Speculations on the Earth's Thermal History
The gross structure of the Earth's interior—mantle and core with their subdivisions—affords evidence of the thermal evolution. The principal event was the formation of a liquid iron core from an initially cool unsorted conglomerate after about 500 m.y. of heating by radioactivity and tidal friction. Core formation was accompanied by conversion of gravitational energy to heat, the deep interior reaching temperatures of 4000°–5000°; this process was completed about 4500 m.y. ago. Fractional melting of the mantle concentrated radioactive and “lithophile” elements in the upper mantle and transition zone, leaving the lower mantle devoid of radioactivity. Formation of stable continental crust became possible after this upward concentration and decay of radioactivity, beginning about 3500 m.y. ago. Further concentration of the radioactive elements in the continental crust has left the subcontinental mantle impoverished by comparison with the suboceanic mantle. Present temperatures are consequently higher in the suboceanic mantle than at the same depths beneath continents, approaching or reaching melting temperatures beneath the oceans while cooling continues beneath continents. The required concentrations of radioactive elements appear to be in reasonable agreement with existing measurements if the primitive undifferentiated Earth resembled the meteorite Orgueil in its radioactive content.