catastrophism and uniformitarianism The terms catastrophism and uniformitarianism refer to concepts and ensuing arguments that preoccupied Earth scientists about two hundred years ago, and which to some extent recur from time to time as our perception of the nature of the geological record changes. Until the early nineteenth century the origin of the crystalline rocks that we know as volcanic or igneous was regarded by some European scholars as being from precipitation in a primeval ocean. The concept has been called ‘neptunism’, but as new knowledge about volcanic activity and the immense heat within the Earth became generally available the Neptunists had to retreat before the criticisms of the Plutonists with their new (modern) views on the origin of igneous materials. This may have cleared away some of the obstacles to understanding the nature of the crust of the Earth and the processes that have produced it, but geological argument remained fierce and widespread. The new dramatic aspect of Earth history inferred from igneous and structural features, and especially from unconformities, required radically novel explanation. It was soon forthcoming, but new arguments were to break out, as described below.

Catastrophism was the early nineteenth-century line of reasoning that ascribed major changes in the physical environment to sudden, violent, and short-lived episodes or events. It was advocated most notably by the French baron Georges Cuvier (1769–1832), the foremost anatomist of the day, who sought to explain the unconformities separating rocks containing very different kinds of fossils. He had, in fact, little field experience outside the Paris basin, but worked with such geologists as A. Brongniart and J. B. Elie de Beaumont who had demonstrated the marine-freshwater cycles in the Tertiary (Cenozoic) rocks there. Cuvier also believed that the entire history of the Earth could be encompassed in 75 000 years, and he rejected his contemporary James Hutton's view that gradual and constant action by geomorphic processes was sufficient to account for great environmental changes. Cuvier's fellow-countryman the Chevalier de Lamarck (1744–1829), a fellow-student of the living world, thought that gradual changes in the physical environment were the cause of changes in animals and plants, with, in time, a progressive ‘improvement’ of living things. That idea, too, was firmly rejected by Cuvier. Elie de Beaumont went on to develop the idea that catastrophic, sudden, but infrequent upheavals of mountain ranges were responsible for the environmental changes and the destruction of much of the contemporary biota. This catastrophism was, on the evidence available, an attractive idea, but there was no discussion of the source of the energy to bring these upheavals about, nor any explanation of why and where they occurred as they did.

Here we turn to Hutton's great contribution to geology. Uniformitarianism has a place in every textbook of Earth science, for it has been regarded as the closest thing to a fundamental ‘law’ that geologists have within their discipline. It has also been referred to as the principal of uniformity, and as perhaps one of geology's major contributions to science. Students of Earth science in Hutton's day saw the need to explain even the simplest geological phenomena in the light of what they knew of contemporary natural processes, and supernatural or divine causes were discounted. This was eventually developed by the Scot (and Plutonist) James Hutton as a critical tenet in his Theory of the Earth, published in 1795. From then on it has become part of the lore of geology, for rather than being adopted as a law, it has been interpreted in so many different ways as to be in disrepute. It has been expressed variously in four common, but not necessarily equivalent, forms:

(1) As another famous Scot, Sir Archibald Geikie, put it, the present is the key to the past.(2) Former changes of the Earth's surface may be explained by reference to processes now in operation.(3) Earth history may be deciphered in terms of present observations on the assumption that the laws of physics do not change with time.(4) Further to (3) above, events and processes in the geological past have proceeded at the same rate and in the same manner as they do today.

Since Hutton's day, and even more since the publication of Charles Lyell's Principles of geology, being an attempt to explain the former changes of the Earth's surface by reference to causes now in operation (1830–3), these postulations have been debated and denied. Hutton himself denied that any sort of supernatural agency has been involved in Earth history or in the functioning of the Earth as a machine or system. He seems, however, to have espoused the view that we should seek to explain the past in terms of causes and processes now existing or known today. It seems an acceptable and simple way of going about Earth history, and Lyell, one of the most influential geologists of all time, tended to agree. The principle has been termed ‘actualism’.

Geological changes brought about by the continuing activity of observable processes require very long periods of time. Hutton's recognition of geological time as being orders of magnitude longer than had previously been considered was one of his most perceptive achievements. Nevertheless, the principle of uniformitarianism that he propounded has now been rejected on a number of grounds. Two considerations lead most obviously to this rejection. The first is that many factors have been changing gradually with time; as examples, heat flow from the Earth has been diminishing since the planet first formed; the composition of the atmosphere has changed several times; and in pre-Silurian times there was virtually no terrestrial life. We know that both atmosphere and terrestrial life are important in bringing about environmental or geomorphic changes. The second objection stresses that the present provides few or only vague indications of the infrequent, but sudden, major changes and events that must have taken place in the past. More than one geologist has emphasized that big geological events are rare, small ones are common. It has even been suggested that we need the concept of ‘neocatastrophism’ to cover the recognition of rare violent events (such as giant meteor impacts) taking place in addition to the continuing activity of ‘normal processes’. Derek Ager likened the incidence of geological catastrophes to the life of a soldier: long periods of boredom and short periods of terror. The American biologist S. Jay Gould (1977) preferred to use the term ‘punctuationalism’ for long periods of tranquility interrupted by brief moments of profound change.

Table 1. Major extinction events

Extinction	Major animal groups affected	Percentage
episode	of extinct
	families
(After N. D. Newell)
Late Cretaceous	Ammonites, belemnites, rudist	26
bivalves, corals, echinoids,
bryozoans, sponges, planktonic
foraminifera, dinosaurs, marine
reptiles, pterosaurs
Late Triassic	Ammonites, brachiopods,	35
conodonts, reptiles, fish
Late Permian	Ammonites, rugose corals,	50
trilobites, blastoids, inadunate,
flexibiliate and camerate
crinoids, productid brachiopods,
fusulinid foraminifera, bryozoans,
reptiles
Late Devonian	Corals, stromatoporodis,	30
trilobites, ammonoids, bryozoans,
brachiopods, fish
Late Ordovician	Trilobites, brachiopods,
crinoids, echinoids
Late Cambrian	Trilobites, sponges, gastropods	52

A summary review of the catastrophes or crises that have taken place during the history of life on Earth reveals that no single repetitive process can be shown to be solely responsible for sudden drastic reduction of animal and plant numbers and diversity. In total, these events number about eighteen, but rather less severe crises are much more numerous—and are that much more difficult to detect in the geological record. It was the Late Permian and late Cretaceous crises that first impressed the early geologists, but those listed in Table 1 have attracted the most attention so far. Smaller events, when smaller particular groups were wiped out, appear now to have been more numerous, but the causes of these extinctions remain as uncertain as those of the bigger mass mortalities.

Geologists have been able to witness and record very few catastrophes on anything more than the local scale: the scenarios put forward to account for sudden great changes in the past are based upon little or no experience of the processes involved. So to explain these events it is necessary to examine the entire geological record. And there is another possible dimension to consider: from a careful study of the timing of major extinctions and others, less obvious, some palaeobiologists have claimed that it is not random but shows a periodicity of about 30 million years (Ma).

The more widely discussed and more plausible causes of these catastrophes include the following:extraterrestrial: bolide impact, cosmic ray flare; volcanic activity; geomagnetic reversal; changes of sea level, salinity changes, anoxic events in the oceans; climatic changes: glacial episodes, increasing aridity; disturbance of the ecological balance; and disease or geochemical poisoning. Each of these has, in theory, been shown to be a possible agent for disaster, but to what extent do they lend themselves, singly or in combination, to a periodicity scenario? What phenomena that might be the ultimate cause(s) are both periodic and global in their influence?

A sharp fall in sea-level seems to be the only likely cause to occur at or shortly before most mass extinctions, and like many of the other possibilities, it may be linked to sudden violent or widespread volcanic activity, changes in the volume of the ocean basins, or to continental collisions and resulting Earth movements (orogeny). In the eyes of many writers, all these stem from the behaviour of the mantle deep below the crust, for it is there that plate motions and volcanic activity originate. Moreover, it has been argued that there is a periodicity of about 30 million years in the frequency of reversals of the Earth's magnetic field, which itself is related to dynamics within the core and mantle. To the mantle, too, may be traced the causes of exceptional phases of volcanicity, and it seems likely that this holds for the entire Phanerozoic Eon or longer.

Early Earth history differs significantly from that of more geologically recent times; some aspects of, say, the Archaean cannot be explained entirely by the processes known from the Phanerozoic record. The changing nature of the interior of the planet during the 4500 or so million years of its existence remains obscure, but it was presumably unaffected by events elsewhere in the Solar System.

Nevertheless, we are becoming more aware of the influence of cyclic changes in the behaviour of the Earth in its motion around the sun (Milankovich cycles), and the idea that other astronomical cycles may have influenced the history of life on Earth has been widely discussed. Some palaeobiologists have claimed to see a regularity rather than a randomness in the timing of extinctions precisely on account of such influences, and even on account of bodies beyond the Solar System. They invoke phantom stars, as yet unobserved planets, and passage of our Solar System through the spiral arms of the Galaxy. Much is hypothesis rather than fact and some of these propositions seem to verge upon the fanciful. Most palaeobiologists and geologists today, taking the cautious approach, favour combinations of volcanic activity, climatic change, and lowering of sea level as providing sufficient environmental stress to bring about crisis in the biosphere. Meteor or comet impact while stress was mounting would perhaps give the final push to an impending mass extinction.

Meteor impacts are not in fact very rare events, geologically speaking, and their influence in early Earth history was probably important. It seems, however, wise to look carefully at terrestrial processes as prime causes of catastrophes, even on the grandest scale. Uniformitarianism in its original sense is no longer tenable; and we acknowledge terrestrial agencies beyond present experience that offer better models of Earth history and greater insight into particular events. This neocatastrophism may serve in the explanation of mass extinctions, but where does uniformitarianism stand today in providing us with the key to the past—to the evolution of the Earth during all those long spells of time between the ‘catastrophes’?

The advent of the paradigm of plate tectonics opened up great attractions as an explanation of how the Earth itself works, that is, how the geological cycle has operated throughout a great part of geological time. Research suggests that plate tectonics has been in operation for the past 4000 million years. In spite of the fact that heat production was so much greater in early Precambrian time than it is now, the tectonic and geochemical processes that produced the early Archaean rocks were not fundamentally different from those we see today. Brian Windley, a British authority on Precambrian geology, reviewed the evidence and concluded that, allowing for greater heat production in early Precambrian times, the basic processes responsible for the geology of continents and ocean basins have not changed significantly throughout the ensuing time. They could also have brought about the conditions of stress which gave rise to mass extinctions. With this concept, he suggests, those great protagonists of uniformitarianism, Hutton and Lyell, would have agreed.

D. L. Dineley

Bibliography

Adams, F. D. (1938) The birth and development of the geological sciences. Dover Publications, New York.
Albritton, C. C. (1989) Catastrophic episodes in Earth history. Chapman and Hall, London.
Berggren, W. A. and Van Couvering, J. A. (eds) (1984) Catastrophes and Earth history. Princeton University Press.
Gould, S. J. (1990) Time's arrow, time's cycle: myth and metaphor in the discovery of geological time. Penguin Books, Harmondsworth.
Stanley, S. M. (1987) Extinction. Scientific American Library, 20. Scientific American Books, New York.
Windley, B. F. (1993) Uniformitarianism today: plate tectonics is the key to the past. Journal of the Geological Society of London, 150, 7–19.