plate tectonics: the history of a paradigm

plate tectonics: the history of a paradigm Through the efforts of James Hutton and his successors the outlines of the geological evolution of the Earth became apparent, yet for 150 years the means by which large-scale crustal change was brought about remained unknown. When Alfred Wegener proposed his continental drift hypothesis in Die Entstehung der Kontinente und Ozeane in 1915 he opened a great controversy on how the continents come to be where they are. Orogeny and volcanism had generally been held to be associated with the contraction of the Earth, but the continents and oceans were regarded as primordial features, fixed in their positions since the crust first formed. Wegener's idea demanded great lateral movement of the continents, and it was definitely not appreciated outside Germany. His book provided no mechanism whereby the drift of continents could come about.

Geophysicists were loud in their objections to any lateral movement of continental blocks. At international conferences on continental drift held in 1922 and 1956 there was only a minority in favour of continental drift in view of the perceived difficulties. At that time advocates of continental drifting were Alex du Toit in South Africa and Arthur Holmes in Britain. Between the two world wars du Toit assembled a great mass of supportive data, while Holmes contributed a mechanism of continental transport in the form of convecting currents in the mantle. After the Second World War advances on two scientific fronts took place; palaeomagnetism and ocean floor surveying and sampling. From P. M. S. Blackett's original discovery of palaeomagnetism came the recognition of past polar reversals and the conclusion that either the poles or the continents had moved.

The second front on which great changes occurred was in our knowledge of what underlies the oceans. Improved (American) instrumentation and the deployment of large resources in submarine bathymetry and geophysics led to the discovery of the world's mid-oceanic ridges. Sampling programmes brought up rock specimens from the oceanic plains as well as the ridges. The volcanic rocks virtually everywhere were basaltic or ultrabasic types, and the sediments were nowhere older than the Cretaceous or possibly the Jurassic. It also became apparent that the ridges are prime sites of seismic activity and basaltic eruption. Geophysics showed the basaltic floor of the oceans to be much thinner than the granitic crust of the continents. Deeply subsided volcanic islands (sea mounts) were found in all oceans.

All this led Harry H. Hess, a distinguished geologist at Princeton University, to suggest that a kind of convective movement beneath the crust brought material up along the axis of the mid-oceanic ridges to spread out across the ocean floors and eventually sink into the trenches at the edges of continents. It had meanwhile been discovered that the ocean floors exhibit a remarkable regular striped pattern of variations in magnetic intensity. In places these are offset along lines that run at right angles to the magnetic lineaments, but the reasons for this remained a mystery. Then F. J. Vine and D. H. Matthews of Cambridge University came up with a possible test of the Hess hypothesis. They had in mind that the Earth's magnetic field had reversed direction several times in recent past ages and that when basalt was ejected at the mid-oceanic ridges it had acquired the imprint of the magnetic direction of the day. If this new strip of basalt were then pushed away from the ridge by subsequently erupted material, a record of the changes would appear from the polarities of the strips outwards from the ridge. Something like a large-scale tape recording of palaeomagnetism during the creation of the ocean floor would thus be made. The idea caught on and many geophysicists took to sea with improved magnetometers to chart the palaeomagnetic patterns of the ocean floors and to improve our detailed view of the ridges themselves. The Vine–Matthews hypothesis was confirmed by the new data.

The mid-ocean ridge, the newly found axis of sea-floor spreading, does not meander and curve at random, but is offset by fracture zones which can extend great distances from the ridge itself. Here originate the many earth tremors associated with the ridge, which occur when one side of the fault zone moves relative to the other. The rate of basalt injection may vary from fault-bound sector to sector, causing stresses and fault-zone movements. Differing rates of spreading from place to place along the ridge cause plate growth to be uneven, though the growth on each side of the ridge seems to be similar throughout. Improvements in isotopic dating of the ocean floor basalts identified the ages of the magnetic strips, and revealed more than 170 reversals of the Earth's magnetic field over about 76 million years. Spreading continues: the Atlantic is at present widening by about 5 cm per year.

Close to the ocean trenches the basaltic plate begins to turn down at an angle of about 45° and to descend into the depths. Soft sediment that has accumulated on the surface of the basalt during its passage from ridge to trench tends not to continue down but lingers as a accretionary wedge on the flank of the trench. Heat and pressure above the descending plate cause island-arc volcanicity.

The basic idea behind plate tectonics was first mooted by a Canadian, J. Tuzo Wilson, in the scientific journal Nature in 1965. He proposed that the elongate ‘mobile belts’ of the crust, where mountain ranges, earthquakes, volcanicity, and thrust faulting occur, commonly end abruptly at a transform fault. Transform faults link a continuous network of ridges, mountain belts, and trenches which divides the entire outer mechanical shell of the Earth into several large lithospheric plates. Soon afterwards Jason Morgan at Princeton and Dan McKenzie at Cambridge independently envisaged the Earth's surface as being divided into plates or blocks to which there were three kinds of boundary: ocean ridge, where new crust was generated; ocean trenches, where it was destroyed; and transform faults. The plates were thought to be about 100 km thick and to rest on a weak or plastic layer, the asthenosphere. On some plate surfaces there are the continents, not drifting but being moved passively as passengers on their host plates.

During the late 1960s and 1970s the explanation of mountain-building (orogenic) cycles came to be attributed to activity along active continental plate margins or to continental collisions. Overthrusting, volcanism, and plutonic igneous intrusion were identified as originating above the subduction zone where one plate is forced beneath the edge of its neighbour. Palaeomagnetic and tectonic studies now point to ancient world geographies and to the rotation and migration of continents. The expanding and contracting oceans, continental collisions, and fracturing can be modelled with increasing conviction on the basis of the plate-tectonic model. The process is recognizable throughout the Phanerozoic Eon far back into the Proterozoic, and even into the Archaean in a somewhat modified form. The impact has been called a ‘revolution in the earth sciences’, the arrival of the long-sought ‘theory of the Earth’: a true paradigm for all Earth scientists.

D. L. Dineley

Bibliography

Hallam, A. (1983) Great geological controversies, ch. 6. Oxford University Press.
Kearey, P. and and Vine, F. J. (1990) Global tectonics. Blackwell Scientific Publications, Oxford.
Muir Wood, R. (1985) The dark side of the Earth. Allen and Unwin, London.
Park, R. G. (1988) Geological structures and moving plates. Blackie, Glasgow.