In geology, uniformitarianism is the belief that Earth's physical structure is the result of currently existing forces that have operated uniformly (in the same way) since Earth formed roughly 4.5 billion years ago. Mountains rise, valleys deepen, and sand grains collect now the same way they uplifted, eroded, and deposited over these millions of years. The activities of the present are a key to those of the past.
Early theories of Earth's formation were based on a literal reading of the Biblical book of Genesis. In the mid-sixteenth century, Irish Catholic bishop James Ussher (1581–1656) counted the ages of Biblical characters and calculated Earth to be only 6,000 years old. Bound by tradition (and even by law) to work within this short time frame, scientists of the time had to explain the placement and composition of rocks with more acceptable theories like catastrophism—the belief that Earth changes suddenly during cataclysmic earthquakes, floods, or eruptions.
In 1785, Scottish geologist James Hutton (1726–1797) electrified the geologic community when he presented a theory on the formation of Earth that contradicted the Bible-based one. The major elements contained in his Theory of the Earth were later termed "uniformitarianism." Hutton maintained that:
1. The fossilized strata (horizontal layers of material) of Earth, originating from the bottom of the sea, were formed by natural processes driven by heat energy from Earth's core.
2. The present continents' shapes indicated that they had once belonged to a singular landmass. Hutton added that the current disintegration and erosion of surface rock would lead to the formation of future continents.
3. These processes that shaped Earth were natural and operated very slowly, and most of this activity predated humankind by much more than a few days.
Eventually, the scientific community embraced uniformitarianism because it explained a majority of geological mysteries and did not rely on any kind of divine intervention to bring about change.
Modern uniformitarianism differs slightly from its original version. It agrees that the laws of nature operate the same way today as they did millions of years ago, with one exception: the processes that shape Earth operate the same as they always have, but the speed and intensity of those processes may vary. Volcanoes erupt as they have all along (as shown in rocks), but there were times of greater volcanic activity than today. Land erodes now as it did millions of year ago, but land eroded faster when there were no plants to stop rocks and soil from washing into the seas.
Uniformitarianism allows us to interpret the events of the past in rocks; it allows us to write the history of Earth. In addition to allowing the interpretation of the past, uniformitarianism allows for the prediction of the future. Understanding how and when rivers flood, what causes earthquakes and where they are likely to occur, or how and when a volcano will erupt can limit damage from these events. Although short-term prediction still eludes geologists, long-range forecasting of such disasters can ultimately saves lives and property.
[ See also Catastrophism ]