Geoscientist 17.5 May 2007
The concept of the surface of the Earth being made up of rigid plates constantly moving relative to each other recast geological thinking in the 1960s, making sense of observations and revealing new patterns, like putting jigsaw pieces together. In the third of her reviews of the great intellectual landmarks of the Society's 200 years, Sue Bowler* reports on a Kuhnian revolution.
The exhilarating rush of discovery that followed the first expositions of plate theory led on to a period of expansion and consolidation while it began to sink in just how enormous the change of viewpoint was. Once Earth scientists (as they became) took the plate tectonic framework for granted, they could extend it into the refined and complex theory that remains an extraordinarily successful description of our planet. Its other great success was the new perspective it gave Earth scientists, who could glimpse the future in terms of multidisciplinary investigations, covering the whole of the planet – its oceans and its continents.
In many ways, the plate tectonics revolution had as much impact as the revelation of the age of the Earth. Systematic mapping, dating and correlation began to gel only when geologists could look back at the history of the Earth in depth. In the first half of the 20th Century, scientists had enough information to be able to “think big” about the Earth, but not enough to reinforce or refute their ideas. Arthur Holmes’s suggestion of convection within the mantle dates from this fruitful time, as does Harold Jeffreys’s interpretation of seismic signals as suggesting a solid Earth, and no mechanism for continents to plough through the solid rock of the oceans.
Despite the work of Alexander du Toit and Alfred Wegener among others, continental drift remained an interpretation of circumstantial evidence without much physical foundation. But as the exploration of the oceans began and new types of geophysical data were collected, there quickly came a point when the balance tipped and drift, in the form of plate movements, became obvious. Around 1965, the seductive simplicity of the idea, the testable predictions it made and the diverse body of observations tipped the balance and established the former mavericks’ favourite theory, continental drift, as “plate tectonics” - the “next big thing”. How did it all happen so quickly?
For geophysicists to be convinced, what they needed was geophysical evidence. After all, the lack of a certain mechanism never prevented geophysicists from believing in the Earth’s magnetic field (see p. ), precisely because it is geophysics that proves the field’s existence. And so it was that geophysical evidence – when it finally arrived - made all the difference in the world. Much of the crucial data came from International Geophysical Year activities (1957/58), with their focus on collecting and collating such information worldwide.
More precise earthquake location brought their association with plate boundaries into sharp focus;. Ocean surveys for bathymetry, and newly developed instruments such as magnetometers, revealed both the startling topography of the mid-ocean ridges and the striped pattern of remanent magnetism in the ocean floor basalts. At the same time, Keith Runcorn’s derivation of polar wander paths argued for major continental movements, despite the odd but persistent reversals of polarity that he found in the records. Survey data from oil and mineral exploration – then as now making good use of expensive new technologies – found further signs that all was not well with established models of the world. In the early 1960s, discoveries and theories came thick and fast: seafloor spreading, first described by Harry Hess in 1960; detailed bathymetric and magnetic maps of the seafloor, such as that of Mason and Raff (1961); the magnetic reversal timescale (McDougall and Tarling 1963) and more.
In 1965, when Tuzo Wilson’s grasp of the essential role of transform faults settled the remaining serious objections to the theory, plate tectonics blossomed. Convection in the Earth became an object of serious study again, because of the seemingly obvious link between plates movements and mantle flow. Dan Mackenzie’s and Bob Parker’s work on mapping plate movements and rotation poles from earthquake slip vectors (1967) and fault plane solutions established that Euler geometry could describe the plate movements on the Earth, and spawned a veritable industry of plate movement mapping.
After a period in which the pattern was established and reinforced, researchers began to look at the oddities - places where the new orthodoxy did not apply. Nowadays, much research still focuses on proving (in the proper sense of “testing”) the rules of plate tectonic theory by examining their apparent exceptions. The whole area of continental deformation has come under scrutiny over the past decades, as the elegant simplicity of the tectonics of rigid plates meshed with the geological reality that continents do deform. This is now a fruitful area, thanks in a large part to the greatly enhanced data collection ability brought about by the latest generation of digital instruments.
Time-consuming repeated surveys of areas around active faults can now be replaced by networks of instruments on the ground, regularly reporting their position via GPS, while satellite data can monitor displacements in centimetres. UK researchers in the NERC COMET collaboration are drawing together the different strands of data that will allow them to assess deformation even in such a complex region as the Mediterranean over decades, linking the historical and geological record of deformation and damaging quakes to the pattern on human and political timescales. The research value of such work is tremendous, but in an increasingly crowded world, it is now being matched by its human and economic value.
Plate tectonics has given us a working model for our planet, based on the whole of the Earth’s surface and embracing diverse Earth processes. Perhaps the most powerful legacy of the plate tectonic revolution is that we need to use all the information from many, fast-moving specialisms in order to solve current problems in Earth sciences. Increasingly, tectonic problems can also be addressed by comparing the Earth with other planets and moons. In fact, we are in a position comparable to that of geologists a century ago, equipped with an understanding of the big picture of planetary dynamics, but without too much inconvenient data.
Who can tell what big ideas will come from this new phase of exploration?