Earth's core.
The Outer Core is the boundary between the mantle and the outer cores called the core-mantle boundary, is the most significant within earth in terms of changes in material properties.

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The Inner Core is at the center of the core is a solid sphere of iron with lesser amounts of nickel.

Earth's temperature

Earth is about 5500° at its center and 0° at its surface so hear is continually flowing toward the surface. Measurements around the planet have shown that the average flow of heat at the surface is 87 milliwatts per square meter. Earth's surface is so large, heat leaves at a rate of 44 terrawatts per year, whic is about three times the total world rate of energy consumption.
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How Did Earth Get So Hot?

Earth has two thermal stages of existence, the fist stage occurred during earths formation and involved a very rapid increase in internal temperature. the second stage has been the very slow process of cooling down. the first stage was very brief. taking only about 50 milion years. the second stage has taken the remaining 4.5 billion years of earth history and will continue for about another 4.5 billion years.

Heat Flow

Heat travels by three different mechanism: radiation, conduction and convection.
Convection the transfer of heat by moving material in a fluid-like manner and carrying the heat with it is called convection.
Conduction the flow of heat through a material is called conduction.