in geology, the doctrine that existing processes acting in the same manner and with essentially the same intensity as at present are sufficient to account for all geologic change. Uniformitarianism posits that natural agents now at work on and within the Earth have operated with general uniformity through immensely long periods of time. When William Whewell (Whewell, William), a University of Cambridge scholar, introduced the term in 1832, the prevailing view (called catastrophism) was that the Earth had originated through supernatural means and had been affected by a series of catastrophic events such as the biblical Flood. In contrast to the catastrophic view of geology, the principle of uniformity postulates that phenomena displayed in the rocks may be entirely accounted for by geologic processes that continue to operate at the present day—in other words, the present is the key to the past. This principle is fundamental to geologic thinking and underlies the whole development of the science of geology. The expression uniformitarianism, however, has passed into history, for the controversy between catastrophists and uniformitarians has largely died. Geology as an applied science draws on the other sciences, but in the early 19th century geologic discovery had outrun the physics and chemistry of the day. As geologic phenomena became explicable in terms of advancing physics, chemistry, and biology, the reality of the principle of uniformity as a major philosophical tenet of geology became established and the controversy ended.

The idea that the laws that govern geologic processes have not changed during the history of the Earth were articulated by the 18th-century Scottish geologist James Hutton (Hutton, James), who in 1785 presented his ideas—later published in two volumes as Theory of the Earth (1795)—at meetings of the Royal Society of Edinburgh. In this work Hutton showed that the Earth had a long history and that this history could be interpreted in terms of processes observed at the present, of which he gave examples. He showed, for instance, how soils were formed by the weathering of rocks and how layers of sediment accumulated. He stated that there was no need of any preternatural cause to explain the geologic record. Hutton's proposal challenged the concept of a biblical Earth (with a history of some 6,000 years) that was created especially to be a home for man; the effect of his ideas on the learned world can be compared only with the earlier revolution in thought brought about by Nicolaus Copernicus, Johannes Kepler, and Galileo when they displaced the concept of a universe centred on the Earth with the concept of a solar system centred on the Sun. Both advances challenged existing thought and were fiercely resisted for many years. In the publication Principles of Geology, 3 vol. (1830–33), the Scottish geologist Sir Charles Lyell (Lyell, Sir Charles, Baronet) deciphered the history of the Earth employing Huttonian principles and made available a host of new geologic evidence in support of the view that physical laws were permanent and that any form of supernaturalism can be rejected. Lyell's work in turn profoundly influenced Charles Darwin, who recognized Lyell as having produced a revolution in science.

The publication in 1859 of the conclusions of Darwin and Alfred Wallace on the origin of species extended the principle of uniformity to the plant and animal kingdoms. Although the catastrophists continued to fight a rearguard action against the Huttonian-Lyellian-Darwinian view until the end of the century, a new criticism was raised by William Thomson (later Lord Kelvin (Kelvin, William Thomson, Baron)), one of the leading researchers on thermodynamics. Thomson pointed out that the Earth is losing heat by conduction and that the nature of geologic processes may have changed as a consequence; he also concluded that this cooling placed an upper limit on the age of the Earth. With the discovery of radioactivity and the recognition that radioactive isotopes within the Earth provide a continuing internal source of heat, it became clear that Thomson's conclusion that the Earth was less than 100 million years old was incorrect, but his argument that the Earth suffers an irreversible loss of energy remains valid.

This heat loss, owing in part to the decay of the heat-producing radionuclides, has an important consequence. Although the principle of uniformity is correct in that physical laws have not changed over geologic time, the behaviour of the Earth has altered as temperatures have fallen. One important consequence is that the extent of igneous activity and movement of the crust has changed during geologic time. It is possible that the plate (plate tectonics) tectonism that operates today and that has operated in past geologic intervals of time was preceded by somewhat different processes of deformation during Precambrian times.