Editing Geology (section)

==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" />