The heat energy produced and exhausted during the formation and development of the earth is considered quantitatively. It includes chemical reactions, energy released during the accretion and core formation of the earth, radioactive energy, thermal convection and heat conduction in the mantle, and volcanic and geothermal activities. When the earth is made of the accumulation of Type I carbonaceous chondrite, the heat energy required to form the molten Fe-Si core is calculated to be 2.23×10
38 ergs. This large amount of energy can be provided by release of the gravitational energy during the accretion, if the accretion time is less than 1×10
4 years. In this case one difficulty is degassing of CO
2 or CO produced by the reduction reactions of iron and silicon oxides. The completion of the reduction before the accretion removes this difficulty. The energy that the earth gains during its evolution is I
GE, heat energy converted from the gravitational energy by the rapid growth of the earth ; R_E, radioactive energy by the long-lived radioactive elements contained ; and C
E, heat energy converted from the gravitational energy by the core formation. I
GE is calculated to be 2.5×10
38 ergs for the accretion time of 1×10
4 years, R
E 0.93×10
38 ergs for the chondritic mantle model, and C
E 1.5×10
38 ergs. The energy lost from the earth’s interior during 4.5×10
9 years is composed of E
A, energy released by volcanic activity ; E
C, energy carried to the earth’s surface by mantle convection ; L
E, heat loss by heat conduction ; and E
W, energy released by hot springs. E
A, L
E, E
C, and E
W are calculated to be 0.003×10
38 ergs, 0.8×10
38 ergs, 2.1×10
38 ergs, and 0.001×10
38 ergs, respectively. The difference between the energy gained and lost, 2.0×10
38 ergs, represents the energy preserved now in the earth’s interior, This raises the mean temperature within the earth by about 2300℃. The energy required to melt the whole earth is approximately 2.9×10
38 ergs. The energy balance calculated above is consistent with the solid earth model. It is emphasized that the energy released during the accretion and core formation of the earth is more effective for the heating of the earth’s interior than that by the long-lived radioactive elements contained, and that the release of heat by mantle convection is most important for the cooling.
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