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Namely, silicas contained in the granitic layer are richer than those contained in the basaltic layer in quantity, those materials gather radioactive elements such as thorium, uranium, potassium and make them in-active.
Besides, since the birth of the earth, thermal energies inside the earth have been originated in nuclear energies generated from nuclear decays of radioactive matters.
Most of those radioactive matters are concentrated on the earth's crust, the remaining matters are contained in the mantle.
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