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outer core
The outer core of the Earth is a liquid layer about 〔http://education.nationalgeographic.com.au/education/encyclopedia/core/?ar_a=1〕 thick and composed of iron and nickel that lies above Earth's solid inner core and below its mantle. Its outer boundary lies beneath Earth's surface. The transition between the inner core and outer core is located approximately beneath the Earth's surface.
==Properties==
The temperature of the outer core ranges from in the outer regions to near the inner core. Because of its high temperature, modeling work has shown that the outer core is a low viscosity fluid (about ten times the viscosity of liquid metals at the surface) that convects turbulently. Eddy currents in the nickel iron fluid of the outer core are believed to influence the Earth's magnetic field. The average magnetic field strength in the Earth's outer core was measured to be 2.5 millitesla, 50 times stronger than the magnetic field at the surface.〔(First Measurement Of Magnetic Field Inside Earth's Core ). Science20.com. Retrieved on 2012-01-27.〕 The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core. Sulfur and oxygen could also 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. This growth rate is estimated to be 1 mm per year.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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