geological controversies Geology emerged as a distinctive science only about two centuries ago. From its earliest days the subject has been associated with controversy. Focusing on controversy can be illuminating because issues tend to become dramatized and the underlying assumptions and attitudes of the protagonists are often brought out into the open. Furthermore, attention is concentrated on matters most critical to growth and development of a given subject. Attention is here confined to four major controversies, which are considered briefly in historical sequence.

Nature versus neptunists and plutonists

In interpreting his rock sequence from ‘primitive’ igneous and metamorphic rocks to unconsolidated sediments, the pioneer German geologist Abraham Gottlob Werner adopted in the late eighteenth century the widely accepted view that initially the Earth had been enveloped by a primeval ocean covering even the highest mountains. All the older rocks we see today, even igneous ones such as granite, he perceived as being chemical precipitates from this ocean. The emphasis upon the role of water in Werner's theory led to its being called neptunist. A major conflict was, however, to arise about the origin of basalt. Werner could not deny, of course, the existence of volcanoes, but he restricted their activity to very recent times, and related them to local melting by the combustion of underlying seams of coal. This interpretation was progressively undermined by detailed researches in the Auvergne of France and elsewhere. More fundamental opposition to the neptunist scheme was provided by the recognition that granite was not necessarily primordial. For example, James Hutton (the original plutonist) recognized intrusive granite veins in Scotland, thereby conclusively establishing that some granite, at least, was younger than the enveloping country rock. The first two decades of the nineteenth century witnessed progressive defections from the neptunist camp, and plutonism became firmly established.

Catastrophists versus uniformitarians

The terms ‘catastrophism’ and ‘uniformitarianism’ were coined in 1832 by William Whewell, but ‘catastrophism’ fails to do justice to the cluster of beliefs that characterized the opposition to Charles Lyell's doctrine, because what can be called ‘directionalism’ was also an important component of their system. ‘Uniformitarianism’, as outlined by Lyell, is both a system and a method. The term has frequently been used as an exact equivalent of the continental term ‘actualism’, and refers to the study of present-day processes as a means of interpreting past events. It is, however, perfectly possible to use actualistic methods and come to ‘catastrophist’ conclusions.

The leading ‘catastrophists’ of the early nineteenth century were the Frenchmen Georges Cuvier and Leonce Elie de Beaumont. Basing his ideas primarily on his stratigraphic work in the Tertiary of the Paris Basin, Cuvier invoked a succession of catastrophes that not only disrupted strata and caused dramatic changes in relative sea level but resulted in mass extinctions of fauna. Elie de Beaumont followed Cuvier in arguing that folded and tilted strata implied sudden disturbance, and that one was not entitled to extrapolate to such ‘catastrophic’ phenomena from the manifestly slow and gradual ‘causes now in operation’.

A lively debate was engaged in England in 1820s and 1830s. Leading catastrophists such as William Buckland and Adam Sedgwick promoted the so-called diluvial theory, which accounted for many geological phenomena by the action of the biblical flood. Although the diluvial theory was quickly abandoned, a stubborn belief persisted that Lyell had overstated his case, and that there was indeed a direction to Earth history rather than a steady-state condition of the sort favoured by Lyell. This was established most clearly from the fossil record, which indicated a kind of organic progression to more complex forms culminating in Man, a fact that was later explained by Darwin's theory of evolution by natural selection.

The age of the Earth

By the middle of the nineteenth century the belief that the Earth was a mere 6000 years old was adhered to only by biblical fundamentalists. While the consensus of geologists, following Lyell, was that the Earth was immensely older than that, there was a general reluctance to undertake even approximate estimates of age. The first serious attempt was made in 1860 by John Phillips, who adopted the cumulative thickness of strata as the best available measure of geological time, using the best current estimates of sedimentation rates. He arrived at an age estimate of nearly 96 million years for the formation of the Earth's crust, which strongly challenged the Lyellian notion of virtually unlimited time.

Only a few years later the Scottish physicist William Thomson, later elevated to the peerage as Lord Kelvin, attempted an estimate on a completely different basis. This was made on the widely held assumption that the Earth was originally a hot, molten sphere that had cooled gradually, with the heat, derived ultimately from gravitational energy, being transmitted solely by conduction. His best estimate of the age was 98 million years, a figure remarkably close to that of Phillips. Kelvin's work was initially well received by geologists. Darwin became very concerned, however, about the limited time allowed for evolution, but ‘Darwin's bulldog’ Thomas Henry Huxley questioned the validity of Kelvin's underlying assumptions.

A growing opposition to Kelvin emerged progressively among geologists, which became especially sharp when he became increasingly dogmatic, reducing his estimate by 1897 to a mere 24 million years. The American geologist Thomas Chamberlin speculated that there might be sources of energy locked up in atoms, of which nineteenth-century physicists were completely unaware. The discovery of radioactivity in 1896 confirmed this prescient thought, and within a few years Kelvin's assumptions had been completely undermined. With radiometric dating well established, it became generally agreed early this century that the Earth must be several thousand million years old.

Continental drift

By the end of nineteenth century a consensus had emerged among geologists that the Earth had both cooled and contracted in volume through time, and many attempted to explain orogenic belts as the consequence of this contraction. The Europeans tended to believe that extensive sectors of ocean were underlain by subsided continent. Isostatic theory had more influence among the Americans, who were impressed by the fact that gravity surveys appeared to indicate that the oceans were underlain by denser crust than the continents, implying that they were permanent features whose underlying crust was not interchangeable with continents. Both groups denied the possibility of any significant lateral movement of continental masses through the oceans, a phenomenon that would have been completely inconsistent with the stabilist Earth model.

Although he was not the first person to propose such lateral movement, the notion of continental drift is irrevocably associated with the name of the German meteorologist and geophysicist Alfred Wegener, because he was the first to put forward substantial evidence for a coherent and logically argued hypothesis that took account of a wide variety of natural phenomena. He challenged the cooling, contracting-Earth model on a number of grounds. It was not clear from the contraction hypothesis why the shrinkage ‘wrinkles’ represented by fold mountains were not distributed uniformly rather than being confined to narrow zones. Further, some basic assumptions about the Earth's supposed cooling had been undermined by the discovery of widespread radioactivity in rocks, leading to the production of considerable amounts of heat acting in opposition to thermal loss into space by radiation.

Wegener postulated that, commencing in the Mesozoic and continuing up to the present, a huge supercontinent, Pangaea, had rifted and the fragmented components had moved apart, creating the Atlantic and Indian Oceans. During the westward drift of the Americas, the western Cordilleran ranges had been produced by compression, as had the Alps and Himalayas as Africa and India had converged with Eurasia. He deployed a variety of arguments in support of his hypothesis, citing a number of notable geological facts, such as matching orogenic belts on the two sides of the Atlantic, and close resemblances among fossils of the southern continents implying former land connections. There was in addition convincing evidence for Late Palaeozoic ice sheets in South America, southern Africa, Australia, and especially India that was not consistent with the present dispersed continental configuration. Wegener also put forward a number of geophysical arguments but was unable to propose a plausible mechanism for drift.

The initial reaction to Wegener's hypothesis was not uniformly hostile, but it became increasingly so in the years between the two world wars. The most formidable opposition came from certain geophysicists, who insisted that the Earth possessed too great a strength for continents to migrate across its surface. These scientists ridiculed Wegener's proposed mechanism for drift. Wegener nevertheless had some distinguished supporters, most notably Emile Argand, Alexander du Toit, and Arthur Holmes, the last of whom provided a plausible mechanism for drift involving convection in the mantle. Despite their efforts, supporters of continental drift were generally dismissed as cranks, and by the middle of the twentieth century the hypothesis had been almost totally rejected by Earth scientists. The situation was radically transformed by research developments after the Second World War, most notably in the study of rock magnetism, and a vastly increased knowledge of what underlies the oceans. These developments led to a posthumous vindication of Wegener. The theory of plate tectonics, an outcome of continental drift, was put forward in the late 1960s and generally accepted by the Earth sciences community within a few years.

Anthony Hallam

Bibliography

Hallam, A. (1973) A revolution in the Earth sciences: from continental drift to plate tectonics. Oxford University Press.
Hallam, A. (1989) Great geological controversies (2nd edn). Oxford University Press.

controversies, geological see geological controversies