Geological concept that substantial change to the Earth's crust occurs gradually via historically familiar mechanisms.

Uniformitarianism posits above all that geological change occurs over long periods, with relatively small effects over shorter periods (such as over the lifespan of individuals), that this process has been going on for very long periods of time, and that the processes that drive this geological change are no different from the geological processes we observe in action today.

Think erosion as well as plate tectonics as the prominent drivers of the geological change posited by Uniformitarianism, though with two major caveats. First is that plate tectonics was only understood long after Uniformitarianism was posited and second that catastrophes, such as a volcanic eruptions, occur as well (i.e., see also, though to a degree by contrast, Catastrophism).

Changes in species diversity within the fossil record, consistent with Uniformitarianism, also can occur over relatively long periods, an idea that is consistent with those of Darwinism. Indeed, Darwin was substantially influenced by the ideas of Uniformitarianism along with the principle prediction from Uniformitarianism that the Earth is extremely old.