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GEOLOGY IS NOT A SCIENCE!!!!!!!!!!!!!!!!!!
{{For|the scientific journal|Geology (journal)}}
{{Geology2}}
'''Geology''' (from the [[Ancient Greek|Greek]] γῆ, ''gē'', i.e. "earth" and -λoγία, ''-logia'', i.e. "study of, discourse"<ref name=OnlineEtDict>{{cite web|title=geology|url=http://www.etymonline.com/index.php?term=geology&allowed_in_frame=0|publisher=[[Online Etymology Dictionary]]}}</ref><ref name=LSJ>{{LSJ|gh{{=}}|γῆ|ref}}</ref>) is the [[science]] comprising the study of solid [[Earth]], the [[rock (geology)|rocks]] of which it is composed, and the processes by which they change. Geology can also refer generally to the study of the solid features of any celestial body (such as the [[geology of the Moon]] or [[Geology of Mars|Mars]]).

Geology gives insight into the [[history of the Earth]], as it provides the primary evidence for [[plate tectonics]], the [[evolutionary history of life]], and [[paleoclimatology|past climates]]. In modern times, geology is commercially important for [[mining|mineral]] and [[petroleum geology|hydrocarbon]] exploration and exploitation and for evaluating [[water resources]]. It is publicly important for the prediction and understanding of [[natural hazard]]s, the remediation of [[Environmental Geology|environmental]] problems, and for providing insights into past [[climate change]]. Geology plays a role in [[geotechnical engineering]] and is a major [[academic discipline]].
{{TOC limit|limit=2}}

==Geologic time==
[[Image:Geologic Clock with events and periods.svg|thumb|300px|Geological time put in a diagram called a [[geological clock]], showing the relative lengths of the [[geologic eon|eons]] of the Earth's history.]]
{{Main|History of the Earth|Geologic time scale}}

The geologic time scale encompasses the history of the Earth.<ref>[http://www.stratigraphy.org/ International Commission on Stratigraphy]</ref> It is bracketed at the old end by the dates of the earliest [[Solar System|solar system]] material at 4.567 [[Gigaannum|Ga]],<ref name="4.567">{{Cite journal |doi=10.1126/science.1073950 |year=2002 |month=Sep |author=Amelin, Y; Krot, An; Hutcheon, Id; Ulyanov, Aa |title=Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions. |volume=297 |issue=5587 |pages=1678–83 |issn=0036-8075 |pmid=12215641 |journal=Science |bibcode=2002Sci...297.1678A}}</ref> (gigaannum: billion years ago) and the age of the Earth at 4.54 Ga<ref name="4.54">Patterson, C., 1956. “Age of Meteorites and the Earth.” Geochimica et Cosmochimica Acta 10: p. 230-237.</ref><ref name="4.54 book">{{Cite book |isbn=0-8047-2331-1 |author=G. Brent Dalrymple |year=1994 |publisher=Stanford Univ. Press |location=Stanford, Calif. |title=The age of the earth}}</ref> at the beginning of the informally recognized [[Hadean eon]]. At the young end of the scale, it is bracketed by the present day in the [[Holocene epoch]].

===Important milestones===
* 4.567 Ga: [[Formation and evolution of the Solar System|Solar system formation]]<ref name="4.567" />
* 4.54 Ga: Accretion of Earth<ref name="4.54" /><ref name="4.54 book" />
* c. 4 Ga: End of [[Late Heavy Bombardment]], first life
* c. 3.5 Ga: Start of [[photosynthesis]]
* c. 2.3 Ga: Oxygenated [[atmosphere]], first [[snowball Earth]]
* 730–635 [[Ma (unit)|Ma]] (megaannum: million years ago): two snowball Earths
* 542± 0.3 Ma: [[Cambrian explosion]] – vast multiplication of hard-bodied life; first abundant [[fossil]]s; start of the [[Paleozoic]]
* c. 380 Ma: First [[vertebrate]] land animals
* 250 Ma: [[Permian-Triassic extinction]] – 90% of all land animals die. End of Paleozoic and beginning of [[Mesozoic]]
* 65 Ma: [[Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction]] – [[Dinosaurs]] die; end of Mesozoic and beginning of [[Cenozoic]]
* c. 7 Ma – Present: Hominins
** c. 7 Ma: First [[hominin]]s appear
** 3.9 Ma: First [[Australopithecus]], direct ancestor to modern [[Homo sapiens]], appear
** 200 [[Ma (unit)|ka]] (kiloannum: thousand years ago): First modern Homo sapiens appear in East Africa

===Brief time scale===

{{Timeline Geological Timescale}}

===Relative and absolute dating===
Geological events can be given a precise date at a point in time, or they can be related to other events that came before and after them. Geologists use a variety of methods to give both relative and absolute dates to geological events. They then use these dates to find the rates at which processes occur.

====Relative dating====
{{Main|Relative dating}}
[[Image:Cross-cutting relations.svg|thumb|350px|[[Cross-cutting relations]] can be used to determine the relative ages of [[stratum|rock strata]] and other geological structures. Explanations: A - [[fold (geology)|folded]] rock strata cut by a [[thrust fault]]; B - large [[intrusion]] (cutting through A); C - [[erosion]]al [[angular unconformity]] (cutting off A & B) on which rock strata were deposited; D - [[dike (geology)|volcanic dyke]] (cutting through A, B & C); E - even younger rock strata (overlying C & D); F - [[normal fault]] (cutting through A, B, C & E).]]

Methods for [[relative dating]] were developed when geology first emerged as a [[formal science]]. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.

'''The [[principle of Uniformitarianism]]''' states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.<ref>Reijer Hooykaas, [http://books.google.com/books?id=3TgYAAAAIAAJ&pgis=1 ''Natural Law and Divine Miracle: The Principle of Uniformity in Geology, Biology, and Theology''], Leiden: [[EJ Brill]], 1963.</ref> A fundamental principle of geology advanced by the 18th century Scottish physician and geologist [[James Hutton]], is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."<ref>{{cite book|last=Levin|first=Harold L.|title=The earth through time|year=2010|publisher=J. Wiley|location=Hoboken, N.J.|isbn=978-0-470-38774-0|edition=9th|page=18}}</ref>

'''[[Intrusion (geology)|The principle of intrusive relationships]]''' concerns crosscutting intrusions. In geology, when an [[igneous rocks|igneous]] intrusion cuts across a formation of [[sedimentary rock]], it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, [[laccolith]]s, [[batholith]]s, [[Sill (geology)|sills]] and [[Dike (geology)|dikes]].

'''The [[principle of cross-cutting relationships]]''' pertains to the formation of [[Fault (geology)|faults]] and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a [[normal fault]] or a [[thrust fault]].<ref name="steno strat">{{cite web |url=http://rainbow.ldeo.columbia.edu/courses/v1001/steno.html |title=Steno's Principles of Stratigraphy |last=Olsen |first=Paul E. |year=2001 |work=Dinosaurs and the History of Life |publisher=Columbia University |accessdate=2009-03-14}}</ref>

'''The [[principle of inclusions and components]]''' states that, with sedimentary rocks, if inclusions (or ''[[Clastic rocks|clasts]]'') are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when [[xenolith]]s are found. These foreign bodies are picked up as [[magma]] or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.

[[Image:SEUtahStrat.JPG|thumb|300px|The [[Permian]] through [[Jurassic]] stratigraphy of the [[Colorado Plateau]] area of southeastern [[Utah]] is a great example of both Original Horizontality and the Law of Superposition. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as [[Capitol Reef National Park]] and [[Canyonlands National Park]]. From top to bottom: Rounded tan domes of the [[Navajo Sandstone]], layered red [[Kayenta Formation]], cliff-forming, vertically jointed, red [[Wingate Sandstone]], slope-forming, purplish [[Chinle Formation]], layered, lighter-red [[Moenkopi Formation]], and white, layered [[Cutler Formation]] sandstone. Picture from [[Glen Canyon National Recreation Area]], Utah.]]
'''The [[principle of original horizontality]]''' states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although [[cross-bedding]] is inclined, the overall orientation of cross-bedded units is horizontal).<ref name="steno strat" />

'''The [[Law of superposition|principle of superposition]]''' states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.<ref name="steno strat" />

'''The [[principle of faunal succession]]''' is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of [[Charles Darwin]]'s [[theory of evolution]], the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat ([[facies]] change in sedimentary strata), and that not all fossils may be found globally at the same time.<ref>As recounted in [[Simon Winchester]], ''The Map that Changed the World'' (New York: HarperCollins, 2001), pp. 59–91.</ref>

====Absolute dating====
{{Main|Absolute dating|Radiometric dating|Geochronology}}
Geologists can also give precise absolute dates to geologic events. These dates are useful on their own, and can also be used in conjunction with relative dating methods or to calibrate relative dating methods.<ref>{{cite doi|10.1016/S0012-821X(98)00050-8}}</ref>

A large advance in geology in the advent of the 20th century was the ability to give precise absolute dates to geologic events through radioactive isotopes and other methods. The advent of radiometric dating changed the understanding of geologic time. Before, geologists could only use fossils to date sections of rock relative to one another. With isotopic dates, [[absolute dating]] became possible, and these absolute dates could be applied fossil sequences in which there was datable material, converting the old relative ages into new absolute ages.

For many geologic applications, [[isotope]] ratios are measured in minerals that give the amount of time that has passed since a rock passed through its particular [[closure temperature]], the point at which different radiometric isotopes stop diffusing into and out of the [[Crystal structure|crystal lattice]].<ref>{{Cite book |isbn=978-0-582-06701-1 |author=Hugh R. Rollinson |year=1996 |publisher=Longman |location=Harlow |title=Using geochemical data evaluation, presentation, interpretation}}</ref><ref>{{Cite book |isbn=978-0-02-336450-1 |author=Gunter Faure. |year=1998 |publisher=Prentice-Hall |location=Upper Saddle River, NJ |title=Principles and applications of geochemistry: a comprehensive textbook for geology students}}</ref> These are used in [[geochronology|geochronologic]] and [[thermochronology|thermochronologic]] studies. Common methods include [[uranium-lead dating]], [[potassium-argon dating]] and [[argon-argon dating]], and [[uranium-thorium dating]]. These methods are used for a variety of applications. Dating of lavas and ash layers can help to date stratigraphy and calibrate relative dating techniques. These methods can also be used to determine ages of [[pluton]] emplacement. Thermochemical techniques can be used to determine temperature profiles within the crust, the uplift of mountain ranges, and paleotopography.

Fractionation of the [[lanthanide series]] elements is used to compute ages since rocks were removed from the mantle.

Other methods are used for more recent events. [[Optically stimulated luminescence]] and [[Cosmogenic isotope#Natural|cosmogenic radionucleide]] dating are used to date surfaces and/or erosion rates. [[Dendrochronology]] can also be used for the dating of landscapes. [[Radiocarbon dating]] is used for young [[organic material]].

==Geologic materials==
The majority of geological data come from research on solid Earth materials. These typically fall into one of two categories: rock and unconsolidated material.

===Rock===
[[File:rock cycle.gif|thumb|300px|This schematic diagram of the rock cycle shows the relationship between magma and sedimentary, metamorphic, and igneous rock]]

{{Main|Rock (geology)|Rock cycle}}

There are three major types of rock: [[igneous]], [[sedimentary]], and [[metamorphic]]. The [[rock cycle]] is an important concept in geology which illustrates the relationships between these three types of rock, and magma. When a rock [[crystallization|crystallizes]] from melt ([[magma]] and/or [[lava]]), it is an igneous rock. This rock can be [[weathering|weathered]] and [[eroded]], and then [[deposition (geology)|redeposited]] and [[lithification|lithified]] into a sedimentary rock, or be turned into a [[metamorphic rock]] due to heat and pressure that change the [[mineral]] content of the rock and give it a characteristic [[fabric (geology)|fabric]]. The sedimentary rock can then be subsequently turned into a metamorphic rock due to heat and pressure, and the metamorphic rock can be weathered, eroded, deposited, and lithified, becoming a sedimentary rock. Sedimentary rock may also be re-eroded and redeposited, and metamorphic rock may also undergo additional metamorphism. All three types of rocks may be re-melted; when this happens, a new magma is formed, from which an igneous rock may once again crystallize.

The majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth.

===Unconsolidated material===
Geologists also study unlithified material, which typically comes from more recent deposits. Because of this, the study of such material is often known as [[Quaternary geology]], after the recent [[Quaternary Period]]. This includes the study of sediment and [[soils]], and is important to some (or many) studies in [[geomorphology]], [[sedimentology]], and [[paleoclimatology]].

==Whole-Earth structure==
[[File:Active Margin.svg|thumb|left|Oceanic-continental convergence resulting in [[subduction]] and [[volcanic arc]]s illustrates one effect of [[plate tectonics]].]]

===Plate tectonics===
{{Main|Plate tectonics}}
[[Image:Farallon Plate.jpg|thumb|right|On this diagram, subducting [[Slab (geology)|slabs]] are in blue, and continental margins and a few plate boundaries are in red. The blue blob in the cutaway section is the seismically imaged [[Farallon Plate]], which is subducting beneath North America. The remnants of this plate on the Surface of the Earth are the [[Juan de Fuca Plate]] and [[Explorer plate]] in the Northwestern USA / Southwestern Canada, and the [[Cocos Plate]] on the west coast of Mexico.]]
In the 1960s, a series of discoveries, the most important of which was seafloor spreading,<ref>H. H. Hess, "[http://repositories.cdlib.org/sio/lib/23 History Of Ocean Basins]" (November 1, 1962). IN: Petrologic studies: a volume in honor of A. F. Buddington. A. E. J. Engel, Harold L. James, and B. F. Leonard, editors. [New York?]: [[Geological Society of America]], 1962. pp. 599–620.</ref><ref name="TDE discovery">{{Cite book |last=Kious |first=Jacquelyne |coauthors=Tilling, Robert I. |others=Kiger, Martha, Russel, Jane |title=This Dynamic Earth: The Story of Plate Tectonics |publisher=United States Geological Survey |location=Reston, Virginia, USA |date=February 1996 |edition=Online |chapter=Developing the Theory |isbn=0-16-048220-8 |url=http://pubs.usgs.gov/gip/dynamic/understanding.html |accessdate=13 March 2009}}</ref> showed that the Earth's [[lithosphere]], which includes the [[Crust (geology)|crust]] and rigid uppermost portion of the [[upper mantle]], is separated into a number of [[tectonic plate]]s that move across the [[Plasticity (physics) | plastically]] deforming, solid, upper mantle, which is called the [[asthenosphere]]. There is an intimate coupling between the movement of the plates on the surface and the [[mantle convection|convection of the mantle]]: oceanic plate motions and mantle [[Convection|convection currents]] always move in the same direction, because the oceanic lithosphere is the rigid upper thermal [[boundary layer]] of the convecting mantle. This coupling between rigid plates moving on the surface of the Earth and the convecting [[Mantle (geology)|mantle]] is called plate tectonics.

The development of plate tectonics provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features could be explained as plate boundaries.<ref name="TDE plates">{{Cite book |last=Kious |first=Jacquelyne |coauthors=Tilling, Robert I. |others=Kiger, Martha, Russel, Jane |title=This Dynamic Earth: The Story of Plate Tectonics |publisher=United States Geological Survey |location=Reston, Virginia, USA |date=February 1996 |edition=Online |chapter=Understanding Plate Motions |isbn=0-16-048220-8 |url=http://pubs.usgs.gov/gip/dynamic/understanding.html |accessdate=13 March 2009}}</ref> [[Mid-ocean ridge]]s, high regions on the seafloor where [[hydrothermal vent]]s and volcanoes exist, were explained as [[divergent boundary|divergent boundaries]], where two plates move apart. Arcs of volcanoes and earthquakes were explained as [[convergent boundary|convergent boundaries]], where one plate [[subduction|subducts]] under another. [[Transform boundary|Transform boundaries]], such as the [[San Andreas fault]] system, resulted in widespread powerful earthquakes. Plate tectonics also provided a mechanism for [[Alfred Wegener|Alfred Wegener's]] theory of [[continental drift]],<ref>{{Cite book |isbn=0-486-61708-4 |author= |year=1999 |publisher=Dover Pub |location=S.l. |title=Origin of continents and oceans}}</ref> in which the [[continents]] move across the surface of the Earth over geologic time. They also provided a driving force for crustal deformation, and a new setting for the observations of structural geology. The power of the theory of plate tectonics lies in its ability to combine all of these observations into a single theory of how the lithosphere moves over the convecting mantle.

===Earth structure===
{{Main|Structure of the Earth}}
[[Image:Jordens inre-numbers.svg|thumb|left|The [[Earth]]'s layered structure. (1) inner core; (2) outer core; (3) lower mantle; (4) upper mantle; (5) lithosphere; (6) crust]]
[[Image:Earthquake wave paths.svg|thumb|right|Earth layered structure. Typical wave paths from earthquakes like these gave early seismologists insights into the layered structure of the Earth]]
Advances in [[seismology]], [[computer modeling]], and [[mineralogy]] and [[crystallography]] at high temperatures and pressures give insights into the internal composition and structure of the Earth.

Seismologists can use the arrival times of [[seismic wave]]s in reverse to image the interior of the Earth. Early advances in this field showed the existence of a liquid [[outer core]] (where [[shear wave]]s were not able to propagate) and a dense solid [[inner core]]. These advances led to the development of a layered model of the Earth, with a [[Crust (geology)|crust]] and [[lithosphere]] on top, the [[Mantle (geology)|mantle]] below (separated within itself by [[Seismic tomography|seismic discontinuities]] at 410 and 660 kilometers), and the outer core and inner core below that. More recently, seismologists have been able to create detailed images of wave speeds inside the earth in the same way a doctor images a body in a CT scan. These images have led to a much more detailed view of the interior of the Earth, and have replaced the simplified layered model with a much more dynamic model.

Mineralogists have been able to use the pressure and temperature data from the seismic and modelling studies alongside knowledge of the elemental composition of the Earth at depth to reproduce these conditions in experimental settings and measure changes in crystal structure. These studies explain the chemical changes associated with the major seismic discontinuities in the mantle, and show the crystallographic structures expected in the inner core of the Earth.

==Geological development of an area==
[[File:Volcanosed.svg|thumb|right|300px|An originally horizontal sequence of sedimentary rocks (in shades of tan) are affected by [[igneous]] activity. Deep below the surface are a [[magma chamber]] and large associated igneous bodies. The magma chamber feeds the [[volcano]], and sends off shoots of [[magma]] that will later crystallize into dikes and sills. Magma also advances upwards to form [[intrusive rock|intrusive igneous bodies]]. The diagram illustrates both a [[cinder cone]] volcano, which releases ash, and a [[composite volcano]], which releases both lava and ash.]]
[[File:Fault types.png|thumb|left|200px| An illustration of the three types of faults. Strike-slip faults occur when rock units slide past one another, normal faults occur when rocks are undergoing horizontal extension, and thrust faults occur when rocks are undergoing horizontal shortening.]]
The geology of an area changes through time as rock units are deposited and inserted and deformational processes change their shapes and locations.

Rock units are first emplaced either by deposition onto the surface or intrusion into the [[Country rock (geology)|overlying rock]]. Deposition can occur when sediments settle onto the surface of the Earth and later [[lithification|lithify]] into sedimentary rock, or when as [[volcanic rock|volcanic material]] such as [[volcanic ash]] or [[lava flow]]s blanket the surface. [[Igneous intrusion]]s such as [[batholith]]s, [[laccolith]]s, [[dike (geology)|dikes]], and [[sill (geology)|sills]], push upwards into the overlying rock, and crystallize as they intrude.

After the initial sequence of rocks has been deposited, the rock units can be [[deformation (mechanics)|deformed]] and/or [[metamorphism|metamorphosed]]. Deformation typically occurs as a result of horizontal shortening, [[extension (geology)|horizontal extension]], or side-to-side ([[strike-slip]]) motion. These structural regimes broadly relate to [[convergent boundary|convergent boundaries]], [[divergent boundary|divergent boundaries]], and transform boundaries, respectively, between tectonic plates.

When rock units are placed under horizontal [[compression (geology)|compression]], they shorten and become thicker. Because rock units, other than muds, [[Incompressible surface|do not significantly change in volume]], this is accomplished in two primary ways: through [[fault (geology)|faulting]] and [[fold (geology)|folding]]. In the shallow crust, where [[brittle deformation]] can occur, thrust faults form, which cause deeper rock to move on top of shallower rock. Because deeper rock is often older, as noted by the [[law of superposition|principle of superposition]], this can result in older rocks moving on top of younger ones. Movement along faults can result in folding, either because the faults are not planar, or because the rock layers are dragged along, forming drag folds, as slip occurs are along the fault. Deeper in the Earth, rocks behave plastically, and fold instead of faulting. These folds can either be those where the material in the center of the fold buckles upwards, creating "[[antiform]]s", or where it buckles downwards, creating "[[synform]]s". If the tops of the rock units within the folds remain pointing upwards, they are called [[anticline]]s and [[syncline]]s, respectively. If some of the units in the fold are facing downward, the structure is called an overturned anticline or syncline, and if all of the rock units are overturned or the correct up-direction is unknown, they are simply called by the most general terms, antiforms and synforms.
[[File:Antecline (PSF).png|thumb|right|A diagram of folds, indicating an [[anticline]] and a [[syncline]].]]
Even higher pressures and temperatures during horizontal shortening can cause both folding and [[metamorphism]] of the rocks. This metamorphism causes changes in the [[mineral|mineral composition]] of the rocks; creates a [[foliation (geology)|foliation]], or planar surface, that is related to mineral growth under stress; and can remove signs of the original textures of the rocks, such as [[bed (geology)|bedding]] in sedimentary rocks, flow features of [[lava]]s, and crystal patterns in [[crystalline rock]]s.

Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through [[normal fault]]ing and through the ductile stretching and thinning. Normal faults drop rock units that are higher below those that are lower. This typically results in younger units being placed below older units. Stretching of units can result in their thinning; in fact, there is a location within the [[Maria Fold and Thrust Belt]] in which the entire sedimentary sequence of the [[Grand Canyon]] can be seen over a length of less than a meter. Rocks at the depth to be ductilely stretched are often also metamorphosed. These stretched rocks can also pinch into lenses, known as [[boudinage|boudins]], after the French word for "sausage", because of their visual similarity.

Where rock units slide past one another, [[strike-slip fault]]s develop in shallow regions, and become [[shear zone]]s at deeper depths where the rocks deform ductilely.

[[File:Kittatinny Mountain Cross Section.jpg|thumb|right|Geologic cross-section of [[Kittatinny Mountain]]. This cross-section shows metamorphic rocks, overlain by younger sediments deposited after the metamorphic event. These rock units were later folded and faulted during the uplift of the mountain.]]

The addition of new rock units, both depositionally and intrusively, often occurs during deformation. Faulting and other deformational processes result in the creation of topographic gradients, causing material on the rock unit that is increasing in elevation to be eroded by hillslopes and channels. These sediments are deposited on the rock unit that is going down. Continual motion along the fault maintains the topographic gradient in spite of the movement of sediment, and continues to create [[accommodation space]] for the material to deposit. Deformational events are often also associated with volcanism and igneous activity. Volcanic ashes and lavas accumulate on the surface, and igneous intrusions enter from below. [[Dike (geology)|Dikes]], long, planar igneous intrusions, enter along cracks, and therefore often form in large numbers in areas that are being actively deformed. This can result in the emplacement of [[dike swarm]]s, such as those that are observable across the Canadian shield, or rings of dikes around the [[lava tube]] of a volcano.

All of these processes do not necessarily occur in a single environment, and do not necessarily occur in a single order. The [[Hawaiian Islands]], for example, consist almost entirely of layered [[basalt]]ic lava flows. The sedimentary sequences of the mid-continental United States and the [[Geology of the Grand Canyon area|Grand Canyon]] in the southwestern United States contain almost-undeformed stacks of sedimentary rocks that have remained in place since [[Cambrian]] time. Other areas are much more geologically complex. In the southwestern United States, sedimentary, volcanic, and intrusive rocks have been metamorphosed, faulted, foliated, and folded. Even older rocks, such as the [[Acasta gneiss]] of the [[Slave craton]] in northwestern [[Canada]], the [[Oldest rock|oldest known rock in the world]] have been metamorphosed to the point where their origin is undiscernable without laboratory analysis. In addition, these processes can occur in stages. In many places, the Grand Canyon in the southwestern United States being a very visible example, the lower rock units were metamorphosed and deformed, and then deformation ended and the upper, undeformed units were deposited. Although any amount of rock emplacement and rock deformation can occur, and they can occur any number of times, these concepts provide a guide to understanding the [[geological history]] of an area.

==Methods of geology==
Geologists use a number of field, laboratory, and numerical modeling methods to decipher Earth history and understand the processes that occur on and in the Earth. In typical geological investigations, geologists use primary information related to [[petrology]] (the study of rocks), stratigraphy (the study of sedimentary layers), and structural geology (the study of positions of rock units and their deformation). In many cases, geologists also study modern soils, [[river]]s, [[landscape]]s, and [[glacier]]s; investigate past and current life and [[biogeochemistry|biogeochemical]] pathways, and use [[geophysics|geophysical methods]] to investigate the subsurface.

[[File:Washington State Land Forms.tif|thumb|Washington State Land Forms]]

===Field methods===
[[Image:Brunton.JPG|thumb|A standard [[:en:Brunton compass|Brunton Pocket Transit]], used commonly by geologists in mapping and surveying]]
[[File:USGS 1950s mapping field camp.jpg|thumb|A typical [[USGS]] field mapping camp in the 1950s]]
[[File:PDA Mapping.jpg|thumb|right|Today, [[handheld computer]]s with [[GPS]] and [[geographic information systems]] software are often used in geological field work ([[digital geologic mapping]]).]]

Geological [[field work]] varies depending on the task at hand. Typical fieldwork could consist of:
* [[Geological map]]ping<ref>{{Cite book |isbn=0-471-82902-1 |author=Robert R. Compton. |year=1985 |publisher=Wiley |location=New York |title=Geology in the field}}</ref>
** Structural mapping: the locations of the major rock units and the faults and folds that led to their placement there.
** Stratigraphic mapping: the locations of [[sedimentary facies]] ([[Lithology|lithofacies]] and [[biofacies]]) or the mapping of [[isopach]]s of equal thickness of sedimentary rock
** Surficial mapping: the locations of soils and surficial deposits
* Surveying of topographic features
** Creation of [[topographic map]]s<ref>{{cite web |url=http://topomaps.usgs.gov/ |title=USGS Topographic Maps |publisher=United States Geological Survey |accessdate=2009-04-11}}</ref>
** Work to understand change across landscapes, including:
*** Patterns of [[erosion]] and [[deposition (geology)|deposition]]
*** River channel change through [[meander|migration]] and [[avulsion (river)|avulsion]]
*** Hillslope processes
* Subsurface mapping through [[Geophysical survey|geophysical methods]]<ref>{{Cite book |isbn=0-393-92637-0 |author=H. Robert Burger, Anne F. Sheehan, Craig H. Jones. |year=2006 |publisher=W.W. Norton |location=New York |title=Introduction to applied geophysics : exploring the shallow subsurface}}</ref>
** These methods include:
*** Shallow [[seismology|seismic]] surveys
*** [[Ground-penetrating radar]]
*** [[Electrical resistivity tomography]]
** They are used for:
*** [[Exploration geophysics|Hydrocarbon exploration]]
*** Finding [[groundwater]]
*** [[Archaeological geophysics|Locating buried archaeological artifacts]]
* High-resolution stratigraphy
** Measuring and describing stratigraphic sections on the surface
** [[Well drilling]] and [[well logging|logging]]
* [[Biogeochemistry]] and [[geomicrobiology]]<ref>{{Cite book |isbn=0-250-40218-1 |author=ed. by Wolfgang E. Krumbein |year=1978 |publisher=Ann Arbor Science Publ. |location=Ann Arbor, Mich. |title=Environmental biogeochemistry and geomicrobiology}}</ref>
** Collecting samples to:
*** Determine [[biochemical pathway]]s
*** Identify new [[species (biology)|species]] of organisms
*** Identify new [[chemical compound]]s
** And to use these discoveries to
*** Understand early life on Earth and how it functioned and metabolized
*** Find important compounds for use in pharmaceuticals.
* [[Paleontology]]: excavation of [[fossil]] material
** For research into past life and [[evolution]]
** For [[museum]]s and education
* Collection of samples for [[geochronology]] and [[thermochronology]]<ref>{{Cite book |isbn= 0-19-510920-1 |author= [[Ian McDougall (geologist)|Ian McDougall]], T. Mark Harrison. |year= 1999 |publisher= Oxford University Press |location= New York |title= Geochronology and thermochronology by the ♯°Ar/©Ar method}}</ref>
* [[Glaciology]]: measurement of characteristics of glaciers and their motion<ref>{{Cite book |isbn=0-470-84426-4 |author=Bryn Hubbard, Neil Glasser. |year=2005 |publisher=J. Wiley |location=Chichester, England |title=Field techniques in glaciology and glacial geomorphology}}</ref>

===Laboratory methods===
[[Image:lecia dmrx.jpg|thumb|A [[petrographic microscope]], which is an [[optical microscope]] fitted with cross-[[Polarizer|polarizing]] lenses, a [[conoscopy|conoscopic lens]], and compensators (plates of anisotropic materials; gypsum plates and quartz wedges are common), for crystallographic analysis.]]

====Petrology====
{{Main|Petrology}}

In addition to the field identification of rocks, petrologists identify rock samples in the laboratory. Two of the primary methods for identifying rocks in the laboratory are through [[optical microscopy]] and by using an [[electron microprobe]]. In an [[optical mineralogy]] analysis, [[thin section]]s of rock samples are analyzed through a [[petrographic microscope]], where the minerals can be identified through their different properties in plane-polarized and cross-polarized light, including their [[birefringence]], [[pleochroism]], [[Crystal twinning|twinning]], and interference properties with a [[Conoscopy|conoscopic lens]].<ref>{{Cite book |isbn=0-19-506024-5 |author=William D. Nesse. |year=1991 |publisher=Oxford University Press |location=New York |title=Introduction to optical mineralogy}}</ref> In the electron microprobe, individual locations are analyzed for their exact chemical compositions and variation in composition within individual crystals.<ref>{{Cite journal |doi=10.1111/j.1365-3091.1985.tb00470.x |title=A new approach to provenance studies: electron microprobe analysis of detrital garnets from Middle Jurassic sandstones of the northern North Sea |year=1985 |author=Morton, ANDREW C. |journal=Sedimentology |volume=32 |page=553 |issue=4|bibcode = 1985Sedim..32..553M }}</ref> [[Stable isotope|Stable]]<ref>{{Cite journal |doi=10.1016/S0012-8252(02)00133-2 |title=Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime |year=2003 |author=Zheng, Y |journal=Earth-Science Reviews |volume=62 |page=105 |bibcode=2003ESRv...62..105Z |last2=Fu |first2=Bin |last3=Gong |first3=Bing |last4=Li |first4=Long}}</ref> and [[radioactive isotope]]<ref>{{Cite journal |doi=10.1016/0012-821X(95)00052-E |title=Magma dynamics at Mt Etna: Constraints from U-Th-Ra-Pb radioactive disequilibria and Sr isotopes in historical lavas |year=1995 |author=Condomines, M |journal=Earth and Planetary Science Letters |volume=132 |page=25 |last2=Tanguy |first2=J |last3=Michaud |first3=V |bibcode=1995E&PSL.132...25C}}</ref> studies provide insight into the [[Geochemistry|geochemical]] evolution of rock units.

Petrologists use [[Fluid inclusions|fluid inclusion]] data<ref>{{Cite book |isbn=0-412-00601-4 |author=T.J. Shepherd, A.H. Rankin, D.H.M. Alderton. |year=1985 |publisher=Blackie |location=Glasgow |title=A practical guide to fluid inclusion studies}}</ref> and perform high temperature and pressure physical experiments<ref>{{Cite journal |doi=10.1007/BF00375521 |title=Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids |year=1987 |author=Sack, Richard O. |journal=Contributions to Mineralogy and Petrology |volume=96 |page=1 |last2=Walker |first2=David |last3=Carmichael |first3=Ian S. E. |bibcode=1987CoMP...96....1S}}</ref> to understand the temperatures and pressures at which different mineral phases appear, and how they change through igneous<ref>{{Cite book |isbn=978-0-7637-3448-0 |author=Alexander R. McBirney. |year=2007 |publisher=Jones and Bartlett Publishers |location=Boston |title=Igneous petrology}}</ref> and metamorphic processes. This research can be extrapolated to the field to understand metamorphic processes and the conditions of crystallization of igneous rocks.<ref>{{Cite book
| isbn = 978-0-939950-34-8
| author = Frank S. Spear
| year = 1995
| publisher = Mineralogical Soc. of America
| location = Washington, DC
| title = Metamorphic phase equilibria and pressure-temperature-time paths}}</ref> This work can also help to explain processes that occur within the Earth, such as [[subduction]] and [[magma chamber]] evolution.

====Structural geology====
[[File:Orogenic wedge.jpg|thumb|left|400px|A diagram of an orogenic wedge. The wedge grows through faulting in the interior and along the main basal fault, called the [[Decollement|décollement]]. It builds its shape into a [[critical taper]], in which the angles within the wedge remain the same as failures inside the material balance failures along the décollement. It is analogous to a bulldozer pushing a pile of dirt, where the bulldozer is the overriding plate.]]
{{Main|Structural geology}}
Structural geologists use microscopic analysis of oriented thin sections of geologic samples to observe the [[fabric (geology)|fabric]] within the rocks which gives information about strain within the crystal structure of the rocks. They also plot and combine measurements of geological structures in order to better understand the orientations of faults and folds in order to reconstruct the history of rock deformation in the area. In addition, they perform analog and numerical experiments of rock deformation in large and small settings.

The analysis of structures is often accomplished by plotting the orientations of various features onto [[stereographic projection|stereonets]]. A stereonet is a stereographic projection of a sphere onto a plane, in which planes are projected as lines and lines are projected as points. These can be used to find the locations of fold axes, relationships between several faults, and relationships between other geologic structures.

Among the most well-known experiments in structural geology are those involving [[orogenic wedge]]s, which are zones in which [[mountain]]s are built along [[convergent boundary|convergent]] tectonic plate boundaries.<ref>{{Cite journal |doi=10.1146/annurev.ea.18.050190.000415 |title=Critical Taper Model of Fold-And-Thrust Belts and Accretionary Wedges |year=1990 |author=Dahlen, F A |journal=Annual Review of Earth and Planetary Sciences |volume=18 |page=55|bibcode = 1990AREPS..18...55D }}</ref> In the analog versions of these experiments, horizontal layers of sand are pulled along a lower surface into a back stop, which results in realistic-looking patterns of faulting and the growth of a [[critical taper|critically tapered]] (all angles remain the same) orogenic wedge.<ref>{{Cite journal |doi=10.1016/S0191-8141(97)00096-5 |title=Material transfer in accretionary wedges from analysis of a systematic series of analog experiments |year=1998 |author=Gutscher, M |journal=Journal of Structural Geology |volume=20 |page=407 |issue=4|bibcode = 1998JSG....20..407G }}</ref> Numerical models work in the same way as these analog models, though they are often more sophisticated and can include patterns of erosion and uplift in the mountain belt.<ref>{{Cite journal |doi=10.1146/annurev.ea.23.050195.002111 |title=Modeling the Topographic Evolution of Collisional Belts |year=1995 |author=Koons, P O |journal=Annual Review of Earth and Planetary Sciences |volume=23 |page=375|bibcode = 1995AREPS..23..375K }}</ref> This helps to show the relationship between erosion and the shape of the mountain range. These studies can also give useful information about pathways for metamorphism through pressure, temperature, space, and time.<ref>{{cite journal |last=Dahlen |first=F. A. |last2=Suppe |first2=J. |last3=Davis |first3=D. |title=Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory |journal=[[Journal of Geophysical Research|J. Geophys. Res.]] |volume=89 |issue=B12 |pages=10087–10101 |year=1984 |doi=10.1029/JB089iB12p10087 |bibcode = 1984JGR....8910087D }}</ref>

====Stratigraphy====
{{Main|Stratigraphy}}
[[File:Exploration geologist.jpg|thumb|250px|Exploration geologists examining a freshly recovered drill core. [[Chile]], 1994]]
In the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those from [[drill core]]s.<ref name="hodell"/> Stratigraphers also analyze data from geophysical surveys that show the locations of stratigraphic units in the subsurface.<ref>{{Cite book |isbn=0-89181-033-1 |author=edited by A.W. Bally. |year=1987 |publisher=American Association of Petroleum Geologists |location=Tulsa, Okla., U.S.A. |title=Atlas of seismic stratigraphy}}</ref> Geophysical data and [[well log]]s can be combined to produce a better view of the subsurface, and stratigraphers often use computer programs to do this in three dimensions.<ref>{{Cite journal |doi=10.1306/02260403062 |title=Three-dimensional reconstruction of geological surfaces: An example of growth strata and turbidite systems from the Ainsa basin (Pyrenees, Spain) |year=2004 |author=Fernández, O. |journal=AAPG Bulletin |volume=88 |page=1049 |last2=Muñoz |first2=J. A. |last3=Arbués |first3=P. |last4=Falivene |first4=O. |last5=Marzo |first5=M. |issue=8}}</ref> Stratigraphers can then use these data to reconstruct ancient processes occurring on the surface of the Earth,<ref>{{Cite journal |doi=10.1130/0016-7606(1998)110<1105:TDSEOT>2.3.CO;2 |title=Three-dimensional stratigraphic evolution of the Miocene Baltimore Canyon region: Implications for eustatic interpretations and the systems tract model |year=1998 |author=Poulsen, Chris J. |journal=Geological Society of America Bulletin |volume=110 |page=1105 |last2=Flemings |first2=Peter B. |last3=Robinson |first3=Ruth A. J. |last4=Metzger |first4=John M. |issue=9|bibcode = 1998GSAB..110.1105P }}</ref> interpret past environments, and locate areas for water, coal, and hydrocarbon extraction.

In the laboratory, [[biostratigraphy|biostratigraphers]] analyze rock samples from outcrop and drill cores for the fossils found in them.<ref name=hodell>{{Cite journal |doi=10.1029/94PA01838 |title=Magnetostratigraphic, Biostratigraphic, and Stable Isotope Stratigraphy of an Upper Miocene Drill Core from the Salé Briqueterie (Northwestern Morocco): A High-Resolution Chronology for the Messinian Stage |year=1994 |author=Hodell, David A. |journal=Paleoceanography |volume=9 |page=835 |last2=Benson |first2=Richard H. |last3=Kent |first3=Dennis V. |last4=Boersma |first4=Anne |last5=Rakic-El Bied |first5=Kruna |bibcode=1994PalOc...9..835H |issue=6}}</ref> These fossils help scientists to date the core and to understand the [[Sedimentary depositional environment|depositional environment]] in which the rock units formed. Geochronologists precisely date rocks within the stratigraphic section in order to provide better absolute bounds on the timing and rates of deposition.<ref>{{Cite journal |doi=10.1016/S0277-3791(98)00077-8 |title=Submerged Late Pleistocene reefs on the tectonically-stable S.E. Florida margin: high-precision geochronology, stratigraphy, resolution of Substage 5a sea-level elevation, and orbital forcing |year=1999 |author=Toscano, M |journal=Quaternary Science Reviews |volume=18 |page=753 |last2=Lundberg |first2=Joyce |issue=6|bibcode = 1999QSRv...18..753T }}</ref> Magnetic stratigraphers look for signs of magnetic reversals in igneous rock units within the drill cores.<ref name="hodell" /> Other scientists perform stable isotope studies on the rocks to gain information about past climate.<ref name="hodell" />

==Planetary geology==
[[File:Mars Viking 21i093.png|thumb|Surface of Mars as photographed by the [[Viking 2]] lander December 9, 1977.]]
{{Main|Planetary geology|Geology of solar terrestrial planets}}
With the advent of [[space exploration]] in the twentieth century, geologists have begun to look at other planetary bodies in the same way as the [[Earth]]. This led to the establishment of the field of [[planetary geology]], sometimes known as astrogeology, in which geologic principles are applied to other bodies of the solar system.

Although the Greek-language-origin prefix ''[[wikt:geo-|geo]]'' refers to Earth, "geology" is often used in conjunction with the names of other planetary bodies when describing their composition and internal processes: examples are "the [[geology of Mars]]" and "[[Lunar geology]]". Specialised terms such as ''selenology'' (studies of the Moon), ''areology'' (of Mars), etc., are also in use.

Although planetary geologists are interested in all aspects of the planets, a significant focus is in the search for past or present life on other worlds. This has led to many missions whose purpose is (or whose purposes include) to examine planetary bodies for evidence of life. One of these is the [[Phoenix lander]], which analyzed [[Mars|Martian]] polar soil for water and chemical and mineralogical constituents related to biological processes.

==Applied geology==

===Economic geology===
{{Main|Economic geology}}
Economic geologists help locate and manage the Earth's [[natural resource]]s, such as petroleum and coal, as well as mineral resources, which include metals such as iron, copper, and uranium.

====Mining geology====
{{Main|Mining}}
Mining geology consists of the extractions of mineral resources from the Earth. Some resources of economic interests include [[gemstone]]s, [[metal]]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]].

====Petroleum geology====
[[Image:Mudlogging.JPG|thumb|Mud log in process, a common way to study the [[lithology]] when drilling oil wells.]]
{{Main|Petroleum geology}}
[[Petroleum geologist]]s study the locations of the subsurface of the Earth which can contain extractable hydrocarbons, especially [[petroleum]] and [[natural gas]]. Because many of these reservoirs are found in [[sedimentary basin]]s,<ref>{{Cite book |isbn=0-12-636370-6 |author=Richard C. Selley. |year=1998 |publisher=Academic Press |location=San Diego |title=Elements of petroleum geology}}</ref> they study the formation of these basins, as well as their sedimentary and tectonic evolution and the present-day positions of the rock units.

===Engineering geology===
{{Main|Engineering geology|Soil mechanics|Geotechnical engineering}}
Engineering geology is the application of the geologic principles to engineering practice for the purpose of assuring that the geologic factors affecting the location, design, construction, operation and maintenance of engineering works are properly addressed.

In the field of [[civil engineering]], geological principles and analyses are used in order to ascertain the mechanical principles of the material on which structures are built. This allows tunnels to be built without collapsing, bridges and skyscrapers to be built with sturdy foundations, and buildings to be built that will not settle in clay and mud.<ref>{{Cite book |isbn=0-534-55144-0 |author=Braja M. Das. |year=2006 |publisher=THOMSON LEARNING (KY) |location=England |title=Principles of geotechnical engineering}}</ref>

===Hydrology and environmental issues===
{{Main|Hydrogeology}}
Geology and geologic principles can be applied to various environmental problems, such as [[stream restoration]], the restoration of [[brownfields]], and the understanding of the interactions between [[Habitat (ecology)|natural habitat]] and the geologic environment. Groundwater hydrology, or [[hydrogeology]], is used to locate groundwater,<ref name="Hamilton, Pixie A. 1995 217">{{Cite journal |doi=10.1111/j.1745-6584.1995.tb00276.x |title=Effects of Agriculture on Ground-Water Quality in Five Regions of the United States |year=1995 |author=Hamilton, Pixie A. |journal=Ground Water |volume=33 |page=217 |last2=Helsel |first2=Dennis R. |issue=2}}</ref> which can often provide a ready supply of uncontaminated water and is especially important in arid regions,<ref>{{Cite journal |doi=10.1080/07900629948916 |title=Water Scarcity in the Twenty-first Century |year=1999 |author=Seckler, David |journal=International Journal of Water Resources Development |volume=15 |page=29 |last2=Barker |first2=Randolph |last3=Amarasinghe |first3=Upali}}</ref> and to monitor the spread of contaminants in groundwater wells.<ref name="Hamilton, Pixie A. 1995 217"/><ref>{{Cite journal |doi=10.1111/j.1745-6584.1988.tb00397.x |title=Arsenic in Ground Water of the Western United States |year=1988 |author=Welch, Alan H. |journal=Ground Water |volume=26 |page=333 |last2=Lico |first2=Michael S. |last3=Hughes |first3=Jennifer L. |issue=3}}</ref>

Geologists also obtain data through stratigraphy, [[boreholes]], [[core sample]]s, and [[ice core]]s. Ice cores<ref>{{Cite journal |doi=10.1038/329408a0 |title=Vostok ice core provides 160,000-year record of atmospheric CO2 |year=1987 |author=Barnola, J. M. |journal=Nature |volume=329 |page=408 |last2=Raynaud |first2=D. |last3=Korotkevich |first3=Y. S. |last4=Lorius |first4=C. |issue=6138|bibcode = 1987Natur.329..408B }}</ref> and sediment cores<ref>{{Cite journal |doi=10.1007/BF00239699 |title=Holocene paleoclimatic evidence and sedimentation rates from a core in southwestern Lake Michigan |year=1990 |author=Colman, S.M. |journal=Journal of Paleolimnology |volume=4 |last2=Jones |first2=G.A. |last3=Forester |first3=R.M. |last4=Foster |first4=D.S. |issue=3}}</ref> are used to for paleoclimate reconstructions, which tell geologists about past and present temperature, precipitation, and [[sea level]] across the globe. These data are our primary source of information on [[global climate change]] outside of instrumental data.<ref>{{Cite journal |doi=10.1029/2003RG000143 |title=Climate over past millennia |year=2004 |author=Jones, P. D. |journal=Reviews of Geophysics |volume=42 |pages=RG2002 |bibcode=2004RvGeo..42.2002J |issue=2}}</ref>

===Natural hazards===
{{Main|Natural hazard}}
Geologists and geophysicists study natural hazards in order to enact safe [[building code]]s and warning systems that are used to prevent loss of property and life.<ref>[http://www.usgs.gov/hazards/ USGS Natural Hazards Gateway]</ref> Examples of important natural hazards that are pertinent to geology (as opposed those that are mainly or only pertinent to meteorology) are:
[[Image:GCRockfall.JPG|thumb|Rockfall in the Grand Canyon]]
{{columns-list|3|
* [[Avalanche]]s
* [[Earthquake]]s
* [[Flood]]s
* [[Landslide]]s and [[debris flow]]s
* [[River channel migration]] and [[Avulsion (river)|avulsion]]
* [[Soil liquefaction|Liquefaction]]
* [[Sinkhole]]s
* [[Subsidence]]
* [[Tsunami]]s
* [[Volcano]]es
}}


==History of Geology==
{{Main|History of geology|Timeline of geology }}
[[Image:Geological map Britain William Smith 1815.jpg|thumb|right|[[William Smith (geologist)|William Smith]]'s [[geologic map]] of [[England]], [[Wales]], and southern [[Scotland]]. Completed in 1815, it was the first national-scale geologic map, and by far the most accurate of its time.<ref name=map>{{Cite book |isbn=0-06-093180-9 |author=Simon Winchester ; |year=2002 |publisher=Perennial |location=New York, NY |title=The map that changed the world: William Smith and the birth of modern geology}}</ref>]]
The study of the physical material of the Earth dates back at least to [[ancient Greece]] when [[Theophrastus]] (372-287 BCE) wrote the work ''Peri Lithon'' (''On Stones''). In the [[Roman Empire|Roman]] period, [[Pliny the Elder]] wrote in detail of the many minerals and metals then in practical use, and correctly noted the origin of [[amber]].

Some modern scholars, such as [[Fielding H. Garrison]], are of the opinion that modern geology began in the [[Islamic Golden Age|medieval Islamic world]].<ref>"The Saracens themselves were the originators not only of algebra, chemistry, and geology, but of many of the so-called improvements or refinements of civilization, such as street lamps, window-panes, fireworks, stringed instruments, cultivated fruits, perfumes, spices, etc." (Fielding H. Garrison, ''An introduction to the history of medicine'', W.B. Saunders, 1921, p. 116)</ref> [[Abū al-Rayhān al-Bīrūnī|Abu al-Rayhan al-Biruni]] (973–1048 CE) was one of the earliest [[Geography in medieval Islam|Muslim geologists]], whose works included the earliest writings on the [[geology of India]], hypothesizing that the [[Indian subcontinent]] was once a sea.<ref>{{cite book |title=The Age of Achievement: A.D. 750 to the End of the Fifteenth Century : The Achievements |series=History of civilizations of Central Asia |editor1-first=M. S. |editor1-last=Asimov |editor2-first=Clifford Edmund |editor2-last=Bosworth |isbn=978-92-3-102719-2 |pages=211&ndash;214}}</ref> Islamic Scholar [[Avicenna|Ibn Sina]] (Avicenna, 981–1037) proposed detailed explanations for the formation of mountains, the origin of earthquakes, and other topics central to modern geology, which provided an essential foundation for the later development of the science.<ref>Toulmin, S. and Goodfield, J. (1965), ’The Ancestry of science: The Discovery of Time’, Hutchinson & Co., London, p. 64</ref><ref>{{cite report|title=The Contribution of Ibn Sina (Avicenna) to the development of Earth Sciences |author=Munin M. Al-Rawi |publisher=Foundation for Science Technology and Civilisation |location=Manchester, UK |date=November 2002 |id=Publication 4039 |url=http://www.muslimheritage.com/uploads/ibnsina.pdf |format=PDF |accessdate=April 2012}}</ref> In China, the [[polymath]] [[Shen Kuo]] (1031–1095) formulated a hypothesis for the process of land formation: based on his observation of fossil animal shells in a geological [[stratum]] in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by [[Deposition (sediment)|deposition]] of [[silt]].<ref>{{cite book|last=Needham |first=Joseph |year=1986 |title=Science and Civilization in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth |location=Taipei |publisher=Caves Books, Ltd.|pages=603&ndash;604}}</ref>

[[Nicolas Steno]] (1638–1686) is credited with the [[law of superposition]], the [[principle of original horizontality]], and the [[principle of lateral continuity]]: three defining principles of [[stratigraphy]].

The word ''geology'' was first used by [[Ulisse Aldrovandi]] in 1603,<ref>[http://books.google.com/books/about/Four_centuries_of_the_word_geology.html?id=Ip-rAAAACAAJ Four centuries of the word geology: Ulisse Aldrovandi 1603 in Bologna]</ref> then by [[Jean-André Deluc]] in 1778 and introduced as a fixed term by [[Horace-Bénédict de Saussure]] in 1779. The word is derived from the [[Ancient Greek|Greek]] γῆ, ''gê'', meaning "earth" and λόγος, ''[[logos]]'', meaning "speech".<ref>{{Cite book | last = Winchester | first = Simon | authorlink = Simon Winchester | title = [[The Map that Changed the World]] | publisher = HarperCollins Publishers | year = 2001 | page = 25 | isbn=0-06-093180-9 }}</ref> But according to another source, the word "geology" comes from a Norwegian, Mikkel Pedersøn Escholt (1600–1699), who was a priest and scholar. Escholt first used the definition in his book titled, ''Geologica Norvegica'' (1657).<ref>[http://books.google.com/books?id=6al2BH438AYC&dq=ductus+stenonianus&source=gbs_navlinks_s Kermit H., (2003). Niels Stensen, 1638-1686: the scientist who was beatified.] Gracewing Publishing. p. 127.</ref>

[[William Smith (geologist)|William Smith]] (1769–1839) drew some of the first geological maps and began the process of ordering [[rock strata]] (layers) by examining the fossils contained in them.<ref name="map" />

[[James Hutton]] is often viewed as the first modern [[geologist]].<ref>[http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_hutton.html James Hutton: The Founder of Modern Geology], American Museum of Natural History</ref> In 1785 he presented a paper entitled ''Theory of the Earth'' to the [[Royal Society of Edinburgh]]. In his paper, he explained his theory that the Earth must be much older than had previously been supposed in order to allow enough time for mountains to be eroded and for [[sediment]]s to form new rocks at the bottom of the sea, which in turn were raised up to become dry land. Hutton published a two-volume version of his ideas in 1795 ([http://www.gutenberg.org/etext/12861 Vol. 1], [http://www.gutenberg.org/etext/14179 Vol. 2]).
[[File:Hutton James portrait Raeburn.jpg|thumb|left|150px|[[Scottish people|Scotsman]] James Hutton, father of modern geology]]
Followers of Hutton were known as ''[[Plutonism|Plutonists]]'' because they believed that some rocks were formed by ''vulcanism'', which is the deposition of lava from volcanoes, as opposed to the ''[[Neptunism|Neptunists]]'', led by [[Abraham Gottlob Werner|Abraham Werner]], who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.

[[Sir Charles Lyell]] first published his famous book, ''[[Principles of Geology]]'',<ref>{{Cite book |isbn=978-0-226-49797-6 |author=Charles Lyell. |year=1991 |publisher=University of Chicago Press |location=Chicago |title=Principles of geology}}</ref> in 1830. This book, which influenced the thought of [[Charles Darwin]], successfully promoted the doctrine of [[uniformitarianism]]. This theory states that slow geological processes have occurred throughout the [[History of Earth|Earth's history]] and are still occurring today. In contrast, [[catastrophism]] is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time.

Much of 19th-century geology revolved around the question of the [[Age of the Earth|Earth's exact age]]. Estimates varied from a few hundred thousand to billions of years.<ref>{{Cite journal |doi=10.1130/GSAT01701A.1 |title=John Perry's neglected critique of Kelvin's age for the Earth: A missed opportunity in geodynamics |year=2007 |author=England, Philip |journal=GSA Today |volume=17 |page=4 |last2=Molnar |first2=Peter |last3=Richter |first3=Frank}}</ref> By the early 20th century, [[radiometric dating]] allowed the Earth's age to be estimated at two billion years. The awareness of this vast amount of time opened the door to new theories about the processes that shaped the planet.

Some of the most significant advances in 20th-century geology have been the development of the theory of [[plate tectonics]] in the 1960s and the refinement of estimates of the planet's age. Plate tectonics theory arose from two separate geological observations: [[seafloor spreading]] and [[continental drift]]. The theory revolutionized the [[Earth sciences]]. Today the Earth is known to be approximately 4.5 billion years old.<ref name="age_earth">{{Cite book |first=G.B. | last=Dalrymple | year=1991 | title=The Age of the Earth | publisher=Stanford University Press | location=California |isbn=0-8047-1569-6 }}</ref>

==Fields or related disciplines==
{{columns-list|3|
* [[Earth science]]
* [[Economic geology]]
** [[Mining|Mining geology]]
** [[Petroleum geology]]
* [[Engineering geology]]
* [[Environmental geology]]
* [[Geoarchaeology]]
* [[Geochemistry]]
** [[Biogeochemistry]]
** [[Isotope geochemistry]]
* [[Geochronology]]
* [[Geodetics]]
* [[Geography]]
* [[Geological modelling]]
* [[Geometallurgy]]
* [[Geomicrobiology]]
* [[Geomorphology]]
* [[Geomythology]]
* [[Geophysics]]
* [[Glaciology]]
* [[Historical geology]]
* [[Hydrogeology]]
* [[Meteorology]]
* [[Mineralogy]]
* [[Oceanography]]
** [[Marine geology]]
* [[Paleoclimatology]]
* [[Paleontology]]
** [[Micropaleontology]]
** [[Palynology]]
* [[Petrology]]
* [[Petrophysics]]
* [[Plate tectonics]]
* [[Sedimentology]]
* [[Seismology]]
* [[Soil science]]
** [[Pedology (soil study)]]
* [[Speleology]]
* [[Stratigraphy]]
** [[Biostratigraphy]]
** [[Chronostratigraphy]]
** [[Lithostratigraphy]]
* [[Structural geology]]
* [[Volcanology]]
}}

==Regional geology==
{{Main|Regional geology}}

==See also==
{{Portal||Earth sciences|Solar System|Energy}}

{{Div col|2}}
* [[Agrogeology]]
* [[Digital geologic mapping]]
* [[Geologic modeling]]
* [[Geoprofessions]]
* [[Glossary of geology terms]]
* [[International Union of Geological Sciences]] ([[IUGS]])
* [[List of fossil sites]] ''(with link directory)''
* [[List of geology topics]]
* [[List of Russian geologists]]
* [[List of important publications in geology]]
* [[List of minerals]]
* [[List of rock textures]]
* [[List of rock types]]
* [[List of soil topics]]
* [[Mineral collecting]]
* [[Systems Geology]]
* [[Timeline of geology]]

{{div col end}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category|Geology}}
{{WVS}}
{{wikibooks|Historical Geology}}
* [http://geology.com/ ''Earth Science News, Maps, Dictionary, Articles, Jobs'']
* [http://www.agu.org/ American Geophysical Union]
* [http://www.egu.eu/ European Geosciences Union]
* [http://www.geosociety.org/ Geological Society of America]
* [http://www.geolsoc.org.uk/ ''Geological Society of London'']
* [http://www.minigeology.com Video-interviews with famous geologists]

{{Geology}}
{{Physical Earth}}
{{Earth science}}
{{Nature nav}}

[[Category:Geology| ]]

{{Link FA|eu}}
{{Link FA|he}}

Revision as of 13:53, 4 October 2013

GEOLOGY IS NOT A SCIENCE!!!!!!!!!!!!!!!!!!

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