Some aspects of the thermal evolution of the earth

Empirical data relating to the thermal history of the earth are examined. Recent astronomic and geochemical evidence strongly suggests that the earth formed by accretion from an initially low-temperature gas-dust cloud of solar composition. The distribution of U, Pb, Th and K within the earth imply that it passed through a melting or partial melting process about 4.5 × 10⁹ years ago. This conclusion is confirmed if the core is assumed to consist dominantly of iron-nickel. Formation of the core, which likewise occurred about 4.5 × 10⁹ years ago would liberate sufficient gravitational energy to cause melting. Evidence in favour of melting is also provided by analogy with meteorites.

An examination is made of possible causes of this early melting stage and it is concluded that gravitational energy is chiefly responsible. Radioactive heating does not appear to be important. A critical factor in the early heating and chemical evolution is the interaction of accreting dust falling with high velocity into the primitive reducing atmosphere surrounding the earth. Because of this interaction, a metallic phase is produced by reduction.

The distribution of temperature within the earth 4.5 × 10⁹ years ago will be given by the melting-point gradient. Recent data on the electrical conductivity of the mantle and the melting point of metals under high pressures suggest that the present temperature distribution is much less than the melting point gradient. This implies that the earth has cooled considerably.

The inferred cooling is consistent with present data on the abundance of radioactive elements in meteorites and in the earth, and also with possible modes of internal heat transfer—particularly convection and radiation.