Tectonics and Continental Drift
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Interpretation of tectonic features through horizontal movements has opened the way to the idea of continental drift. Argand precedes Wegener. One should speak of continental-block drift rather than of continental drift. But this interpretation has been open to question. Gliding tectonics explain horizontal com pression and cover folding, resulting from a basement upheaval. The importance of the great transcurrents faults is that the horizontal displacements they prove are not subject to such
... t subject to such discussion. One of the main facts brought out by tectonics is the chronology of the movements. The idea of individual, more or less world-wide, phases of discontinuous activity, should be discarded. The isostatic equilibrium of the crust is one of the most important facts to take into account. It is likely that the existence of the asthenosphere, inferred from vertical displacements, also makes horizontal movements possible. Isostatic equilibrium of the crust is consistent with any set of horizontal stresses, which must be the cause of horizontal deformations. Inquiry about the causes of such horizontal stresses leads only to very hypothetical conclusions. The best guide is the evaluation of the amount of energy involved. The energy produced by earthquakes if of the order of 1025 ergs/y, and the power needed to allow tectonic phenomena seems not to be much higher. Geothermal flow amounts to 1028 ergs/y. Thus, we can see how tectonic phenomena may derive their energy from geothermal flow. Two mechanisms have been suggested: contraction by cooling may build up stresses, but not provide the necessary amount of energy. But the convection current hypothesis seems to work. All the models of convection currents that have been considered share these two characteristics: (a) Between ascending and descending branches, the deviation of density or temperature from their mean value at the same depth are very small; thus, structure of the Earth as a whole may be studied without taking these deviations into account. (b) All these models involve velocities so small that inertia plays absolutely no part. I favour the idea that convection currents have been discontinuous, both in time and space.