Uniformitarianism
As the scientific discipline of geology began to take shape during the late eighteenth century, different schools of thought contributed to the debate over the Earth's creation and development. Among the more popular theories in the debate was one based on a slow formation and erosion of the Earth's landscape and another espousing a more rapid, but still natural origin for the rock and fossil record. A third, subscribed to a history consisting of a series of catastrophes, thought by some to be of supernatural (divine) origin.
Philosophers and naturalist historians who subscribed to the gradual evolution of the Earth's landscape became known as Plutonists , or Vulcanists. They believed that the Earth consisted of rock types of several different origins including those such as granite produced by internal heat and cooling, as well as accumulations of materials from erosion at the Earth's surface, such as sandstones.
The main philosophical opponents to Plutonists were called Neptunists. They were led by Abraham Gottlob Werner, a renowned German geologist who believed that all rocks formed by either chemical reactions or mechanical deposition in seawater. Based upon this theory of formation, Werner maintained that after the early oceans flooded the earth, seawater caused rocks (i.e., the continents) to form. Georges Cuvier, a French paleontologist and anatomist, was the best known proponent of the third view--catastrophism. He believed that the Earth's surface had been carved into its current landscape through a series of catastrophes, which also accounted for the complexity of the fossil record. Some adherants considered these catastrophes to be of divine origin and believed the Earth to be only 6000 years, more-or-less according to church doctrine. However, biblical catastrophists were probably in the minority. These three geologic schools of thought represented the state of the art in mid- to late-eighteenth century Europe.
In 1785 James Hutton, a Scottish geologist, presented a paper called System of the Earth to the Royal Society of Edinburgh, in which he refuted Neptunist theory, and indirectly, catastrophist theory as well. Hutton's paper contained the major elements of what was later termed uniformitarianism , and became a cornerstone of geologic thought. He maintained that the rocks of the earth were formed by a variety of natural processes driven by heat energy from the earth's core. These processes included uplift, erosion and deposition, volcanism, and mountain-building. He added that weathering and erosion of today's continental rock would lead to the formation of future continents in a seemingly endless cycle. He stressed that these earth-shaping processes were natural and operated very slowly, and most of this activity predated humankind. Hutton's theory sent ripples through the geologic community because it challenged the commonly accepted premise that the Earth was relatively young. Even some plutonists did not agree with the limitless age proposed by Hutton, and many were critical of the cyclical, almost static, nature of landscape development that he envisioned.
Even though Hutton did not live to see the profound effect of his theory on the geologic community, uniformitarianism gained momentum after his ideas were published by John Playfair. He elaborated on Hutton's ideas, as well as clarifying them, in the influential Illustrations of the Huttonian Theory (1802). Playfair coined the uniformitarian phrase "the present is the key to the past" meaning that by observing geologic processes today, we can understand how they worked to form rocks and landscapes in the past. Although a few of Hutton's other concepts still have some significance in geology today, this one was perhaps the most influential and lasting. About thirty years later, the first great geological textbook, Principals of Geology, was written by Sir Charles Lyell, an English geologist. In it, he explained the principle of uniformitarianism so persuasively, using illustrations and examples from around the world, that for many decades, it was considered the most authoritative geological reference book in the Western world.
Today, geologists apply a more tempered version of uniformitarianism to their work. While geologists still believe, in large part, that "the present is the key to the past", there is now widespread recognition that rare events--natural catastrophes such as massive earthquakes--are important in shaping both the rock record and the fossil record. A good example of this is the catastrophic event that caused the extinction of the dinosaurs at the end of the Mesozoic Era. It resulted in worldwide deposition of a layer of iridium, supposedly from an asteroid impact, and the demise of many types of plants and animals.
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