetic field strength in the Earth's outer core was estimated to be 2.5 millitesla, 50 times stronger than the magnetic field at the surface. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to the inner core. Sulfur and oxygen could be present in the outer core.
As heat is transferred outward toward the mantle, the net trend is for the inner boundary of the liquid region to freeze, causing the solid inner core to grow at expense of the outer core. This rate is estimated to be 1 mm per year.
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