Uniformitarianism

Uniformitarianism is one of the most basic principles of modern geology, the belief that fundamentally the same geological processes that operated in the distant past also operate today. It exists in contrast with catastrophism, which states that earth surface features originated suddenly in the past, by radically different geological processes than those currently occurring.

In the 18th and 19th centuries, the debate between the two theories was very controversial, since uniformitarianism seemed to be incompatible with the prevailing religious beliefs of the time. Today, however, all mainstream scientists support uniformitarianism.

The known slow processes have changed in several ways. Before continental drift was recognized in the 20th century, the surface of Earth was believed to have remained generally unchanged since creation. Cooling from a molten state was believed to have caused shrinkage, which caused mountains and folding of the surface. Now it is believed the Earth did not form in a molten state, much of the inner material is now molten, and the surface crust is floating and slowly moving.

In recent decades, the theory of uniformitarianism has been modified to reflect the discovery of certain catastrophic events in the earth's past. This modification, which could be phrased as "the geologic forces are almost always functioning slowly and the same through time", is called actualism.