If early twentieth-century geologists and geo-physicists had heeded the fundamental axiom of the Greek philosopher Heraclitus. "Everything flows." the sterile and sometimes bitter controversy that divided them in the first half of the twentieth century might have been avoided.
At the time, some geologists argued that the history of past climates, reconstructed from examination of rock strata, and the distribution of past fauna, documented through analysis of the fossil record, were inexplicable if the continents had never moved. Noting that some of the continents could be fitted together reasonably well as a kind of crustal jigsaw puzzle, they theorized that during some part of the Earth's history, the continents must have moved.
Geophysicists, looking at different types of data, reached a very different conclusion. When a major earthquake occurred, they noted, the Earth behaved like a gigantic bell struck by a hammer. it rang, and the reverberations echoed around the Earth for several hours thereafter. They inferred from this that the outer part of the Earth was strong and rigid. This inference seemed to be confirmed by the evidence of mountains. Rocks at the base of mountains like ten-kilometer-high Everest had to be able to withstand enormous stress or they would crack and the mountains collapse. Because the height of any structure is limited by the strength of its supporting materials, the stability of mountains seemed to corroborate the geophysicists' conclusion: the Earth was simply too strong for the continents to move.
There followed a classic confrontation, pitting "movement" against "rigidity," which in retrospect need never have occurred. The "strength paradox" had been familiar to generations of geologists from the study of rock deformations in mountain belts, where it had been observed that some quite rigid rocks had in the past been highly ductile, on occasion even viscous. But both geologists and geophysicists failed to connect this evidence with a phenomenon they knew in the context of practical problems of structural engineering "creep". Creep is observed in materials that are subjected to relatively low stresses for very long periods of time; the materials deform continuously, but very slowly, like fluids with an extremely high viscosity. The process operates most rapidly in materials near their melting point.
Thus, before talking of the "strength" of rocks, both groups of scientists should have known something of the temperature of the rocks they were studying and should have specified the time scale under consideration. Rocks at the Earth's surface are between 600° and 1,000° C below their melting temperatures and thus creep so slowly that even on geological time scales of millions of years, they may be regarded as brittle and strong solids. Within the Earth, however, temperature increases relatively rapidly with depth and, below a few hundred kilometers, creep occurs so readily that on time scales of more than a few million years, rocks underneath the Earth's crust must be considered as fluids even though they are perfectly normal crystalline solids.
