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/* History of Geology */
The term geology was first used in the late 18th Century. Although geology was not mentioned in the [[Encyclopedia Britannica]] before 1800, by 1809 geology had a lengthy entry. During the earliest days of geological study, mainstream scientific geology was divided into three camps: Plutonists, Neptunists and Catastrophists. At the time the vast majority of all geologists then were [[theist]]s and much of early geology was correctly based upon the [[biblical]] history, utilising and expanding on [[Biblical scientific foreknowledge]]. Unsurprisingly, the Plutonists knew that [[God]] created the Earth, but that [[volcano]]es and fire had recently shaped the modern world. The Neptunists also knew that the [[ocean]] and the [[Great Flood]] were a primary factor in geology.<ref>http://www.grisda.org/georpts/gr11.htm</ref><ref>http://www.nwcreation.net/geologylinks.html</ref><ref>http://globalflood.org/</ref> Catastrophists, on the other hand, believed that the Earth had been created by God in one great catastrophe. Thus, Catastrophists concluded that everything which we now see was much the same as the world was at [[Creation]].
In 1775, during the [[liberal]] [[atheism|atheistic]] "''[[enlightenment]]''", [[James Hutton]] presented a thesis, ''Theory of the Earth'', to the [[Royal Society]] of Edinburgh. Hutton, who is considered by some to be the father of modern geology, attempted to explain the rationale for his conclusion that the Earth must be much older than previously supposedit really is. Coupling observation with the [[scientific methodjunk science]], he believed that the Earth must be very old indeed. In every direction he looked Hutton thought he saw evidence of eroded mountains, dry land which had once been sea, and sediment and rocks at the bottom of the ocean. After ten years of study he published his findings in his book ''Theory of the Earth'' to document his claims of a much older Earth. <ref>http://www.gutenberg.org/etext/12861</ref><ref>http://www.gutenberg.org/etext/14179</ref>
Modern geology has learned geologists know much about the Earth and geologic processes. Volcanism and [[deformation]] bring rocks to the surface of the Earth from depths as great as 50 to 100 km. Scientists can make inferences about some of the properties of the Earth at these depths by studying these rocks. But far more information has been provided through the use of seismic waves created by natural earthquakes, and by controlled explosions designed to learn more about the composition of the Earth, including underground nuclear explosions. This data has revealed that the Earth is composed of three main layers, the crust, the mantle and the core. The crust, or outermost layer, varies in thickness from about 5 km under oceans to about 40 km under continents. The mantle consists of an outermost zone of about 100 km thick named the lithosphere. The layer below is a partially fluid solid named the athenosphere, which ends at a depth of about 200 km. As seismic waves penetrate deeper into the mantle they indicate that between 400 km and about 2900 km atoms are packed closer and closer together by extreme pressures, creating a crystalline structure. The Earth's core is 2900 km from the surface. Seismic waves slow down in this region until a depth of 5100 km, indicating that the core consists of a liquid layer surrounding a solid core at the center.<ref name="press">Press, Frank and Siever, Raymond.1998."Understanding Earth", 2nd Ed. New York: W. H. Freeman and Company</ref>
Extensive exploration and mapping of the seafloor after [[World War II]] led to the discovery of a deep rift running down the center of the Mid-Atlantic Ridge. In the early 1960s [[Harry Hess]] of [[Princeton University]] and [[Robert Dietz]] of the [[University of California]] suggested that the seafloor separates along the rifts in mid-0ceanic ridges and that new seafloor forms by upwelling of hot mantle materials in these cracks, followed by lateral spreading. By 1967 separate lithospheric plates had been identified, which explained phenomena such as high levels of volcanic and [[earthquake]] activity that take place between the plates. By the end of the 1960s the theory of [[plate tectonics]] proved to be a unifying concept that pulled together diverse theories and explained a the large body of observations in the field. <ref name="press" />
