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{{Short description|Scientific study of Earth's physical composition}}
{{hatnote group|
{{about|the Earth science|the scientific journal|Geology (journal){{!}}''Geology'' (journal)}}
{{Distinguish|Geography}}
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[[File:Book-hawaii-vtorov-142.jpg|thumb|Solidified lava flow in Hawaii]]
[[File:Badlands at Sunset.jpg|thumb|Sedimentary layers in [[Badlands National Park]], South Dakota]]
[[File:Cheshire Cat (5121607167).jpg|thumb|Metamorphic rock, Nunavut, Canada]]
'''Geology''' ({{etymology|grc|''{{wikt-lang|grc|γῆ}}'' ({{grc-transl|γῆ}})|earth||''{{wikt-lang|grc|-|λoγία}}'' ({{grc-transl|[[-logy|-λoγία]]}})|study of, discourse}})<ref name=OnlineEtDict>{{OEtymD|geology|access-date=}}</ref><ref name="LSJ">{{LSJ|gh{{=}}|γῆ|ref}}.</ref> is a branch of [[natural science]] concerned with the Earth and other [[astronomical object]]s, the rocks of which they are composed, and the processes by which they change over time.<ref name = 'Geology Definition'>{{cite web|url=https://www.geolsoc.org.uk/Geology-Career-Pathways/What-is-Geology|title = What is geology?| publisher = The Geological Society| access-date = 31 May 2023}}</ref> Modern geology significantly overlaps all other [[Earth science]]s, including [[hydrology]]. It is integrated with [[Earth system science]] and [[planetary science]].

Geology describes the [[structure of the Earth]] on and beneath its surface and the processes that have shaped that structure. [[Geologists]] study the mineralogical composition of rocks in order to get insight into their history of formation. Geology determines the [[relative ages]] of rocks found at a given location; [[geochemistry]] (a branch of geology) determines their [[Geochronology|absolute ages]].<ref>{{cite journal|last1=Gunten|first1=Hans R. von|title=Radioactivity: A Tool to Explore the Past|journal=Radiochimica Acta|volume=70–71|issue=s1|year=1995|pages=305–413|issn=2193-3405|doi=10.1524/ract.1995.7071.special-issue.305|s2cid=100441969|url=http://doc.rero.ch/record/292801/files/ract.1995.7071.special-issue.305.pdf|access-date=2019-06-29|archive-date=2019-12-12|archive-url=https://web.archive.org/web/20191212005652/http://doc.rero.ch/record/292801/files/ract.1995.7071.special-issue.305.pdf|url-status=live}}</ref> By combining various petrological, crystallographic, and paleontological tools, [[geologist]]s are able to chronicle the geological [[history of the Earth]] as a whole. One aspect is to demonstrate the [[age of the Earth]]. Geology provides evidence for [[plate tectonics]], the [[evolutionary history of life]], and the Earth's [[past climates]].

[[Geologist]]s broadly study the properties and processes of Earth and other terrestrial planets. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including [[fieldwork]], [[Petrology|rock description]], [[Geophysical survey|geophysical techniques]], [[Geochemistry|chemical analysis]], [[physical experiment]]s, and [[numerical modelling]]. In practical terms, geology is important for [[mining|mineral]] and [[petroleum geology|hydrocarbon]] exploration and exploitation, evaluating [[water resources]], understanding [[natural hazard]]s, remediating [[Environmental Geology|environmental]] problems, and providing insights into past [[Climate variability and change|climate change]]. Geology is a major [[academic discipline]], and it is central to [[geological engineering]] and plays an important role in [[geotechnical engineering]].

==Geological material==
[[File:Or Venezuela.jpg|thumb| [[Native metal|Native]] [[Gold#Occurrence|gold]] from [[Venezuela]]]]
[[File:Quartz, Tibet.jpg|thumb|[[Quartz]] from [[Tibet]]. Quartz makes up more than 10% of the Earth's crust by mass.]]
The majority of geological data comes from research on solid Earth materials. Meteorites and other extraterrestrial natural materials are also studied by geological methods.

===Minerals===
{{main|Mineral}}
Minerals are naturally occurring [[Chemical element|element]]s and [[Chemical compound|compound]]s with a definite homogeneous chemical composition and an ordered atomic arrangement.

Each mineral has distinct physical properties, and there are many tests to determine each of them. Minerals are often identified through these tests. The specimens can be tested for:<ref>{{cite web|title=Mineral Identification Tests|url=http://jersey.uoregon.edu/~mstrick/MinRockID/MinTests.html|website=Geoman's Mineral ID Tests|access-date=17 April 2017|archive-date=9 May 2017|archive-url=https://web.archive.org/web/20170509215641/http://jersey.uoregon.edu/~mstrick/MinRockID/MinTests.html|url-status=live}}</ref>
* Color: Minerals are grouped by their color. Mostly diagnostic but impurities can change a mineral's color.
* [[Streak (mineralogy)|Streak]]: Performed by scratching the sample on a [[porcelain]] plate. The color of the streak can help identify the mineral.
* Hardness: The resistance of a mineral to scratching or indentation.
* Breakage pattern: A mineral can either show fracture or [[Cleavage (crystal)|cleavage]], the former being breakage of uneven surfaces, and the latter a breakage along closely spaced parallel planes.
* [[Lustre (mineralogy)|Luster]]: Quality of light reflected from the surface of a mineral. Examples are metallic, pearly, waxy, dull.
* [[Specific gravity]]: the weight of a specific volume of a mineral.
* Effervescence: Involves dripping [[hydrochloric acid]] on the mineral to test for fizzing.
* Magnetism: Involves using a magnet to test for [[magnetism]].
* Taste: Minerals can have a distinctive taste such as [[halite]] (which tastes like [[table salt]]).

===Rock===
[[File:Cycle of rocks 2.png|thumb|upright=1.4|The [[rock cycle]] shows the relationship between [[igneous]], [[sedimentary]], and [[metamorphic]] [[rock (geology)|rock]]s.]]

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

A rock is any naturally occurring solid mass or aggregate of minerals or [[mineraloid]]s. Most research in geology is associated with the study of rocks, as they provide the primary record of the majority of the geological history of the Earth. There are three major types of rock: [[igneous]], [[sedimentary]], and [[metamorphic]]. The [[rock cycle]]
illustrates the relationships among them (see diagram).

When a rock [[solidifies]] or [[crystallizes]] from melt ([[magma]] or [[lava]]), it is an [[igneous rock]]. This rock can be [[weathering|weathered]] and [[eroded]], then [[deposition (geology)|redeposited]] and [[lithified]] into a sedimentary rock. Sedimentary rocks are mainly divided into four categories: sandstone, shale, carbonate, and evaporite. This group of classifications focuses partly on the size of sedimentary particles (sandstone and shale), and partly on mineralogy and formation processes (carbonation and evaporation).<ref>{{Cite book |last1=Guéguen |first1=Yves |url=https://books.google.com/books?id=fCP5qyRyX-oC&dq=rocks&pg=PP7 |title=Introduction to the Physics of Rocks |last2=Palciauskas |first2=Victor |date=1994 |publisher=Princeton University Press |isbn=978-0-691-03452-2 |location=Princeton University Press |pages=10 |language=en}}</ref> Igneous and sedimentary rocks can then be turned into [[metamorphic rock]]s by heat and pressure that change its [[mineral]] content, resulting in a [[fabric (geology)|characteristic fabric]]. All three types may melt again, and when this happens, new magma is formed, from which an igneous rock may once again solidify.
Organic matter, such as coal, bitumen, oil, and natural gas, is linked mainly to organic-rich sedimentary rocks.

To study all three types of rock, geologists evaluate the minerals of which they are composed and their other physical properties, such as [[Texture (geology)|texture]] and [[Fabric (geology)|fabric]].

===Unlithified material===
Geologists also study unlithified materials (referred to as ''[[superficial deposits]]'') that lie above the [[bedrock]].<ref>[http://des.nh.gov/organization/commissioner/gsu/gmp/categories/overview.htm "Surficial Geologic Maps"]. {{Webarchive|url=https://web.archive.org/web/20160216095807/http://des.nh.gov/organization/commissioner/gsu/gmp/categories/overview.htm|date=2016-02-16}}, in New Hampshire Geological Survey, Geologic maps. des.nh.gov.</ref> This study is often known as [[Quaternary geology]], after the [[Quaternary period]] of geologic history, which is the most recent period of geologic time.

====Magma====
{{main|Magma}}

[[Magma]] is the original unlithified source of all [[igneous rocks]]. The active flow of molten rock is closely studied in [[volcanology]], and [[igneous petrology]] aims to determine the history of [[igneous rock]]s from their original molten source to their final crystallization.

==Whole-Earth structure==

===Plate tectonics===
{{Main|Plate tectonics}}
[[File:Plates tect2 en.svg|thumb|upright=1.35|The major [[tectonic plates]] of the [[Earth]]<ref>{{Cite web|url=https://ocre-geoscience.com/ocre-geomap/|title=OCRE GeoMap|website=OCRE Geoscience Services}}</ref>]]

In the 1960s, it was discovered that the Earth's [[lithosphere]], which includes the [[Crust (geology)|crust]] and rigid uppermost portion of the [[upper mantle]], is separated into [[tectonic plate]]s that move across the [[Plasticity (physics)|plastically]] deforming, solid, upper mantle, which is called the [[asthenosphere]]. This theory is supported by several types of observations, including seafloor spreading<ref>Hess, H. H. (November 1, 1962) "[http://repositories.cdlib.org/sio/lib/23 History Of Ocean Basins]. {{Webarchive|url=https://web.archive.org/web/20091016025000/http://repositories.cdlib.org/sio/lib/23|date=2009-10-16}}", pp. 599–620 in ''Petrologic studies: a volume in honor of A. F. Buddington''. A. E. J. Engel, Harold L. James, and B. F. Leonard (eds.). [[Geological Society of America]].</ref><ref name="TDE discovery">{{Cite book |last1=Kious |first1=Jacquelyne |author2=Tilling, Robert I. |others=Kiger, Martha, Russel, Jane |title=This Dynamic Earth: The Story of Plate Tectonics |publisher=United States Geological Survey |location=Reston |year=1996 |edition=Online |chapter=Developing the Theory |isbn=978-0-16-048220-5 |chapter-url=http://pubs.usgs.gov/gip/dynamic/understanding.html |access-date=13 March 2009 |archive-date=10 August 2011 |archive-url=https://web.archive.org/web/20110810162308/http://pubs.usgs.gov/gip/dynamic/understanding.html |url-status=live }}</ref> and the global distribution of mountain terrain and seismicity.

There is an intimate coupling between the movement of the plates on the surface and the [[mantle convection|convection of the mantle]] (that is, the [[heat]] transfer caused by the slow movement of ductile mantle rock). Thus, oceanic parts of plates and the adjoining mantle [[convection currents]] always move in the same direction – because the oceanic lithosphere is actually 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 has provided a physical basis for many  observations of the solid [[Earth]]. Long linear regions of geological features are explained as plate boundaries:<ref name="TDE plates">{{Cite book |last1=Kious |first1=Jacquelyne |author2=Tilling, Robert I. |others=Kiger, Martha, Russel, Jane |title=This Dynamic Earth: The Story of Plate Tectonics |publisher=United States Geological Survey |location=Reston, VA |date=1996 |edition=Online |chapter=Understanding Plate Motions |isbn=978-0-16-048220-5 |chapter-url=http://pubs.usgs.gov/gip/dynamic/understanding.html |access-date=13 March 2009 |archive-date=10 August 2011 |archive-url=https://web.archive.org/web/20110810162308/http://pubs.usgs.gov/gip/dynamic/understanding.html |url-status=live }}</ref>
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[[File:Active Margin.svg|thumb|Oceanic-continental convergence resulting in [[subduction]] and [[volcanic arc]]s illustrates one effect of [[plate tectonics]].]]
* [[Mid-ocean ridge]]s, high regions on the seafloor where [[hydrothermal vent]]s and volcanoes exist, are seen as [[divergent boundary|divergent boundaries]], where two plates move apart.
* Arcs of volcanoes and earthquakes are theorized as [[convergent boundaries]], where one plate [[subducts]], or moves, under another.
* [[Transform boundary|Transform boundaries]], such as the [[San Andreas Fault]] system, are where plates slide horizontally past each other.

Plate tectonics has provided a mechanism for [[Alfred Wegener]]'s theory of [[continental drift]],<ref>{{Cite book |author=Wegener, A. |url=https://archive.org/details/originsofcontine0000unse |title=Origin of continents and oceans |publisher=Courier Corporation |year=1999 |isbn=978-0-486-61708-4 |url-access=registration}}</ref> in which the [[continents]] move across the surface of the Earth over geological 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}}
[[File: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 (uppermost part of the lithosphere)]]
[[File:Earthquake wave paths.svg|thumb|upright=1.35|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 to image the interior of the Earth. Early advances in this field showed the existence of a liquid [[outer core]] (where [[S-wave|shear waves]] 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 [[lithosphere]] (including crust) 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 modeling studies alongside knowledge of the elemental composition of the Earth to reproduce these conditions in experimental settings and measure changes within the 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.
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==Geological time==
{{Main|Geological history of Earth|Geologic time scale}}

The geological time scale encompasses the history of the Earth.<ref>[http://www.stratigraphy.org/ International Commission on Stratigraphy]. {{webarchive|url=https://web.archive.org/web/20050920105136/http://www.stratigraphy.org/|date=September 20, 2005}}. stratigraphy.org.</ref> It is bracketed at the earliest by the dates of the first [[Solar System]] material at 4.567 [[Gigaannum|Ga]]<ref name="4.567">{{Cite journal | doi = 10.1126/science.1073950| title = Lead Isotopic Ages of Chondrules and Calcium-Aluminum-Rich Inclusions| journal = Science| volume = 297| issue = 5587| pages = 1678–1683| year = 2002| last1 = Amelin | first1 = Y.|bibcode = 2002Sci...297.1678A| pmid=12215641| s2cid = 24923770}}</ref> (or 4.567 billion years ago) and the formation of the Earth at
4.54 Ga<ref name="4.54">{{cite journal|author=Patterson, C.|year= 1956|title=Age of Meteorites and the Earth|journal= Geochimica et Cosmochimica Acta |volume=10|issue= 4|pages= 230–237|bibcode = 1956GeCoA..10..230P |doi = 10.1016/0016-7037(56)90036-9 }}</ref><ref name="4.54 book">{{Cite book |author=Dalrymple |first=G. Brent |title=The age of the earth |publisher=Stanford University Press |year=1994 |isbn=978-0-8047-2331-2 |location=Stanford, California}}</ref>
(4.54 billion years), which is the beginning of the [[Hadean eon]]{{snd}}a division of geological time. At the later end of the scale, it is marked by the present day (in the [[Holocene epoch]]).

===Timescale of the Earth===

{{Timeline Geological Timescale}}

===Important milestones on Earth===
[[File:Geologic Clock with events and periods.svg|thumb|upright=1.35|Geological time in a diagram called a [[geological clock]], showing the relative lengths of the [[geologic eon|eon]]s and [[era (geology)|era]]s of the Earth's history]]
* 4.567 [[Ga (unit)|Ga]] (gigaannum: billion years ago): [[Solar system formation]]<ref name="4.567" />
* 4.54 Ga: [[Accretion (astrophysics)|Accretion, or formation]], of Earth<ref name="4.54" /><ref name="4.54 book" />
* c. 4 Ga: End of [[Late Heavy Bombardment]], the 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): second snowball Earth
* 541 ± 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]]
* 66 Ma: [[Cretaceous–Paleogene extinction]] – [[Dinosaur]]s die; end of Mesozoic and beginning of [[Cenozoic]]
* 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

===Timescale of the Moon===
{{main|Lunar geologic timescale}}
{{Timeline Lunar Geological Timescale}}

===Timescale of Mars===
{{main|Geological history of Mars}}
{{Mars timescale}}

==Dating methods==

===Relative dating===
{{Main|Relative dating}}
[[File:Cross-cutting relations.svg|thumb|upright=1.35|[[Cross-cutting relations]] can be used to determine the relative ages of [[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 [[natural science]]. Geologists still use the following principles today as a means to provide information about geological history and the timing of geological events.

The ''[[principle of uniformitarianism]]'' states that the geological processes observed in operation that modify the Earth's crust at present have worked in much the same way over geological time.<ref>Reijer Hooykaas, [https://books.google.com/books?id=3TgYAAAAIAAJ ''Natural Law and Divine Miracle: The Principle of Uniformity in Geology, Biology, and Theology'']. {{Webarchive|url=https://web.archive.org/web/20170119224821/https://books.google.com/books?id=3TgYAAAAIAAJ&pgis=1|date=2017-01-19}}, 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|last1=Levin|first1=Harold L.|title=The earth through time|year=2010|publisher=J. Wiley|location=Hoboken, NJ|isbn=978-0-470-38774-0|edition=9th|page=18}}</ref>

The ''[[Intrusion (geology)|principle of intrusive relationships]]'' concerns crosscutting intrusions. In geology, when an [[igneous]] intrusion cuts across a formation of [[sedimentary rock]], it can be determined that the igneous intrusion is younger than the sedimentary rock. Different types of intrusions include 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 |last1=Olsen |first1=Paul E. |year=2001 |work=Dinosaurs and the History of Life |publisher=Columbia University |access-date=2009-03-14 |archive-date=2008-05-09 |archive-url=https://web.archive.org/web/20080509122724/http://rainbow.ldeo.columbia.edu/courses/v1001/steno.html |url-status=live }}</ref>

The ''[[principle of inclusions and components]]'' states that, with sedimentary rocks, if inclusions (or ''[[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 that contains them.

[[File:SEUtahStrat.JPG|thumb|upright=1.35|The [[Permian]] through [[Jurassic]] stratigraphy of the [[Colorado Plateau]] area of southeastern [[Utah]] is an 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 the vertical timeline, 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 during the same period throughout the world, their presence or (sometimes) absence provides a relative age of the formations where they appear. Based on principles that William Smith laid out almost a hundred years before the publication of [[Charles Darwin]]'s theory of [[evolution]], the principles of succession developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, localization of fossil types due to lateral changes in habitat ([[facies]] change in sedimentary strata), and that not all fossils formed 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}}
[[File:Zircon-tuc1001b.jpg|thumb|upright|The [[mineral]] [[zircon]] is often used in [[radiometric dating]].]]

Geologists also use methods to determine the absolute age of rock samples and geological events. These dates are useful on their own and may also be used in conjunction with relative dating methods or to calibrate relative methods.<ref>{{Cite journal |last1=Tucker |first1=R. D. |last2=Bradley |first2=D. C. |last3=Ver Straeten |first3=C. A. |last4=Harris |first4=A. G. |last5=Ebert |first5=J. R. |last6=McCutcheon |first6=S. R. |year=1998 |title=New U–Pb zircon ages and the duration and division of Devonian time |url=https://alaska.usgs.gov/staff/geology/bradley/bradley_pubs/Tucker_Dev_time_scale_1998.pdf |url-status=dead |journal=Earth and Planetary Science Letters |volume=158 |issue=3–4 |pages=175–186 |bibcode=1998E&PSL.158..175T |citeseerx=10.1.1.498.7372 |doi=10.1016/S0012-821X(98)00050-8 |archive-url=https://web.archive.org/web/20161226203632/https://alaska.usgs.gov/staff/geology/bradley/bradley_pubs/Tucker_Dev_time_scale_1998.pdf |archive-date=2016-12-26 |access-date=2018-01-29}}</ref>

At the beginning of the 20th century, advancement in geological science was facilitated by the ability to obtain accurate absolute dates to geological events using [[radioactive isotope]]s and other methods. This changed the understanding of geological time. Previously, geologists could only use fossils and stratigraphic correlation to date sections of rock relative to one another. With isotopic dates, it became possible to assign [[absolute ages]] to rock units, and these absolute dates could be applied to fossil sequences in which there was datable material, converting the old relative ages into new absolute ages.

For many geological applications, [[isotope ratio]]s of radioactive elements 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 lattice]].<ref>{{Cite book |isbn=978-0-582-06701-1 |author=Rollinson, Hugh R.  |year=1996 |publisher=Longman |location=Harlow |title=Using geochemical data evaluation, presentation, interpretation}}</ref><ref>{{Cite book |isbn=978-0-02-336450-1 |author=Faure, Gunter |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 [[geochronologic]] and [[thermochronology|thermochronologic]] studies. Common methods include [[uranium–lead dating]], [[potassium–argon dating]], [[argon–argon dating]] and [[uranium–thorium dating]]. These methods are used for a variety of applications. Dating of [[lava]] and [[volcanic ash]] layers found within a stratigraphic sequence can provide absolute age data for sedimentary rock units that do not contain radioactive isotopes 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 paleo-topography.

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 radionuclide]] 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 geologically young materials containing [[Organic matter|organic carbon]].

==Geological development of an area==
[[File:Volcanosed.svg|thumb|upright=1.35|An originally horizontal sequence of sedimentary rocks (in shades of tan) are affected by [[igneous]] activity. Deep below the surface is a [[magma chamber]] and large associated igneous bodies. The magma chamber feeds the [[volcano]], and sends offshoots 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.svg|thumb|left|upright=0.9| An illustration of the three types of faults.<br>
A. Strike-slip faults occur when rock units slide past one another.<br>
B. Normal faults occur when rocks are undergoing horizontal extension.<br>
C. Reverse (or thrust) faults occur when rocks are undergoing horizontal shortening.]]

[[File:San Andreas.jpg|thumb|upright|The [[San Andreas Fault]] in [[California]]]]

The geology of an area changes through time as rock units are deposited and inserted, and deformational processes alter 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 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 [[faulting]] and [[fold (geology)|folding]]. In the shallow crust, where [[brittle deformation]] can occur, thrust faults form, which causes the deeper rock to move on top of the 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 rock layers are dragged along, forming drag folds as slip occurs 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|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. This 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 ending up below older units. Stretching of units can result in their thinning. In fact, at one location within the [[Maria Fold and Thrust Belt]], the entire sedimentary sequence of the [[Grand Canyon]] appears 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|upright=1.4|Geological [[cross section (geology)|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 indiscernible 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 [[Historical geology|geological history]] of an area.

==Investigative methods==
[[File:Brunton.JPG|thumb|A standard [[:en:Brunton compass|Brunton Pocket Transit]], commonly used by geologists for mapping and surveying]]

Geologists use a number of fields, laboratory, and numerical modeling methods to decipher Earth history and to understand the processes that occur on and inside 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 [[biogeochemical]] pathways, and use [[geophysics|geophysical methods]] to investigate the subsurface. Sub-specialities of geology may distinguish '''endogenous''' and '''exogenous''' geology.<ref>Compare: {{cite book
| last1 = Hansen
| first1 = Jens Morten
| chapter = On the origin of natural history: Steno's modern, but forgotten philosophy of science
| editor1-last = Rosenberg
| editor1-first = Gary D.
| title = The Revolution in Geology from the Renaissance to the Enlightenment
| chapter-url = https://books.google.com/books?id=4gGAgHcKX6YC
| series = Geological Society of America Memoir
| volume = 203
| location = Boulder, CO
| publisher = Geological Society of America
| publication-date = 2009
| page = 169
| isbn = 978-0-8137-1203-1
| access-date = 2016-08-24
| quote = [...] the historic dichotomy between 'hard rock' and 'soft rock' geologists, i.e. scientists working mainly with endogenous and exogenous processes, respectively [...] endogenous forces mainly defining the developments below Earth's crust and the exogenous forces mainly defining the developments on top of and above Earth's crust.
| date = 2009-01-01
| archive-date = 2017-01-20
| archive-url = https://web.archive.org/web/20170120031800/https://books.google.com/books?id=4gGAgHcKX6YC
| url-status = live
}}</ref>

===Field methods===
[[File:USGS 1950s mapping field camp.jpg|thumb|A typical [[USGS]] field mapping camp in the 1950s]]
[[File:PDA Mapping.jpg|thumb|Today, [[handheld computer]]s with [[GPS]] and [[geographic information systems]] software are often used in geological field work ([[digital geological mapping]]).]]
[[File:Petrified forest log 1 md.jpg|thumb|upright|A [[petrified]] log in [[Petrified Forest National Park]], [[Arizona]], US]]

Geological [[field work]] varies depending on the task at hand. Typical fieldwork could consist of:
* [[Geological map]]ping<ref>{{Cite book |isbn=978-0-471-82902-7 |author= Compton, Robert R. |year= 1985 |publisher= Wiley |location= New York |title= Geology in the field}}</ref>
** Structural mapping: identifying the locations of major rock units and the faults and folds that led to their placement there.
** Stratigraphic mapping: pinpointing the locations of [[sedimentary facies]] ([[Lithology|lithofacies]] and [[biofacies]]) or the mapping of [[isopach]]s of equal thickness of sedimentary rock
** Surficial mapping: recording the locations of soils and surficial deposits
* Surveying of topographic features
** compilation of [[topographic map]]s<ref>{{cite web |url=http://topomaps.usgs.gov/ |title= USGS Topographic Maps |publisher= United States Geological Survey |access-date= 2009-04-11 |archive-url=https://web.archive.org/web/20090412214110/http://topomaps.usgs.gov/ |archive-date= 2009-04-12 |url-status=dead }}</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= 978-0-393-92637-8 |author1= Burger, H. Robert |author2= Sheehan, Anne F. |author-link2=Anne Sheehan|author3= Jones, Craig H. |year= 2006 |publisher= W.W. Norton |location= New York |title= Introduction to applied geophysics : exploring the shallow subsurface}}</ref>
** These methods include:
*** Shallow [[seismic]] surveys
*** [[Ground-penetrating radar]]
*** [[Aeromagnetic survey]]s
*** [[Electrical resistivity tomography]]
** They aid in:
*** [[Exploration geophysics|Hydrocarbon exploration]]
*** Finding [[groundwater]]
*** [[Archaeological geophysics|Locating buried archaeological artifacts]]
* High-resolution stratigraphy
** Measuring and describing [[stratigraphic section]]s on the surface
** [[Well drilling]] and [[well logging|logging]]
* [[Biogeochemistry]] and [[geomicrobiology]]<ref>{{Cite book |isbn= 978-0-250-40218-2 |editor= Krumbein, Wolfgang E. |year= 1978 |publisher= Ann Arbor Science Publ. |location= Ann Arbor, MI |title= Environmental biogeochemistry and geomicrobiology}}</ref>
** Collecting samples to:
*** determine [[biochemical pathway]]s
*** identify new [[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= 978-0-19-510920-7 |author1= McDougall, Ian |author2= Harrison, T. Mark |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= 978-0-470-84426-7 |author1= Hubbard, Bryn  |author2= Glasser, Neil |year= 2005 |publisher= J. Wiley |location= Chichester, England |title= Field techniques in glaciology and glacial geomorphology}}</ref>

 {{multiple image
 | total_width = 350
 | footer = In [[optical mineralogy]], thin sections are used to study rocks. The method is based on the distinct refractive indexes of different minerals.
 | image1 = Leica DMRX.jpg
 | caption1 = A [[petrographic microscope]]
 | image2 = Thin section scan crossed polarizers Siilinjärvi R636-105.90.jpg
 | caption2 = A [[thin section]] in cross polarized light
 }}

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

In addition to identifying rocks in the field ([[lithology]]), 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, petrologists analyze [[thin section]]s of rock samples using 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= 978-0-19-506024-9 |author= Nesse, William D. |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 |author=Morton |first=A. C. |year=1985 |title=A new approach to provenance studies: electron microprobe analysis of detrital garnets from Middle Jurassic sandstones of the northern North Sea |journal=Sedimentology |volume=32 |issue=4 |pages=553–566 |bibcode=1985Sedim..32..553M |doi=10.1111/j.1365-3091.1985.tb00470.x}}</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 |issue= 1 |pages= 105–161 |bibcode= 2003ESRv...62..105Z |last2= Fu |first2= Bin |last3= Gong |first3= Bing |last4= Li |first4= Long}}</ref> and [[radioactive isotope]]<ref>{{Cite journal |author=Condomines |first1=M. |last2=Tanguy |first2=J. |last3=Michaud |first3=V. |year=1995 |title=Magma dynamics at Mt Etna: Constraints from U-Th-Ra-Pb radioactive disequilibria and Sr isotopes in historical lavas |journal=Earth and Planetary Science Letters |volume=132 |issue=1 |pages=25–41 |bibcode=1995E&PSL.132...25C |doi=10.1016/0012-821X(95)00052-E}}</ref> studies provide insight into the [[geochemical]] evolution of rock units.

Petrologists can also use [[fluid inclusion]] data<ref>{{Cite book |last1=Shepherd |first1=T. J. |title=A practical guide to fluid inclusion studies |last2=Rankin |first2=A. H. |last3=Alderton |first3=D. H. M. |journal=Mineralogical Magazine |publisher=Blackie |year=1985 |volume=50 |issue=356 |page=352 |isbn=978-0-412-00601-2 |location=Glasgow |doi=10.1180/minmag.1986.050.356.32 |bibcode=1986MinM...50..352P | url=https://books.google.com/books?id=CVSGAAAAIAAJ }}</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 |issue= 1 |pages= 1–23 |last2= Walker |first2= David |last3= Carmichael |first3= Ian S.E. |bibcode= 1987CoMP...96....1S|s2cid= 129193823 }}</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= McBirney, Alexander R. |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 = Spear, Frank S.
 | 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.<ref>{{Cite journal|last1=Deegan|first1=F. M.|last2=Troll|first2=V. R.|last3=Freda|first3=C.|last4=Misiti|first4=V.|last5=Chadwick|first5=J. P.|last6=McLeod|first6=C. L.|last7=Davidson|first7=J. P.|date=May 2010|title=Magma–Carbonate Interaction Processes and Associated CO2 Release at Merapi Volcano, Indonesia: Insights from Experimental Petrology|url=https://doi.org/10.1093/petrology/egq010|journal=Journal of Petrology|volume=51|issue=5|pages=1027–1051|doi=10.1093/petrology/egq010|issn=1460-2415}}</ref>
[[File:Agiospavlos DM 2004 IMG003 Felsenformation nahe.JPG|thumb|Folded [[rock (geology)|rock]] strata]]

===Structural geology===
{{Main|Structural geology}}
[[File:Orogenic wedge.jpg|thumb|upright=1.8|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.]]

Structural geologists use microscopic analysis of oriented thin sections of geological samples to observe the [[fabric (geology)|fabric]] within the rocks, which gives information about strain within the crystalline structure of the rocks. They also plot and combine measurements of geological structures to better understand the orientations of faults and folds to reconstruct the history of rock deformation in the area. In addition, they perform [[analogue modelling (geology)|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 faults, and relationships between other geological structures.

Among the most well-known experiments in structural geology are those involving [[Accretionary wedge|orogenic wedges]], which are zones in which [[mountain]]s are built along [[convergent boundary|convergent]] tectonic plate boundaries.<ref>{{Cite journal |author=Dahlen |first=F. A. |year=1990 |title=Critical Taper Model of Fold-and-Thrust Belts and Accretionary Wedges |journal=Annual Review of Earth and Planetary Sciences |volume=18 |pages=55–99 |bibcode=1990AREPS..18...55D |doi=10.1146/annurev.ea.18.050190.000415}}</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 |pages= 407–416 |issue= 4|bibcode = 1998JSG....20..407G |last2= Kukowski |first2= Nina |last3= Malavieille |first3= Jacques |last4= Lallemand |first4= Serge }}</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 |author=Koons |first=P. O. |year=1995 |title=Modeling the Topographic Evolution of Collisional Belts |journal=Annual Review of Earth and Planetary Sciences |volume=23 |pages=375–408 |bibcode=1995AREPS..23..375K |doi=10.1146/annurev.ea.23.050195.002111}}</ref> This helps to show the relationship between erosion and the shape of a mountain range. These studies can also give useful information about pathways for metamorphism through pressure, temperature, space, and time.<ref>{{cite journal |last1=Dahlen |first1=F. A. |last2=Suppe |first2=J. |last3=Davis |first3=D. |year=1984 |title=Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory |journal=[[Journal of Geophysical Research]] |volume=89 |issue=B12 |pages=10087–10101 |bibcode=1984JGR....8910087D |doi=10.1029/JB089iB12p10087}}</ref>

===Stratigraphy===
[[File:Linze, Zhangye, Gansu, China - panoramio (4).jpg|thumb|Different colors caused by the different minerals in tilted layers of sedimentary rock in [[Zhangye National Geopark]], China]]
{{Main|Stratigraphy}}
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 |title=Atlas of seismic stratigraphy |publisher=American Association of Petroleum Geologists |year=1987 |isbn=978-0-89181-033-9 |editor=Bally |editor-first=A. W. |location=Tulsa, Oklahoma}}</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 |author=Fernández |first1=O. |last2=Muñoz |first2=J. A. |last3=Arbués |first3=P. |last4=Falivene |first4=O. |last5=Marzo |first5=M. |year=2004 |title=Three-dimensional reconstruction of geological surfaces: An example of growth strata and turbidite systems from the Ainsa basin (Pyrenees, Spain) |journal=AAPG Bulletin |volume=88 |issue=8 |pages=1049–1068 |bibcode=2004BAAPG..88.1049F |doi=10.1306/02260403062}}</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 |pages= 1105–1122 |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 |pages= 835–855 |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 [[depositional environment]] in which the rock units formed. Geochronologists precisely date rocks within the stratigraphic section 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 |pages= 753–767 |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}}
With the advent of [[space exploration]] in the twentieth century, geologists have begun to look at other planetary bodies in the same ways that have been developed to study the [[Earth]]. This new field of study is called [[planetary geology]] (sometimes known as astrogeology) and relies on known geological principles to study other bodies of the [[Solar System]]. This is a major aspect of [[planetary science]], and largely focuses on the [[terrestrial planets]], [[icy moon]]s, [[asteroid]]s, [[comet]]s, and [[meteorite]]s. However, some planetary geophysicists study the [[giant planet]]s and [[exoplanet]]s.<ref>{{cite book |last1=Laughlin |first1=Gregory |last2=Lissauer |first2=Jack |title=Treatise on Geophysics |chapter=Exoplanetary Geophysics: An Emerging Discipline |year=2015 |pages=673–694 |doi=10.1016/B978-0-444-53802-4.00186-X |arxiv=1501.05685 |isbn=9780444538031 |s2cid=118743781 }}</ref>

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]]". Specialized terms such as ''selenology'' (studies of the Moon), ''areology'' (of Mars), etc., are also in use.

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

==Applied geology==
[[File:Panning on the Mokelumne.jpg|thumb|upright|Man panning for [[gold]] on the [[Mokelumne]]. [[Harper's Weekly]]: How We Got Gold in California. 1860 ]]

===Economic geology===
{{Main|Economic geology}}

Economic geology is a branch of geology that deals with aspects of economic minerals that humankind uses to fulfill various needs. Economic minerals are those extracted profitably for various practical uses. 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 and ore resources from the Earth. Some resources of economic interests include [[gemstone]]s, [[metal]]s such as [[gold]] and [[copper]], and many minerals such as [[asbestos]], [[Magnesite]], [[perlite]], [[mica]], [[phosphate]]s, [[zeolites]], [[clay]], [[pumice]], [[quartz]], and [[silica]], as well as elements such as [[sulfur]], [[chlorine]], and [[helium]].

====Petroleum geology====
[[File: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 that can contain extractable hydrocarbons, especially [[petroleum]] and [[natural gas]]. Because many of these reservoirs are found in [[sedimentary basin]]s,<ref>{{Cite book |author=Selley |first=Richard C. |title=Elements of petroleum geology |publisher=Academic Press |year=1998 |isbn=978-0-12-636370-8 |location=San Diego, California}}</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 geological principles to engineering practice for the purpose of assuring that the geological factors affecting the location, design, construction, operation, and maintenance of engineering works are properly addressed. Engineering geology is distinct from [[geological engineering]], particularly in North America.
[[File:New water well opens in Shant Abak DVIDS92609.jpg|thumb|upright|A child drinks water from a [[well]] built as part of a hydrogeological humanitarian project in [[Kenya]].]]

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=978-0-534-55144-5 |author=Das, Braja M. |year=2006 |publisher=Thomson Learning|location=England |title=Principles of geotechnical engineering}}</ref>

===Hydrology===
{{Main|Hydrogeology}}

Geology and geological principles can be applied to various environmental problems such as [[stream restoration]], the restoration of [[brownfields]], and the understanding of the interaction between [[natural habitat]] and the geological 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 |pages=217–226 |last2=Helsel |first2=Dennis R. |issue=2 |bibcode=1995GrWat..33..217H |url=https://zenodo.org/record/1230722 |access-date=2020-08-29 |archive-date=2020-10-31 |archive-url=https://web.archive.org/web/20201031082104/https://zenodo.org/record/1230722 |url-status=live }}</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 |issue=1–2 |pages=29–42 |last2=Barker |first2=Randolph |last3=Amarasinghe |first3=Upali|bibcode=1999IJWRD..15...29S }}</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 |pages=333–347 |last2=Lico |first2=Michael S. |last3=Hughes |first3=Jennifer L. |issue=3|bibcode=1988GrWat..26..333W }}</ref>

===Paleoclimatology===
{{Main|Paleoclimatology}}

Geologists also obtain data through stratigraphy, [[boreholes]], [[core sample]]s, and [[ice core]]s. Ice cores<ref>{{Cite journal |author=Barnola |first1=J. M. |last2=Raynaud |first2=D. |last3=Korotkevich |first3=Y. S. |last4=Lorius |first4=C. |year=1987 |title=Vostok ice core provides 160,000-year record of atmospheric CO2 |journal=Nature |volume=329 |issue=6138 |pages=408–414 |bibcode=1987Natur.329..408B |doi=10.1038/329408a0 |s2cid=4268239}}</ref> and sediment cores<ref>{{Cite journal |author=Colman |first1=S. M. |last2=Jones |first2=G. A. |last3=Forester |first3=R. M. |last4=Foster |first4=D. S. |year=1990 |title=Holocene paleoclimatic evidence and sedimentation rates from a core in southwestern Lake Michigan |journal=Journal of Paleolimnology |volume=4 |issue=3 |pages=269 |bibcode=1990JPall...4..269C |doi=10.1007/BF00239699 |s2cid=129496709}}</ref> are used for paleoclimate reconstructions, which tell geologists about past and present temperature, precipitation, and [[sea level]] across the globe. These datasets are our primary source of information on [[global climate change]] outside of instrumental data.<ref>{{Cite journal |last1=Jones |first1=P. D. |last2=Mann |first2=M. E. |date=6 May 2004 |title=Climate over past millennia |url=http://www.meteo.psu.edu/holocene/public_html/shared/articles/JonesMannROG04.pdf |url-status=live |journal=Reviews of Geophysics |volume=42 |issue=2 |page=RG2002 |bibcode=2004RvGeo..42.2002J |doi=10.1029/2003RG000143 |archive-url=https://web.archive.org/web/20190411223135/http://www.meteo.psu.edu/holocene/public_html/shared/articles/JonesMannROG04.pdf |archive-date=11 April 2019 |access-date=28 August 2015 |doi-access=free}}</ref>

===Natural hazards===
[[File:GCRockfall.JPG|thumb|Rockfall in the Grand Canyon]]
{{Main|Natural hazard#Geological hazards}}
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]. {{Webarchive|url=https://web.archive.org/web/20100923133848/http://www.usgs.gov/hazards/|date=2010-09-23}}. usgs.gov.</ref> Examples of important natural hazards that are pertinent to geology (as opposed those that are mainly or only pertinent to meteorology) are:
{{columns-list|colwidth=22em|
* [[Avalanche]]s
* [[Earthquake]]s
* [[Flood]]s
* [[Landslide]]s and [[debris flow]]s
* [[River channel migration]] and [[Avulsion (river)|avulsion]]
* [[Rockfall]]s
* [[Sinkhole]]s
* [[Soil liquefaction]]
* [[Subsidence]]
* [[Tsunami]]s
* [[Volcano]]es
}}

==History==
{{Main|History of geology|Timeline of geology }}
[[File:Geological map Britain William Smith 1815.jpg|thumb|[[William Smith (geologist)|William Smith]]'s [[geological map]] of [[England]], [[Wales]], and southern [[Scotland]]. Completed in 1815, it was the second national-scale geologic map, and by far the most accurate of its time.<ref name=map>{{Cite book |isbn=978-0-06-093180-3 |author=Winchester, Simon |year=2002 |publisher=Perennial |location=New York |title=The map that changed the world: William Smith and the birth of modern geology |url=https://archive.org/details/mapthatchanged00winc }}</ref>{{failed verification|reason=failed on 'second map'|date=September 2019}}]]
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''). During the [[Roman Empire|Roman]] period, [[Pliny the Elder]] wrote in detail of the many minerals and metals, then in practical use – even correctly noting the origin of [[amber]]. Additionally, in the 4th century BCE [[Aristotle]] made critical observations of the slow rate of geological change. He observed the composition of the land and formulated a theory where the Earth changes at a slow rate and that these changes cannot be observed during one person's lifetime. Aristotle developed one of the first evidence-based concepts connected to the geological realm regarding the rate at which the Earth physically changes.<ref>Moore, Ruth. ''The Earth We Live On''. New York: Alfred A. Knopf, 1956. p. 13.</ref><ref>Aristotle. ''Meteorology''. Book 1, Part 14.</ref>

[[Abū al-Rayhān al-Bīrūnī|Abu al-Rayhan al-Biruni]] (973–1048 CE) was one of the earliest [[Persian people|Persian]] 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 |year=1992 |isbn=978-92-3-102719-2 |editor1-last=Asimov |editor1-first=M. S. |series=History of civilizations of Central Asia |pages=211–214 |editor2-last=Bosworth |editor2-first=Clifford Edmund}}</ref> Drawing from Greek and Indian scientific literature that were not destroyed by the [[Early Muslim conquests|Muslim conquests]], the Persian scholar [[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 & Company, London, England, p. 64.</ref><ref>{{cite report |title=The Contribution of Ibn Sina (Avicenna) to the development of Earth Sciences |author=Al-Rawi, Munin M. |publisher=Foundation for Science Technology and Civilisation |location=Manchester, UK |date=November 2002 |id=Publication 4039 |url=http://www.muslimheritage.com/uploads/ibnsina.pdf |access-date=2008-07-22 |archive-date=2012-10-03 |archive-url=https://web.archive.org/web/20121003004835/http://www.muslimheritage.com/uploads/ibnsina.pdf |url-status=live }}</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 the erosion of the mountains and by [[Deposition (sediment)|deposition]] of [[silt]].<ref>{{cite book|last1=Needham |first1=Joseph |year=1986 |title=Science and Civilisation in China|volume=3|location=Taipei |publisher=Caves Books, Ltd.|pages=603–604|isbn=978-0-521-31560-9|title-link=Science and Civilisation in China }}</ref>

[[Georgius Agricola]] (1494–1555) published his groundbreaking work ''[[De Natura Fossilium]]'' in 1546 and is seen as the founder of geology as a scientific discipline.<ref>{{cite web | url=https://ucmp.berkeley.edu/history/agricola.html | title=Georgius Agricola (1494–1555) }}</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>From his will (''Testamento d'Ullisse Aldrovandi'') of 1603, which is reproduced in: Fantuzzi, Giovanni, ''Memorie della vita di Ulisse Aldrovandi, medico e filosofo bolognese'' … (Bologna, Italy:  Lelio dalla Volpe, 1774). [https://books.google.com/books?id=ArggVT7zGk4C&pg=PA81 From p. 81:] {{Webarchive|url=https://web.archive.org/web/20170216115915/https://books.google.com/books?id=ArggVT7zGk4C&pg=PA81|date=2017-02-16}}  " … ''& anco la Giologia, ovvero de Fossilibus;'' … " ( … and likewise geology, or [the study] of things dug from the earth; … )</ref><ref>{{cite book|author1=Vai, Gian Battista|author2=Cavazza, William|title=Four centuries of the word geology: Ulisse Aldrovandi 1603 in Bologna|url=https://books.google.com/books?id=Ip-rAAAACAAJ|year=2003|publisher=Minerva|isbn=978-88-7381-056-8|access-date=2015-11-14|archive-date=2016-04-20|archive-url=https://web.archive.org/web/20160420110353/https://books.google.com/books?id=Ip-rAAAACAAJ|url-status=live}}</ref> then by [[Jean-André Deluc]] in 1778<ref>Deluc, Jean André de, ''Lettres physiques et morales sur les montagnes et sur l'histoire de la terre et de l'homme. …'' [Physical and moral letters on mountains and on the history of the Earth and man. … ], vol. 1 (Paris, France:  V. Duchesne, 1779), pp. 4, 5, and 7.  [https://babel.hathitrust.org/cgi/pt?id=mdp.39015067148760;view=1up;seq=20 From p. 4:] {{Webarchive|url=https://web.archive.org/web/20181122140010/https://babel.hathitrust.org/cgi/pt?id=mdp.39015067148760;view=1up;seq=20 |date=2018-11-22 }}  ''"Entrainé par les liaisons de cet objet avec la Géologie, j'entrepris dans un second voyage de les développer à SA MAJESTÉ; … "'' (Driven by the connections between this subject and geology, I undertook a second voyage to develop them for Her Majesty [viz, [[Charlotte of Mecklenburg-Strelitz]], Queen of Great Britain and Ireland]; … )  [https://babel.hathitrust.org/cgi/pt?id=mdp.39015067148760;view=1up;seq=21 From p. 5:] {{Webarchive|url=https://web.archive.org/web/20181122141605/https://babel.hathitrust.org/cgi/pt?id=mdp.39015067148760;view=1up;seq=21 |date=2018-11-22 }}  ''"Je vis que je faisais un Traité, et non une equisse de ''Géologie''."''  (I see that I wrote a treatise, and not a sketch of geology.)  [https://babel.hathitrust.org/cgi/pt?id=mdp.39015067148760;view=1up;seq=23 From the footnote on p. 7:] {{Webarchive|url=https://web.archive.org/web/20181122141258/https://babel.hathitrust.org/cgi/pt?id=mdp.39015067148760;view=1up;seq=23 |date=2018-11-22 }}  ''"Je répète ici, ce que j'avois dit dans ma première ''Préface'', sur la substitution de mot ''Cosmologie'' à celui de ''Géologie'', quoiqu'il ne s'agisse pas de l'Univers, mais seulement de la ''Terre'': … "''  (I repeat here what I said in my first preface about the substitution of the word "cosmology" for that of "geology", although it is not a matter of the universe but only of the Earth: … ) [Note:  A pirated edition of this book was published in 1778.]</ref> and introduced as a fixed term by [[Horace-Bénédict de Saussure]] in 1779.<ref>Saussure, Horace-Bénédict de, ''Voyages dans les Alpes'', … (Neuchatel, (Switzerland):  Samuel Fauche, 1779).  [http://gallica.bnf.fr/ark:/12148/bpt6k102951m/f4 From pp. i–ii:] {{Webarchive|url=https://web.archive.org/web/20170206185724/http://gallica.bnf.fr/ark:/12148/bpt6k102951m/f4 |date=2017-02-06 }}  ''"La science qui rassemble les faits, qui seuls peuvent servir de base à la Théorie de la Terre ou à la ''Géologie'', c'est la Géographie physique, ou la description de notre Globe; … "'' (The science that assembles the facts which alone can serve as the basis of the theory of the Earth or of "geology", is physical geography, or the description of our globe; … )</ref><ref>On the controversy regarding whether Deluc or Saussure deserves priority in the use the term "geology":

* Zittel, Karl Alfred von, with Maria M. Ogilvie-Gordon, trans., ''History of Geology and Paleontology to the End of the Nineteenth Century'' (London, England:  Walter Scott, 1901), [https://archive.org/stream/cu31924012130534#page/n95/mode/2up p. 76.]
* Geikie, Archibald, ''The Founders of Geology'', 2nd ed. (London, England:  Macmillan and Company, 1905), [https://books.google.com/books?id=QFMPAAAAIAAJ&pg=PA186 p. 186.]. {{Webarchive|url=https://web.archive.org/web/20170216124226/https://books.google.com/books?id=QFMPAAAAIAAJ&pg=PA186|date=2017-02-16}}.
* Eastman, Charles Rochester (12 August 1904). [https://www.jstor.org/stable/1629832?seq=2#page_scan_tab_contents Letter to the Editor: "Variæ Auctoritatis"]. {{Webarchive|url=https://web.archive.org/web/20170207034250/http://www.jstor.org/stable/1629832?seq=2#page_scan_tab_contents|date=2017-02-07}}, ''Science'', 2nd series, '''20''' (502): 215–217; see p. 216.
* Emmons, Samuel Franklin (21 October 1904). [https://www.jstor.org/stable/1631128?seq=1#page_scan_tab_contents Letter to the Editor:  "Variæ Auctoritatis"]. {{Webarchive|url=https://web.archive.org/web/20170207034248/http://www.jstor.org/stable/1631128?seq=1#page_scan_tab_contents|date=2017-02-07}}, ''Science'', 2nd series, '''20''' (512): 537.
* Eastman, C.R. (25 November 1904). [https://www.jstor.org/stable/1630549?seq=3#page_scan_tab_contents Letter to the Editor:  "Notes on the History of Scientific Nomenclature"], {{Webarchive|url=https://web.archive.org/web/20170207134257/http://www.jstor.org/stable/1630549?seq=3#page_scan_tab_contents|date=2017-02-07}}. ''Science'', 2nd series, '''20''' (517): 727–730; see p. 728.
* Emmons, S. F. (23 December 1904). [https://archive.org/stream/jstor-1631724/1631724#page/n1/mode/2up Letter to the Editor: "The term 'geology' "], ''Science'', 2nd series, '''20''' (521): 886–887.
* Eastman, C. R. (20 January 1905). [https://books.google.com/books?id=7_hDAQAAMAAJ&pg=PA111 Letter to the Editor:  "Deluc's 'Geological Letters'"]. {{Webarchive|url=https://web.archive.org/web/20170216123418/https://books.google.com/books?id=7_hDAQAAMAAJ&pg=PA111|date=2017-02-16}}, ''Science'', 2nd series, '''21''' (525): 111.
* Emmons, S. F. (17 February 1905). [https://books.google.com/books?id=7_hDAQAAMAAJ&pg=PA274 Letter to the Editor:  "Deluc versus de Saussure"]. {{Webarchive|url=https://web.archive.org/web/20170216124011/https://books.google.com/books?id=7_hDAQAAMAAJ&pg=PA274|date=2017-02-16}}, ''Science'', 2nd series, '''21''' (529): 274–275.</ref> The word is derived from the [[Ancient Greek|Greek]] γῆ, ''gê'', meaning "earth" and λόγος, ''[[logos]]'', meaning "speech".<ref>{{Cite book | last1 = Winchester | first1 = Simon | author-link = Simon Winchester | title = The Map that Changed the World | publisher = HarperCollins Publishers | year = 2001 | page = [https://archive.org/details/mapthatchanged00winc/page/25 25] | isbn = 978-0-06-093180-3 | title-link = The Map that Changed the World }}</ref> But according to another source, the word "geology" comes from a Norwegian, [[Mikkel Pedersøn Escholt]] (1600–1669), who was a priest and scholar. Escholt first used the definition in his book titled, ''Geologia Norvegica'' (1657).<ref>Escholt, Michel Pedersøn, [https://books.google.com/books?id=FodAAQAAMAAJ&pg=PA1 ''Geologia Norvegica : det er, En kort undervisning om det vitt-begrebne jordskelff som her udi Norge skeedemesten ofuer alt Syndenfields den 24. aprilis udi nærværende aar 1657: sampt physiske, historiske oc theologiske fundament oc grundelige beretning om jordskellfs aarsager oc betydninger''] {{Webarchive|url=https://web.archive.org/web/20170216123747/https://books.google.com/books?id=FodAAQAAMAAJ&pg=PA1|date=2017-02-16}} [Norwegian geology:  that is, a brief lesson about the widely-perceived earthquake which happened here in Norway across all southern parts [on] the 24th of April in the present year 1657: together with physical, historical, and theological bases and a basic account of earthquakes' causes and meanings] (Christiania (now:  Oslo), Norway:  Mickel Thomesøn, 1657). (in Danish).
* Reprinted in English as:  Escholt, Michel Pedersøn with Daniel Collins, trans., [https://books.google.com/books?id=UKlkAAAAcAAJ&pg=PR4 ''Geologia Norvegica'' … ]. {{Webarchive|url=https://web.archive.org/web/20170216124138/https://books.google.com/books?id=UKlkAAAAcAAJ&pg=PR4|date=2017-02-16}}. (London, England:  S. Thomson, 1663).</ref><ref>Kermit H., (2003) [https://books.google.com/books?id=6al2BH438AYC&q=ductus+stenonianus ''Niels Stensen, 1638–1686: the scientist who was beatified'']. {{Webarchive|url=https://web.archive.org/web/20170120163806/https://books.google.com/books?id=6al2BH438AYC&dq=ductus+stenonianus&source=gbs_navlinks_s|date=2017-01-20}}. 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" />

In 1763, [[Mikhail Lomonosov]] published his treatise ''On the Strata of Earth''.<ref>{{cite book |title=On the Strata of the Earth. |others=Translation and commentary by S.M. Rowland and S. Korolev |last=Lomonosov |first=Mikhail |year=2012 |publisher=The Geological Society of America, Special Paper 485 |isbn=978-0-8137-2485-0 |url=https://books.google.com/books?id=o5x3jgEACAAJ |access-date=2021-06-19 |archive-date=2021-06-24 |archive-url=https://web.archive.org/web/20210624205930/https://books.google.com/books?id=o5x3jgEACAAJ |url-status=live }}</ref> His work was the first narrative of modern geology, based on the unity of processes in time and explanation of the Earth's past from the present.<ref>Vernadsky, V. (1911). Pamyati M.V. Lomonosova. Zaprosy zhizni, 5: 257–262 (in Russian) [In memory of M.V. Lomonosov].</ref>

[[James Hutton]] (1726–1797) is often viewed as the first modern geologist.<ref>[http://www.amnh.org/explore/resource-collections/earth-inside-and-out/james-hutton-the-founder-of-modern-geology/ James Hutton: The Founder of Modern Geology]. {{Webarchive|url=https://web.archive.org/web/20160827192049/http://www.amnh.org/explore/resource-collections/earth-inside-and-out/james-hutton-the-founder-of-modern-geology|date=2016-08-27}}. 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 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.<ref>Gutenberg ebook links: ([[gutenberg:12861|Vol. 1]]. {{Cite book |url=http://www.gutenberg.org/ebooks/12861 |title=Theory of the Earth, with Proofs and Illustrations, Volume 1 (Of 4) |access-date=2022-07-30 |archive-date=2020-09-14 |archive-url=https://web.archive.org/web/20200914043214/http://www.gutenberg.org/ebooks/12861 |url-status=live }}, [[gutenberg:14179|Vol. 2]]. {{Cite book |url=https://www.gutenberg.org/ebooks/14179 |title=Theory of the Earth, with Proofs and Illustrations, Volume 2 (Of 4) |access-date=2020-08-28 |archive-date=2020-08-09 |archive-url=https://web.archive.org/web/20200809183159/https://www.gutenberg.org/ebooks/14179 |url-status=live }}).</ref>

Followers of Hutton were known as ''[[Plutonists]]'' because they believed that some rocks were formed by ''vulcanism'', which is the deposition of lava from volcanoes, as opposed to the ''[[Neptunists]]'', led by [[Abraham Werner]], who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.

The first [[Geologic map of Georgia|geological map of the U.S.]] was produced in 1809 by [[William Maclure]].<ref name="Maclure1817">{{cite book|author=Maclure, William|title=Observations on the Geology of the United States of America: With Some Remarks on the Effect Produced on the Nature and Fertility of Soils, by the Decomposition of the Different Classes of Rocks; and an Application to the Fertility of Every State in the Union, in Reference to the Accompanying Geological Map.|url=https://books.google.com/books?id=_bgQAAAAIAAJ|year=1817|publisher=Abraham Small|location=Philadelphia|access-date=2015-11-14|archive-date=2015-10-27|archive-url=https://web.archive.org/web/20151027152452/https://books.google.com/books?id=_bgQAAAAIAAJ|url-status=live}}</ref> In 1807, Maclure commenced the self-imposed task of making a geological survey of the United States. Almost every state in the Union was traversed and mapped by him, the [[Allegheny Mountains]] being crossed and recrossed some 50 times.<ref>{{cite journal |author=Greene |first1=J. C. |last2=Burke |first2=J. G. |year=1978 |title=The Science of Minerals in the Age of Jefferson |journal=Transactions of the American Philosophical Society |series=New Series |volume=68 |issue=4 |pages=1–113 [39] |doi=10.2307/1006294 |jstor=1006294}}</ref> The results of his unaided labours were submitted to the [[American Philosophical Society]] in a memoir entitled ''Observations on the Geology of the United States explanatory of a Geological Map'', and published in the ''Society's Transactions'', together with the nation's first geological map.<ref>[http://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~959~60260:A-Map-of-the-United-States-of-Ameri?sort=Pub_Date%2CPub_List_No%2CSeries_No&qvq=q:Geology;sort:Pub_Date%2CPub_List_No%2CSeries_No;lc:RUMSEY~8~1&mi=2&trs=1282 Maclure's 1809 Geological Map]. {{Webarchive|url=https://web.archive.org/web/20140814020815/http://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~959~60260:A-Map-of-the-United-States-of-Ameri?sort=Pub_Date,Pub_List_No,Series_No&qvq=q:Geology;sort:Pub_Date,Pub_List_No,Series_No;lc:RUMSEY~8~1&mi=2&trs=1282|date=2014-08-14}}. davidrumsey.com.</ref> This antedates [[William Smith (geologist)|William Smith]]'s geological map of England by six years, although it was constructed using a different classification of rocks.

[[Sir Charles Lyell]] (1797–1875) first published his famous book, ''[[Principles of Geology]]'',<ref>{{Cite book |isbn=978-0-226-49797-6 |author=Lyell, Charles |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 [[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|issue=1 |bibcode=2007GSAT...17R...4E |doi-access=free }}</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="4.54 book" />

<gallery>
File:Georgius Agricola.jpg|[[Georgius Agricola]], German [[mineralogist]], founder of geology as a scientific field
File:M.V. Lomonosov by L.Miropolskiy after G.C.Prenner (1787, RAN).jpg|[[Mikhail Lomonosov]], Russian [[polymath]], author of the first systematic treatise in scientific geology ([[1763]])
File:Hutton James portrait Raeburn.jpg|[[James Hutton]], Scottish [[geologist]] and father of [[modern geology]]
File:John Tuzo Wilson in 1992.jpg|[[John Tuzo Wilson]], Canadian [[geophysicist]] and father of [[plate tectonics]]
File:MSH80 david johnston at camp 05-17-80 med (cropped).jpg|The [[volcanologist]] [[David A. Johnston]] 13 hours before his death at the<br>[[1980 eruption of Mount St. Helens]]
</gallery>

==Fields or related disciplines==
{{div col|colwidth=15em}}
* [[Earth system science]]
* [[Economic geology]]
** [[Mining|Mining geology]]
** [[Petroleum geology]]
* [[Engineering geology]]
* [[Environmental geology]]
* [[Environmental science]]
* [[Geoarchaeology]]
* [[Geochemistry]]
** [[Biogeochemistry]]
** [[Isotope geochemistry]]
* [[Geochronology]]
* [[Geodetics]]
* [[Geography]]
** [[Physical geography]]
** [[Technical geography]]
* [[Geological engineering]]
* [[Geological modelling]]
* [[Geometallurgy]]
* [[Geomicrobiology]]
* [[Geomorphology]]
* [[Geomythology]]
* [[Geophysics]]
* [[Glaciology]]
* [[Historical geology]]
* [[Hydrogeology]]
* [[Meteorology]]
* [[Mineralogy]]
* [[Oceanography]]
** [[Marine geology]]
* [[Paleoclimatology]]
* [[Paleontology]]
** [[Micropaleontology]]
** [[Palynology]]
* [[Petrology]]
* [[Petrophysics]]
* [[Planetary geology]]
* [[Plate tectonics]]
* [[Regional geology]]
* [[Sedimentology]]
* [[Seismology]]
* [[Soil science]]
** [[Pedology (soil study)]]
* [[Speleology]]
* [[Stratigraphy]]
** [[Biostratigraphy]]
** [[Chronostratigraphy]]
** [[Lithostratigraphy]]
* [[Structural geology]]
* [[Systems geology]]
* [[Tectonics]]
* [[Volcanology]]
{{Div col end}}

==See also==
{{portal|Geology|Earth sciences}}
{{Div col}}
* {{annotated link|Glossary of geology}}
* {{annotated link|Geoprofessions}}
* {{annotated link|Geotourism}}
* {{annotated link|Index of geology articles}}
* {{annotated link|International Union of Geological Sciences|abbreviation=IUGS}}
* [[List of individual rocks]]
* {{annotated link|Outline of geology}}
* {{annotated link|Timeline of geology}}
{{div col end}}

==References==
{{Reflist}}

==External links==
{{Commons category|Geology}}
{{Wikisource portal|Geology}}
{{WVS}}
{{wikibooks|Historical Geology}}
{{wikiquote}}
* [http://www.onegeology.org/ One Geology: This interactive geological map of the world is an international initiative of the geological surveys around the globe. This groundbreaking project was launched in 2007 and contributed to the 'International Year of Planet Earth', becoming one of their flagship projects. ]
* [http://geology.com/ ''Earth Science News, Maps, Dictionary, Articles, Jobs'']
* [http://www.agu.org/ American Geophysical Union]
* [http://www.americangeosciences.org/ American Geosciences Institute]
* [http://www.egu.eu/ European Geosciences Union]
* [http://www.eurogeologists.eu/ European Federation of Geologists]
* [https://web.archive.org/web/20111029134635/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]
* [https://opentextbc.ca/geology/ Geology OpenTextbook]
* [https://ghkclass.com/ Chronostratigraphy benchmarks]
* [https://www.gutenberg.org/ebooks/73557  The principles and objects of geology, with special reference to the geology of Egypt] (1911), W. F. Hume

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{{Earth science}}
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{{Structural geology}}
{{Planetary Geology}}
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[[Category:Geology| ]]