13 - Convective and tectonic stresses

Summary

Preamble

Decades of general disbelief in continental drift preceded recognition of plate tectonics in the 1960s. In the 1800s, Kelvin noted that the Earth's response to tidal forces indicated an average rigidity modulus exceeding that of steel and, when seismology provided details of internal structure, it showed that the Earth is solid to nearly 2900 km depth. Early proponents of continental drift faced the difficulty that this evidence of solidity appeared incompatible with yielding to any stresses then envisaged. Convection had been contemplated in the 1800s as a means of conveying heat from the deep interior and resolving the age-of-the–Earth problem (Section 4.2). But the idea was stifled, first by difficulty with solar energy and then, when radioactivity was discovered, by recognition that its concentration in continental rocks suggested that the Earth's heat sources were shallow. But by the 1960s, paleomagnetic evidence of continental drift had become overwhelming, mobility of the ocean floors had been recognized and an explanation in terms of convection had become unavoidable. As we now understand, all tectonic processes are ultimately driven by convection. What we see is the surface expression of motion that is necessarily deep.

Thermodynamic arguments (Chapter 22) are central to understanding convective energy and tectonic stresses. The mechanical energy can be derived only by upward transport of heat from very deep sources. This energy is the product of heat flux and a thermodynamic efficiency, giving an estimated power of 7.7 × 10¹² W (Section 22.4).