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{{Etymology|From the [[Greek]] γη- (ge-, "the [[earth]]") and λογος (logos, "word", "[[reason]]")}}
'''Geology''' is the [[science]] and study of the solid matter of the [[Earth]], its composition, structure, physical properties, [[history]] and the processes that shape it. It is one of the Earth sciences.

==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. Of course, the vast majority of all geologists then were [[theist]]s, but much of early geology was based upon the [[biblical]] history. At that time, the Plutonists believed [[God]] created the Earth, but [[volcano]]es and fire had shaped the modern world. At the same time, the Neptunists believed that the [[ocean]], perhaps even the [[Great Flood]], was the primary factor in geology. 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]]. At that time, it was generally presumed by geologists and theologians that the Earth was around six thousand years old. Many [[Young Earth Creationists]] still agree with this interpretation.<ref>http://www.answersingenesis.org/home/area/faq/geology.asp</ref><ref>http://www.creationism.org/topbar/geology.htm</ref><ref>http://www.grisda.org/georpts/gr11.htm</ref><ref>http://www.nwcreation.net/geologylinks.html</ref><ref>http://globalflood.org/</ref>

In 1775, [[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 supposed. Coupling observation with the [[scientific method]], he believed that the Earth must be very old indeed. In every direction he looked Hutton 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>

While met with initial controversy, over time much of Hutton's conclusions were borne out by observations collected from around the world during the rise of the British Empire and in the [[Royal Geographic Society]]. Based upon mounting evidence in 1830, Sir [[Charles Lyell]] published the first of what would eventually grow into twelve editions of ''Principles of Geology''. Charles Lyell reported geological processes occurring every day. He documented geological process which have occurred during the recorded history of man. By inference he concluded that geological processes must have occurred before the recorded history of man.<ref>http://www.britannica.com/eb/article-9049503/Sir-Charles-Lyell-Baronet</ref><ref>http://www.mnsu.edu/emuseum/information/biography/klmno/lyell_charles.html</ref> In time, most geologists came to agree with his theory of [[uniformitarianism]]. 

Modern geology has learned 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" />

Twelve or so plates slide over a partially molten lithosphere, and the continents, embedded in some of the plates, are carried by them. Where plates collide, mountains such as the [[Himalayas]] or the [[Rocky Mountains]] are formed. In some places, called subduction zones, one plate sinks beneath another plate and returns to the molten regions of the athenosphere. Along mid-oceanic ridges magma emerges to expand the plate and create new sea floor.

==Age of the Earth==
There is not universal agreement on how old the Earth is.  Modern geologists calculate an age for the Earth based on their observations and assumptions.
Most geologists conclude that the Earth is approximately 4.6 billion years old

[[Young earth creationism|Young earth creationists]] believe that the earth is approximately 6,000 years old.<ref>[http://www.creationontheweb.com/content/view/3007/ Geology Questions and Answers] (Creation Ministries International)<!-- A more-specific reference would be better --></ref><ref>[http://www.allaboutcreation.org/geologic-time-scale.htm Geologic Time Scale - The Misconceptions] (All about Creation)</ref>
Unlike geologists (where their views are based on science, experiments, observations and cumulative effort), this is based on their observations, assumptions and beliefs, but in their case that includes the assumption that the Bible gives an accurate eye-witness account of history, that that account in the [[Book of Genesis|Genesis]] leads to the age of around 6,000 years.
Young Earth creationists point to unexplained anomalies and other questions in standard geology as evidence of creation geology. Though this view is controversial within the larger scientific community because most scientists do not acknowledge the [[Bible]] as an historical authority. Despite this, many recent studies by creation scientists have corroborated geologic events with their Biblical accounts, such as the [[Great Flood]], the destruction of [[Sodom]] and [[Gommorah]], and the crossing of the Red Sea<ref>[http://www.sfbsc.com/dead-sea-geology Dead Sea Geology]</ref>.

==Geology Today==
Geologists help locate and manage the Earth's natural resources, such as [[petroleum]] and [[coal]], as well as metals such as [[iron]], [[copper]], and [[uranium]]. Additional economic interests include [[gemstone]]s and many minerals such as [[asbestos]], [[perlite]], [[mica]], [[phosphate]]s, [[zeolites]], [[clay]], [[pumice]], [[quartz]], and [[silica]], as well as elements such as [[sulfur]], [[chlorine]], and [[helium]].

Planetary geology (sometimes known as Astrogeology) refers to the application of geologic principles to other bodies of the [[solar system]]. However, specialised terms such as selenology (studies of the [[Moon]]), areology (of [[Mars]]), etc., are also in use.

==References==
<references/>

==External links==
*[http://www.agiweb.org/workforce/brochure.html Careers in the Geosciences]

[[Category:Geology]]