Thermal regime of the Earth's interior

Abstract

SEVERAL recent and apparently unrelated geophysical observations and deductions, when considered together, necessitate a fundamental re-examination of our ideas about the thermal state of the Earth's deep interior.

In summary they are:

(1) Precession must be discounted as a power source for the geomagnetic dynamo¹, leaving core convection as the only plausible basic mechanism. Therefore the temperature gradient in the outer core cannot be less than adiabatic.

(2) The thermodynamic Grüneisen parameter for the outer core is about 1.4 and is only slightly pressure dependent (R. D. Irvine and F.D.S., unpublished).

(3) The resulting adiabatic gradient is much steeper than the Fe melting point gradient² or the Fe–S eutectic gradient³.

(4) Continents and ocean floors are both mobile, although the ocean floors are much more so⁴, so that equality of the continental and oceanic heat fluxes which is observed⁵ (in spite of gross differences between the radioactive constituents of the continental and oceanic crusts) cannot be explained in terms of an association of depleted regions of the mantle with continents.

(5) Hot spots, across which the tectonic plates move, are not themselves moving perceptibly with respect to one another and are presumed to mark features of a rigid lower mantle⁴.

(6) A weak layer (asthenosphere) in the upper mantle allows virtually perfect isostatic balance of continent-sized blocks⁶ but the lower mantle must have greater strength. Low degree harmonic features of the geoid are supported by stresses at depths of several hundred kilometres⁷.

(7) Radiative transfer does not cause a dramatic increase in the thermal conductivity of mantle material at high temperatures^8,9.