Geology

For the scientific journal, see Geology (journal).

Geology (from the Ancient Greek γῆ, gē, i.e. “earth” and-λoγία, -logia, i.e. “study of, discourse”[1][2]) is an earthscience comprising the study of solid Earth, the rocks ofwhich it is composed, and the processes by which theychange. Geology can also refer generally to the studyof the solid features of any celestial body (such as thegeology of the Moon or Mars).Geology gives insight into the history of the Earth byproviding the primary evidence for plate tectonics, theevolutionary history of life, and past climates. Geol-ogy is important for mineral and hydrocarbon explorationand exploitation, evaluating water resources, understand-ing of natural hazards, the remediation of environmentalproblems, and for providing insights into past climatechange. Geology also plays a role in geotechnical engi-neering and is a major academic discipline.

1 Geologic materials

The majority of geological data comes from research onsolid Earth materials. These typically fall into one of twocategories: rock and unconsolidated material.

1.1 Rock

Main articles: Rock (geology) and Rock cycle

There are three major types of rock: igneous,sedimentary, and metamorphic. The rock cycle is animportant concept in geology which illustrates the rela-tionships between these three types of rock, and magma.When a rock crystallizes from melt (magma and/or lava),it is an igneous rock. This rock can be weathered anderoded, and then redeposited and lithified into a sedimen-tary rock, or be turned into a metamorphic rock due toheat and pressure that change the mineral content of therock which gives it a characteristic fabric. The sedimen-tary rock can then be subsequently turned into a meta-morphic rock due to heat and pressure and is then weath-ered, eroded, deposited, and lithified, ultimately becom-ing a sedimentary rock. Sedimentary rock may also bere-eroded and redeposited, and metamorphic rock mayalso undergo additional metamorphism. All three typesof rocks may be re-melted; when this happens, a new

This schematic diagram of the rock cycle shows the relationshipbetween magma and sedimentary, metamorphic, and igneousrock

magma is formed, from which an igneous rock may onceagain crystallize.The majority of research in geology is associated with thestudy of rock, as rock provides the primary record of themajority of the geologic history of the Earth.

1.2 Unconsolidated material

Geologists also study unlithified material, which typicallycomes from more recent deposits. These materials aresuperficial deposits which lie above the bedrock.[3] Be-cause of this, the study of such material is often knownas Quaternary geology, after the recent Quaternary Pe-riod. This includes the study of sediment and soils, in-cluding studies in geomorphology, sedimentology, andpaleoclimatology.

2 Whole-Earth structure

2.1 Plate tectonics

Main article: Plate tectonicsIn the 1960s, a series of discoveries, the most impor-tant of which was seafloor spreading,[4][5] showed that

1

2 2 WHOLE-EARTH STRUCTURE

Asthenosphere

Lithosphere Lithosphere

Continental crustOceanic crust

Tren

ch

Vol

cani

car

c

Oceanic-continental convergence resulting in subduction andvolcanic arcs illustrates one effect of plate tectonics.

the Earth’s lithosphere, which includes the crust andrigid uppermost portion of the upper mantle, is sepa-rated into a number of tectonic plates that move acrossthe plastically deforming, solid, upper mantle, which iscalled the asthenosphere. There is an intimate couplingbetween the movement of the plates on the surface andthe convection of the mantle: oceanic plate motions andmantle convection currents always move in the same di-rection, because the oceanic lithosphere is the rigid upperthermal boundary layer of the convecting mantle. Thiscoupling between rigid plates moving on the surface ofthe Earth and the convecting mantle is called plate tec-tonics.

On this diagram, subducting slabs are in blue, and continentalmargins and a few plate boundaries are in red. The blue blobin the cutaway section is the seismically imaged Farallon Plate,which is subducting beneath North America. The remnants ofthis plate on the Surface of the Earth are the Juan de FucaPlate and Explorer plate in the Northwestern USA / SouthwesternCanada, and the Cocos Plate on the west coast of Mexico.

The development of plate tectonics provided a physicalbasis for many observations of the solid Earth. Long lin-ear regions of geologic features could be explained asplate boundaries.[6] Mid-ocean ridges, high regions onthe seafloor where hydrothermal vents and volcanoes ex-ist, were explained as divergent boundaries, where twoplates move apart. Arcs of volcanoes and earthquakeswere explained as convergent boundaries, where one platesubducts under another. Transform boundaries, such as

the San Andreas Fault system, resulted in widespreadpowerful earthquakes. Plate tectonics also provided amechanism for Alfred Wegener's theory of continentaldrift,[7] in which the continents move across the surfaceof the Earth over geologic time. They also provided adriving force for crustal deformation, and a new settingfor the observations of structural geology. The power ofthe theory of plate tectonics lies in its ability to combineall of these observations into a single theory of how thelithosphere moves over the convecting mantle.

2.2 Earth structure

Main article: Structure of the EarthAdvances in seismology, computer modeling, and

1

2

3

4

5

6

The Earth's layered structure. (1) inner core; (2) outer core; (3)lower mantle; (4) upper mantle; (5) lithosphere; (6) crust (partof the lithosphere)

Innercore

Outer core

Lower mantle Upper mantle

Crust

Focus of earthquake

Kilometers

0 10,000

S

SSSS

SSKS

SKP

SP

P

P

P

P

PP

PKPPPP

P

P

K

K

Earth layered structure. Typical wave paths from earthquakeslike these gave early seismologists insights into the layered struc-ture of the Earth

3.1 Important milestones 3

mineralogy and crystallography at high temperatures andpressures give insights into the internal composition andstructure of the Earth.Seismologists can use the arrival times of seismic wavesin reverse to image the interior of the Earth. Early ad-vances in this field showed the existence of a liquid outercore (where shear waves were not able to propagate) anda dense solid inner core. These advances led to the devel-opment of a layered model of the Earth, with a crust andlithosphere on top, the mantle below (separated withinitself by seismic discontinuities at 410 and 660 kilome-ters), and the outer core and inner core below that. Morerecently, seismologists have been able to create detailedimages of wave speeds inside the earth in the same waya doctor images a body in a CT scan. These images haveled to a much more detailed view of the interior of theEarth, and have replaced the simplified layered modelwith a much more dynamic model.Mineralogists have been able to use the pressure andtemperature data from the seismic and modelling stud-ies alongside knowledge of the elemental composition ofthe Earth to reproduce these conditions in experimentalsettings and measure changes in crystal structure. Thesestudies explain the chemical changes associated with themajor seismic discontinuities in the mantle and show thecrystallographic structures expected in the inner core ofthe Earth.

3 Geologic time

4550 Ma:

HomininsMammalsLand plantsAnimalsMulticellular lifeEukaryotesProkaryotes

Hadean

Arc

heanProterozoic

Paleozoic

Mesozoic

Cenozoic

4527 Ma:Formation of the Moon

4.6 Ga

4 Ga

4.0 Ga

3 Ga

2.5 Ga

2 Ga

1 Ga

541 M

a

252 Ma66 Ma c. 4000 Ma: End of the

Late Heavy Bombardment;first life

c. 3200 Ma:Earliest start

of Photosynthesis

c. 2300 Ma:Atmosphere becomes oxygen-rich;

first Snowball Earth

750-635 Ma:Two Snowball Earths

c. 530 Ma:Cambrian explosion

c. 380 Ma:First vertebrate land animals

230-66 Ma:Non-avian dinosaurs

2 Ma:First Hominins

Geological time put in a diagram called a geological clock, show-ing the relative lengths of the eons of the Earth’s history.

Main articles: History of Earth and Geologic time scale

The geologic time scale encompasses the history of theEarth.[8] It is bracketed at the old end by the dates of

the earliest Solar Systemmaterial at 4.567 Ga,[9] (gigaan-num: billion years ago) and the age of the Earth at 4.54Ga[10][11] at the beginning of the informally recognizedHadean eon. At the young end of the scale, it is brack-eted by the present day in the Holocene epoch.

3.1 Important milestones• 4.567 Ga: Solar system formation[9]

• 4.54 Ga: Accretion of Earth[10][11]

• c. 4 Ga: End of Late Heavy Bombardment, first life• c. 3.5 Ga: Start of photosynthesis• c. 2.3 Ga: Oxygenated atmosphere, first snowballEarth

• 730–635 Ma (megaannum: million years ago): sec-ond snowball Earth

• 542 ± 0.3 Ma: Cambrian explosion – vast multi-plication of hard-bodied life; first abundant fossils;start of the Paleozoic

• c. 380 Ma: First vertebrate land animals• 250 Ma: Permian-Triassic extinction – 90% of allland animals die; end of Paleozoic and beginning ofMesozoic

• 66 Ma: Cretaceous–Paleogene extinction –Dinosaurs die; end of Mesozoic and beginning ofCenozoic

• c. 7 Ma: First hominins appear• 3.9 Ma: First Australopithecus, direct ancestor tomodern Homo sapiens, appear

• 200 ka (kiloannum: thousand years ago): First mod-ern Homo sapiens appear in East Africa

3.2 Brief time scale

The following four timelines show the geologic time scale.The first shows the entire time from the formation of theEarth to the present, but this compresses the most recenteon. Therefore, the second scale shows the most recenteon with an expanded scale. The second scale compressesthe most recent era, so the most recent era is expandedin the third scale. Since the Quaternary is a very shortperiod with short epochs, it is further expanded in thefourth scale. The second, third, and fourth timelines aretherefore each subsections of their preceding timeline asindicated by asterisks. The Holocene (the latest epoch)is too small to be shown clearly on the third timeline onthe right, another reason for expanding the fourth scale.The Pleistocene (P) epoch. Q stands for the Quaternaryperiod.

Millions of Years

4 4 DATING METHODS

4 Dating methods

Geologists use a variety of methods to give both relativeand absolute dates to geological events. They then usethese dates to find the rates at which processes occur.

4.1 Relative dating

Main article: Relative datingMethods for relative dating were developed when geol-

C

AD

B

EC

F

Cross-cutting relations can be used to determine the relative agesof rock strata and other geological structures. Explanations: A –folded rock strata cut by a thrust fault; B – large intrusion (cuttingthrough A); C – erosional angular unconformity (cutting off A &B) on which rock strata were deposited; D – volcanic dyke (cut-ting through A, B & C); E – even younger rock strata (overlyingC & D); F – normal fault (cutting through A, B, C & E).

ogy first emerged as a formal science. Geologists still usethe following principles today as ameans to provide infor-mation about geologic history and the timing of geologicevents.The principle of Uniformitarianism states that the ge-ologic processes observed in operation that modify theEarth’s crust at present have worked in much the sameway over geologic time.[12] A fundamental principle ofgeology advanced by the 18th century Scottish physicianand geologist James Hutton, is that “the present is the keyto the past.” In Hutton’s words: “the past history of ourglobe must be explained by what can be seen to be hap-pening now.”[13]

The principle of intrusive relationships concernscrosscutting intrusions. In geology, when an igneous in-trusion cuts across a formation of sedimentary rock, itcan be determined that the igneous intrusion is youngerthan the sedimentary rock. There are a number of dif-ferent types of intrusions, including stocks, laccoliths,batholiths, sills and dikes.The principle of cross-cutting relationships pertainsto the formation of faults and the age of the sequencesthrough which they cut. Faults are younger than the rocksthey cut; accordingly, if a fault is found that penetratessome formations but not those on top of it, then the for-mations that were cut are older than the fault, and the onesthat are not cut must be younger than the fault. Find-

ing the key bed in these situations may help determinewhether the fault is a normal fault or a thrust fault.[14]

The principle of inclusions and components statesthat, with sedimentary rocks, if inclusions (or clasts) arefound in a formation, then the inclusions must be olderthan the formation that contains them. For example,in sedimentary rocks, it is common for gravel from anolder formation to be ripped up and included in a newerlayer. A similar situation with igneous rocks occurs whenxenoliths are found. These foreign bodies are picked up asmagma or lava flows, and are incorporated, later to coolin the matrix. As a result, xenoliths are older than therock which contains them.

The Permian through Jurassic stratigraphy of the ColoradoPlateau area of southeastern Utah is a great example of bothOriginal Horizontality and the Law of Superposition. Thesestrata make up much of the famous prominent rock formations inwidely spaced protected areas such as Capitol Reef National Parkand Canyonlands National Park. From top to bottom: Roundedtan domes of the Navajo Sandstone, layered red Kayenta For-mation, cliff-forming, vertically jointed, red Wingate Sandstone,slope-forming, purplish Chinle Formation, layered, lighter-redMoenkopi Formation, and white, layered Cutler Formation sand-stone. Picture from Glen Canyon National Recreation Area,Utah.

The principle of original horizontality states that thedeposition of sediments occurs as essentially horizon-tal beds. Observation of modern marine and non-marine sediments in a wide variety of environments sup-ports this generalization (although cross-bedding is in-clined, the overall orientation of cross-bedded units ishorizontal).[14]

The principle of superposition states that a sedimen-tary rock layer in a tectonically undisturbed sequence isyounger than the one beneath it and older than the oneabove it. Logically a younger layer cannot slip beneatha layer previously deposited. This principle allows sed-imentary layers to be viewed as a form of vertical timeline, a partial or complete record of the time elapsed fromdeposition of the lowest layer to deposition of the highestbed.[14]

The principle of faunal succession is based on the ap-pearance of fossils in sedimentary rocks. As organismsexist at the same time period throughout the world, theirpresence or (sometimes) absence may be used to provide

5

a relative age of the formations in which they are found.Based on principles laid out by William Smith almost ahundred years before the publication of Charles Darwin'stheory of evolution, the principles of succession were de-veloped independently of evolutionary thought. The prin-ciple becomes quite complex, however, given the uncer-tainties of fossilization, the localization of fossil types dueto lateral changes in habitat (facies change in sedimentarystrata), and that not all fossils may be found globally at thesame time.[15]

4.2 Absolute dating

Main articles: Absolute dating, radiometric dating, andgeochronology

Geologists also use methods to determine the absoluteage of rock samples and geological events. These datesare useful on their own and may also be used in conjunc-tion with relative dating methods or to calibrate relativemethods.[16]

At the beginning of the 20th century, important advance-ment in geological science was facilitated by the abilityto obtain accurate absolute dates to geologic events usingradioactive isotopes and other methods. This changedthe understanding of geologic time. Previously, geolo-gists could only use fossils and stratigraphic correlationto date sections of rock relative to one another. With iso-topic dates it became possible to assign absolute ages torock units, and these absolute dates could be applied tofossil sequences in which there was datable material, con-verting the old relative ages into new absolute ages.For many geologic applications, isotope ratios of ra-dioactive elements are measured in minerals that givethe amount of time that has passed since a rock passedthrough its particular closure temperature, the point atwhich different radiometric isotopes stop diffusing intoand out of the crystal lattice.[17][18] These are used ingeochronologic and thermochronologic studies. Com-mon methods include uranium-lead dating, potassium-argon dating, argon-argon dating and uranium-thoriumdating. These methods are used for a variety of applica-tions. Dating of lava and volcanic ash layers found withina stratigraphic sequence can provide absolute age data forsedimentary rock units which do not contain radioactiveisotopes and calibrate relative dating techniques. Thesemethods can also be used to determine ages of pluton em-placement. Thermochemical techniques can be used todetermine temperature profiles within the crust, the up-lift of mountain ranges, and paleotopography.Fractionation of the lanthanide series elements is used tocompute ages since rocks were removed from the mantle.Other methods are used for more recent events. Opticallystimulated luminescence and cosmogenic radionucleidedating are used to date surfaces and/or erosion rates.

Dendrochronology can also be used for the dating oflandscapes. Radiocarbon dating is used for geologicallyyoung materials containing organic carbon.

5 Geological development of anarea

magma chamber batholithpluton

conduit

sill dike

silllaccolith

stock

palisade

ActiveVolcanism

Ancient Volcanic Featuresash plume

volcano

cindercone recent

lava flowsstock with

radiating dikes

An originally horizontal sequence of sedimentary rocks (inshades of tan) are affected by igneous activity. Deep below thesurface are a magma chamber and large associated igneous bod-ies. The magma chamber feeds the volcano, and sends offshootsof magma that will later crystallize into dikes and sills. Magmaalso advances upwards to form intrusive igneous bodies. The di-agram illustrates both a cinder cone volcano, which releases ash,and a composite volcano, which releases both lava and ash.

The geology of an area changes through time as rock unitsare deposited and inserted and deformational processeschange their shapes and locations.Rock units are first emplaced either by deposition ontothe surface or intrusion into the overlying rock. Deposi-tion can occur when sediments settle onto the surface ofthe Earth and later lithify into sedimentary rock, or whenas volcanic material such as volcanic ash or lava flowsblanket the surface. Igneous intrusions such as batholiths,laccoliths, dikes, and sills, push upwards into the overly-ing rock, and crystallize as they intrude.After the initial sequence of rocks has been deposited,the rock units can be deformed and/or metamorphosed.Deformation typically occurs as a result of horizontalshortening, horizontal extension, or side-to-side (strike-slip) motion. These structural regimes broadly relate toconvergent boundaries, divergent boundaries, and trans-form boundaries, respectively, between tectonic plates.When rock units are placed under horizontalcompression, they shorten and become thicker. Be-cause rock units, other than muds, do not significantlychange in volume, this is accomplished in two primaryways: through faulting and folding. In the shallow crust,where brittle deformation can occur, thrust faults form,which cause deeper rock to move on top of shallowerrock. Because deeper rock is often older, as noted bythe principle of superposition, this can result in olderrocks moving on top of younger ones. Movement along

6 5 GEOLOGICAL DEVELOPMENT OF AN AREA

An illustration of the three types of faults. Strike-slip faults occurwhen rock units slide past one another, normal faults occur whenrocks are undergoing horizontal extension, and thrust faults occurwhen rocks are undergoing horizontal shortening.

faults can result in folding, either because the faults arenot planar or because rock layers are dragged along,forming drag folds as slip occurs along the fault. Deeperin the Earth, rocks behave plastically, and fold insteadof faulting. These folds can either be those where thematerial in the center of the fold buckles upwards,creating "antiforms", or where it buckles downwards,creating "synforms". If the tops of the rock units within

the folds remain pointing upwards, they are calledanticlines and synclines, respectively. If some of theunits in the fold are facing downward, the structure iscalled an overturned anticline or syncline, and if all ofthe rock units are overturned or the correct up-directionis unknown, they are simply called by the most generalterms, antiforms and synforms.

A diagram of folds, indicating an anticline and a syncline.

Even higher pressures and temperatures during horizon-tal shortening can cause both folding and metamorphismof the rocks. This metamorphism causes changes in themineral composition of the rocks; creates a foliation, orplanar surface, that is related to mineral growth understress. This can remove signs of the original textures ofthe rocks, such as bedding in sedimentary rocks, flow fea-tures of lavas, and crystal patterns in crystalline rocks.Extension causes the rock units as a whole to becomelonger and thinner. This is primarily accomplishedthrough normal faulting and through the ductile stretch-ing and thinning. Normal faults drop rock units that arehigher below those that are lower. This typically results inyounger units being placed below older units. Stretchingof units can result in their thinning; in fact, there is a lo-cation within the Maria Fold and Thrust Belt in which theentire sedimentary sequence of the Grand Canyon can beseen over a length of less than a meter. Rocks at the depthto be ductilely stretched are often also metamorphosed.These stretched rocks can also pinch into lenses, knownas boudins, after the French word for “sausage”, becauseof their visual similarity.Where rock units slide past one another, strike-slip faultsdevelop in shallow regions, and become shear zones atdeeper depths where the rocks deform ductilely.The addition of new rock units, both depositionally andintrusively, often occurs during deformation. Faultingand other deformational processes result in the creationof topographic gradients, causing material on the rockunit that is increasing in elevation to be eroded by hill-slopes and channels. These sediments are deposited onthe rock unit that is going down. Continual motionalong the fault maintains the topographic gradient in spiteof the movement of sediment, and continues to createaccommodation space for the material to deposit. Defor-mational events are often also associated with volcanismand igneous activity. Volcanic ashes and lavas accumu-

6.1 Field methods 7

Geologic cross section of Kittatinny Mountain. This cross sec-tion shows metamorphic rocks, overlain by younger sedimentsdeposited after the metamorphic event. These rock units werelater folded and faulted during the uplift of the mountain.

late on the surface, and igneous intrusions enter from be-low. Dikes, long, planar igneous intrusions, enter alongcracks, and therefore often form in large numbers in ar-eas that are being actively deformed. This can result inthe emplacement of dike swarms, such as those that areobservable across the Canadian shield, or rings of dikesaround the lava tube of a volcano.All of these processes do not necessarily occur in a sin-gle environment, and do not necessarily occur in a sin-gle order. The Hawaiian Islands, for example, consistalmost entirely of layered basaltic lava flows. The sed-imentary sequences of the mid-continental United Statesand the Grand Canyon in the southwestern United Statescontain almost-undeformed stacks of sedimentary rocksthat have remained in place since Cambrian time. Otherareas are much more geologically complex. In the south-western United States, sedimentary, volcanic, and intru-sive rocks have been metamorphosed, faulted, foliated,and folded. Even older rocks, such as the Acasta gneissof the Slave craton in northwestern Canada, the oldestknown rock in the world have been metamorphosed tothe point where their origin is undiscernable without lab-oratory analysis. In addition, these processes can occur instages. In many places, the Grand Canyon in the south-western United States being a very visible example, thelower rock units were metamorphosed and deformed, andthen deformation ended and the upper, undeformed unitswere deposited. Although any amount of rock emplace-ment and rock deformation can occur, and they can occurany number of times, these concepts provide a guide tounderstanding the geological history of an area.

6 Methods of geology

Geologists use a number of field, laboratory, and numer-ical modeling methods to decipher Earth history and tounderstand the processes that occur on and inside theEarth. In typical geological investigations, geologists use

primary information related to petrology (the study ofrocks), stratigraphy (the study of sedimentary layers), andstructural geology (the study of positions of rock unitsand their deformation). In many cases, geologists alsostudy modern soils, rivers, landscapes, and glaciers; in-vestigate past and current life and biogeochemical path-ways, and use geophysical methods to investigate the sub-surface. Sub-specialities of geology may distinguish en-dogenous and exogenous geology.[19]

6.1 Field methods

A standard Brunton Pocket Transit, commonly used by geologistsfor mapping and surveying.

A typical USGS field mapping camp in the 1950s

Geological field work varies depending on the task athand. Typical fieldwork could consist of:

• Geological mapping[20]

• Structural mapping: identifying the locationsof major rock units and the faults and folds thatled to their placement there.

• Stratigraphic mapping: pinpointing the loca-tions of sedimentary facies (lithofacies andbiofacies) or the mapping of isopachs of equalthickness of sedimentary rock

• Surficial mapping: recording the locations ofsoils and surficial deposits

8 6 METHODS OF GEOLOGY

Today, handheld computers with GPS and geographic informa-tion systems software are often used in geological field work(digital geologic mapping).

• Surveying of topographic features

• compilation of topographic maps[21]

• Work to understand change across landscapes,including:• Patterns of erosion and deposition• River-channel change through migrationand avulsion

• Hillslope processes

• Subsurface mapping through geophysical meth-ods[22]

• These methods include:• Shallow seismic surveys• Ground-penetrating radar• Aeromagnetic surveys• Electrical resistivity tomography

• They aid in:• Hydrocarbon exploration• Finding groundwater• Locating buried archaeological artifacts

• High-resolution stratigraphy

• Measuring and describing stratigraphic sec-tions on the surface

• Well drilling and logging

• Biogeochemistry and geomicrobiology[23]

• Collecting samples to:• determine biochemical pathways• identify new species of organisms• identify new chemical compounds

• and to use these discoveries to:

• understand early life on Earth and how itfunctioned and metabolized

• find important compounds for use inpharmaceuticals

• Paleontology: excavation of fossil material

• For research into past life and evolution• For museums and education

• Collection of samples for geochronology andthermochronology[24]

• Glaciology: measurement of characteristics ofglaciers and their motion[25]

6.2 Petrology

Main article: PetrologyIn addition to identifying rocks in the field, petrolo-

A petrographic microscope - an optical microscope fitted withcross-polarizing lenses, a conoscopic lens, and compensators(plates of anisotropic materials; gypsum plates and quartzwedges are common), for crystallographic analysis.

gists identify rock samples in the laboratory. Two ofthe primary methods for identifying rocks in the labo-ratory are through optical microscopy and by using anelectron microprobe. In an optical mineralogy analy-sis, petrologists analyze thin sections of rock samplesusing a petrographic microscope, where the mineralscan be identified through their different properties inplane-polarized and cross-polarized light, including their

6.4 Stratigraphy 9

birefringence, pleochroism, twinning, and interferenceproperties with a conoscopic lens.[26] In the electron mi-croprobe, individual locations are analyzed for their ex-act chemical compositions and variation in compositionwithin individual crystals.[27] Stable[28] and radioactiveisotope[29] studies provide insight into the geochemicalevolution of rock units.Petrologists can also use fluid inclusion data[30] andperform high temperature and pressure physicalexperiments[31] to understand the temperatures andpressures at which different mineral phases appear, andhow they change through igneous[32] and metamorphicprocesses. This research can be extrapolated to the fieldto understand metamorphic processes and the conditionsof crystallization of igneous rocks.[33] This work can alsohelp to explain processes that occur within the Earth,such as subduction and magma chamber evolution.

6.3 Structural geology

Main article: Structural geologyStructural geologists use microscopic analysis of ori-

A diagram of an orogenic wedge. The wedge grows throughfaulting in the interior and along the main basal fault, calledthe décollement. It builds its shape into a critical taper, in whichthe angles within the wedge remain the same as failures inside thematerial balance failures along the décollement. It is analogousto a bulldozer pushing a pile of dirt, where the bulldozer is theoverriding plate.

ented thin sections of geologic samples to observe thefabric within the rocks which gives information aboutstrain within the crystalline structure of the rocks. Theyalso plot and combine measurements of geological struc-tures in order to better understand the orientations offaults and folds in order to reconstruct the history of rockdeformation in the area. In addition, they perform analogand numerical experiments of rock deformation in largeand small settings.The analysis of structures is often accomplished by plot-ting the orientations of various features onto stereonets.A stereonet is a stereographic projection of a sphere ontoa plane, in which planes are projected as lines and linesare projected as points. These can be used to find thelocations of fold axes, relationships between faults, andrelationships between other geologic structures.Among the most well-known experiments in structuralgeology are those involving orogenic wedges, which are

zones in which mountains are built along convergent tec-tonic plate boundaries.[34] In the analog versions of theseexperiments, horizontal layers of sand are pulled along alower surface into a back stop, which results in realistic-looking patterns of faulting and the growth of a criticallytapered (all angles remain the same) orogenic wedge.[35]Numerical models work in the same way as these ana-log models, though they are often more sophisticated andcan include patterns of erosion and uplift in the mountainbelt.[36] This helps to show the relationship between ero-sion and the shape of amountain range. These studies canalso give useful information about pathways for metamor-phism through pressure, temperature, space, and time.[37]

6.4 Stratigraphy

Main article: Stratigraphy

In the laboratory, stratigraphers analyze samples of strati-graphic sections that can be returned from the field, suchas those from drill cores.[38] Stratigraphers also analyzedata from geophysical surveys that show the locations ofstratigraphic units in the subsurface.[39] Geophysical dataand well logs can be combined to produce a better viewof the subsurface, and stratigraphers often use computerprograms to do this in three dimensions.[40] Stratigrapherscan then use these data to reconstruct ancient processesoccurring on the surface of the Earth,[41] interpret pastenvironments, and locate areas for water, coal, and hy-drocarbon extraction.In the laboratory, biostratigraphers analyze rock sam-ples from outcrop and drill cores for the fossils found inthem.[38] These fossils help scientists to date the core andto understand the depositional environment in which therock units formed. Geochronologists precisely date rockswithin the stratigraphic section in order to provide betterabsolute bounds on the timing and rates of deposition.[42]Magnetic stratigraphers look for signs of magnetic rever-sals in igneous rock units within the drill cores.[38] Otherscientists perform stable-isotope studies on the rocks togain information about past climate.[38]

7 Planetary geology

Main articles: Planetary geology and Geology of solarterrestrial planets

With the advent of space exploration in the twentieth cen-tury, geologists have begun to look at other planetary bod-ies in the same ways that have been developed to study theEarth. This new field of study is called planetary geology(sometimes known as astrogeology) and relies on knowngeologic principles to study other bodies of the solar sys-tem.

10 8 APPLIED GEOLOGY

Surface of Mars as photographed by the Viking 2 lander Decem-ber 9, 1977.

Although the Greek-language-origin prefix geo refers toEarth, “geology” is often used in conjunction with thenames of other planetary bodies when describing theircomposition and internal processes: examples are “thegeology ofMars" and "Lunar geology". Specialised termssuch as selenology (studies of the Moon), areology (ofMars), etc., are also in use.Although planetary geologists are interested in studyingall aspects of other planets, a significant focus is to searchfor evidence of past or present life on other worlds. Thishas led to many missions whose primary or ancillarypurpose is to examine planetary bodies for evidence oflife. One of these is the Phoenix lander, which analyzedMartian polar soil for water, chemical, and mineralogicalconstituents related to biological processes.

8 Applied geology

8.1 Economic geology

Main article: Economic geology

Economic geology is an important branch of geologywhich deals with different aspects of economic miner-als being used by humankind to fulfill its various needs.The economic minerals are those which can be extractedprofitably. Economic geologists help locate and managethe Earth’s natural resources, such as petroleum and coal,as well as mineral resources, which include metals suchas iron, copper, and uranium.

8.1.1 Mining geology

Main article: Mining

Mining geology consists of the extractions of mineral re-sources from the Earth. Some resources of economicinterests include gemstones, metals, and many mineralssuch as asbestos, perlite, mica, phosphates, zeolites, clay,pumice, quartz, and silica, as well as elements such assulfur, chlorine, and helium.

8.1.2 Petroleum geology

Mud log in process, a common way to study the lithology whendrilling oil wells.

Main article: Petroleum geology

Petroleum geologists study the locations of the subsurfaceof the Earth which can contain extractable hydrocarbons,especially petroleum and natural gas. Because many ofthese reservoirs are found in sedimentary basins,[43] theystudy the formation of these basins, as well as their sedi-mentary and tectonic evolution and the present-day posi-tions of the rock units.

8.2 Engineering geology

Main articles: Engineering geology, Soil mechanics, andGeotechnical engineering

Engineering geology is the application of the geologicprinciples to engineering practice for the purpose of as-suring that the geologic factors affecting the location, de-sign, construction, operation, and maintenance of engi-neering works are properly addressed.In the field of civil engineering, geological principles andanalyses are used in order to ascertain the mechanicalprinciples of the material on which structures are built.This allows tunnels to be built without collapsing, bridges

11

and skyscrapers to be built with sturdy foundations, andbuildings to be built that will not settle in clay andmud.[44]

8.3 Hydrology and environmental issues

Main article: Hydrogeology

Geology and geologic principles can be applied to vari-ous environmental problems such as stream restoration,the restoration of brownfields, and the understanding ofthe interaction between natural habitat and the geologicenvironment. Groundwater hydrology, or hydrogeology,is used to locate groundwater,[45] which can often providea ready supply of uncontaminated water and is especiallyimportant in arid regions,[46] and to monitor the spread ofcontaminants in groundwater wells.[45][47]

Geologists also obtain data through stratigraphy,boreholes, core samples, and ice cores. Ice cores[48] andsediment cores[49] are used to for paleoclimate recon-structions, which tell geologists about past and presenttemperature, precipitation, and sea level across the globe.These datasets are our primary source of information onglobal climate change outside of instrumental data.[50]

8.4 Natural hazards

Main article: Natural hazard

Geologists and geophysicists study natural hazards in or-der to enact safe building codes and warning systems thatare used to prevent loss of property and life.[51] Examplesof important natural hazards that are pertinent to geology(as opposed those that are mainly or only pertinent to me-teorology) are:

Rockfall in the Grand Canyon

• Avalanches

• Earthquakes

• Floods

• Landslides and debris flows

• River channel migration and avulsion

• Liquefaction

• Sinkholes

• Subsidence

• Tsunamis

• Volcanoes

9 History of geology

Main articles: History of geology and Timeline of geol-ogyThe study of the physical material of the Earth dates back

William Smith's geologic map of England, Wales, and southernScotland. Completed in 1815, it was the second national-scalegeologic map, and by far the most accurate of its time.[52]

at least to ancient Greece when Theophrastus (372–287BCE) wrote the work Peri Lithon (On Stones). During theRoman period, Pliny the Elder wrote in detail of themanyminerals and metals then in practical use – even correctlynoting the origin of amber.Some modern scholars, such as Fielding H. Garrison, areof the opinion that the origin of the science of geologycan be traced to Persia after the Muslim conquests hadcome to an end.[53] Abu al-Rayhan al-Biruni (973–1048

12 9 HISTORY OF GEOLOGY

CE) was one of the earliest Persian geologists, whoseworks included the earliest writings on the geology of In-dia, hypothesizing that the Indian subcontinent was oncea sea.[54] Drawing fromGreek and Indian scientific litera-ture that were not destroyed by theMuslim conquests, thePersian scholar Ibn Sina (Avicenna, 981–1037) proposeddetailed explanations for the formation of mountains, theorigin of earthquakes, and other topics central to mod-ern geology, which provided an essential foundation forthe later development of the science.[55][56] In China, thepolymath Shen Kuo (1031–1095) formulated a hypothe-sis for the process of land formation: based on his obser-vation of fossil animal shells in a geological stratum in amountain hundreds of miles from the ocean, he inferredthat the land was formed by erosion of the mountains andby deposition of silt.[57]

Nicolas Steno (1638–1686) is credited with the law ofsuperposition, the principle of original horizontality, andthe principle of lateral continuity: three defining princi-ples of stratigraphy.The word geology was first used by Ulisse Aldrovandi in1603,[58] then by Jean-André Deluc in 1778 and intro-duced as a fixed term by Horace-Bénédict de Saussure in1779. The word is derived from the Greek γῆ, gê, mean-ing “earth” and λόγος, logos, meaning “speech”.[59] Butaccording to another source, the word “geology” comesfrom a Norwegian, Mikkel Pedersøn Escholt (1600–1699), who was a priest and scholar. Escholt first usedthe definition in his book titled, Geologica Norvegica(1657).[60]

William Smith (1769–1839) drew some of the first geo-logical maps and began the process of ordering rock strata(layers) by examining the fossils contained in them.[52]

James Hutton is often viewed as the first moderngeologist.[61] In 1785 he presented a paper entitled The-ory of the Earth to the Royal Society of Edinburgh. Inhis paper, he explained his theory that the Earth must bemuch older than had previously been supposed in orderto allow enough time for mountains to be eroded and forsediments to form new rocks at the bottom of the sea,which in turn were raised up to become dry land. Huttonpublished a two-volume version of his ideas in 1795 (Vol.1, Vol. 2).Followers of Hutton were known as Plutonists becausethey believed that some rocks were formed by vulcan-ism, which is the deposition of lava from volcanoes, asopposed to the Neptunists, led by Abraham Werner, whobelieved that all rocks had settled out of a large oceanwhose level gradually dropped over time.The first geological map of the U.S. was produced in1809 by William Maclure.[62][63] In 1807, Maclure com-menced the self-imposed task of making a geological sur-vey of the United States. Almost every state in the Unionwas traversed and mapped by him, the Allegheny Moun-tains being crossed and recrossed some 50 times.[64]The results of his unaided labours were submitted to the

Scotsman James Hutton, father of modern geology

American Philosophical Society in a memoir entitled Ob-servations on the Geology of the United States explanatoryof a Geological Map, and published in the Society’s Trans-actions, together with the nation’s first geological map.[65]This antedates William Smith's geological map of Eng-land by six years, although it was constructed using a dif-ferent classification of rocks.Sir Charles Lyell first published his famous book,Principles of Geology,[66] in 1830. This book, whichinfluenced the thought of Charles Darwin, successfullypromoted the doctrine of uniformitarianism. This the-ory states that slow geological processes have occurredthroughout the Earth’s history and are still occurring to-day. In contrast, catastrophism is the theory that Earth’sfeatures formed in single, catastrophic events and re-mained unchanged thereafter. Though Hutton believedin uniformitarianism, the idea was not widely accepted atthe time.Much of 19th-century geology revolved around the ques-tion of the Earth’s exact age. Estimates varied from afew hundred thousand to billions of years.[67] By the early20th century, radiometric dating allowed the Earth’s ageto be estimated at two billion years. The awareness ofthis vast amount of time opened the door to new theoriesabout the processes that shaped the planet.Some of the most significant advances in 20th-centurygeology have been the development of the theory of platetectonics in the 1960s and the refinement of estimates ofthe planet’s age. Plate tectonics theory arose from two

13

separate geological observations: seafloor spreading andcontinental drift. The theory revolutionized the Earth sci-ences. Today the Earth is known to be approximately 4.5billion years old.[68]

10 Fields or related disciplines• Earth science

• Earth system science

• Economic geology

• Mining geology• Petroleum geology

• Engineering geology

• Environmental geology

• Environmental science

• Geoarchaeology

• Geochemistry

• Biogeochemistry• Isotope geochemistry

• Geochronology

• Geodetics

• Geography

• Geological modelling

• Geometallurgy

• Geomicrobiology

• Geomorphology

• Geomythology

• Geophysics

• Glaciology

• Historical geology

• Hydrogeology

• Meteorology

• Mineralogy

• Oceanography

• Marine geology

• Paleoclimatology

• Paleontology

• Micropaleontology

• Palynology

• Petrology

• Petrophysics

• Physical geography

• Plate tectonics

• Sedimentology

• Seismology

• Soil science

• Pedology (soil study)

• Speleology

• Stratigraphy

• Biostratigraphy• Chronostratigraphy• Lithostratigraphy

• Structural geology

• Systems geology

• Volcanology

11 Regional geology

Main article: Regional geology

12 See also

• Geologic modeling

• Geoprofessions

• Glossary of geology terms

• International Union of Geological Sciences (IUGS)

• List of geology topics

• Timeline of geology

13 References[1] “geology”. Online Etymology Dictionary.

[2] γῆ. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project

[3] New Hampshire Geological Survey, Geologic maps, sec-tion Surficial Geologic Maps

14 13 REFERENCES

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[49] Colman, S.M.; Jones, G.A.; Forester, R.M.; Fos-ter, D.S. (1990). “Holocene paleoclimatic evidenceand sedimentation rates from a core in southwesternLake Michigan”. Journal of Paleolimnology. 4 (3).doi:10.1007/BF00239699.

[50] Jones, P. D.; Mann, M. E. (6 May 2004). “Climateover past millennia” (PDF). Reviews of Geo-physics. 42 (2). Bibcode:2004RvGeo..42.2002J.doi:10.1029/2003RG000143.

[51] USGS Natural Hazards Gateway. usgs.gov

[52] Winchester, Simon (2002). The map that changed theworld: William Smith and the birth of modern geology.New York, NY: Perennial. ISBN 0-06-093180-9.

[53] “The Saracens themselves were the originators not only ofalgebra, chemistry, and geology, but of many of the so-called improvements or refinements of civilization, suchas street lamps, window-panes, fireworks, stringed instru-ments, cultivated fruits, perfumes, spices, etc.” (FieldingH. Garrison, An introduction to the history of medicine,W.B. Saunders, 1921, p. 116)

16 14 EXTERNAL LINKS

[54] Asimov, M. S.; Bosworth, Clifford Edmund (eds.). TheAge of Achievement: A.D. 750 to the End of the FifteenthCentury : The Achievements. History of civilizations ofCentral Asia. pp. 211–214. ISBN 978-92-3-102719-2.

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[57] Needham, Joseph (1986). Science and Civilization inChina: Volume 3, Mathematics and the Sciences of theHeavens and the Earth. Taipei: Caves Books, Ltd. pp.603–604.

[58] Vai, Gian Battista; Cavazza, William (2003). Four cen-turies of the word geology: Ulisse Aldrovandi 1603 inBologna. Minerva. ISBN 978-88-7381-056-8.

[59] Winchester, Simon (2001). The Map that Changed theWorld. HarperCollins Publishers. p. 25. ISBN 0-06-093180-9.

[60] Kermit H., (2003) Niels Stensen, 1638–1686: the scientistwho was beatified. Gracewing Publishing. p. 127.

[61] James Hutton: The Founder of Modern Geology Ameri-can Museum of Natural History

[62] Maclure, William (1817). Observations on the Geology ofthe United States of America: With Some Remarks on theEffect Produced on the Nature and Fertility of Soils, by theDecomposition of the Different Classes of Rocks; and anApplication to the Fertility of Every State in the Union, inReference to the Accompanying Geological Map ... author.

[63] Observations on the Geology of the United States(Maclure). David Rumsey map collection

[64] Greene, J.C.; Burke, J.G. (1978). “The Science of Min-erals in the Age of Jefferson”. Transactions of the Ameri-can Philosophical Society, New Series. 68 (4): 1–113 (39).doi:10.2307/1006294.

[65] Maclure’s 1809 Geological Map. davidrumsey.com

[66] Lyell, Charles (1991). Principles of geology. Chicago:University of Chicago Press. ISBN 978-0-226-49797-6.

[67] England, Philip; Molnar, Peter; Richter, Frank (2007).“John Perry’s neglected critique of Kelvin’s age for theEarth: A missed opportunity in geodynamics”. GSA To-day. 17: 4. doi:10.1130/GSAT01701A.1.

[68] Dalrymple, G.B. (1991). The Age of the Earth. Califor-nia: Stanford University Press. ISBN 0-8047-1569-6.

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15 Text and image sources, contributors, and licenses

15.1 Text

• Geology Source: https://en.wikipedia.org/wiki/Geology?oldid=737580047 Contributors: AxelBoldt, The Epopt, Derek Ross, Mav, Berek,Koyaanis Qatsi, Drj, Andre Engels, Youssefsan, Christian List, Fubar Obfusco, Chd~enwiki, DavidLevinson, Ben-Zin~enwiki, Anthere,Graham, Heron, Camembert, Olivier, Edward, Bob frasier, Infrogmation, Michael Hardy, Zocky, Chris-martin, GUllman, Liftarn, Menchi,Alfio, Mdebets, Ahoerstemeier, Cferrero, Whkoh, Mykdavies, Cimon Avaro, BRG, Conti, Nikola Smolenski, Crusadeonilliteracy, Dino,Tpbradbury, Big Bob the Finder, Maximus Rex, Phoebe, Samsara, Shizhao, Frazzydee, Pollinator, PuzzletChung, Robbot, Mirv, Hen-rygb, Academic Challenger, Flauto Dolce, Sunray, Hadal, Fuelbottle, Seth Ilys, Mythrandir, Stirling Newberry, Ancheta Wis, Giftlite,Graeme Bartlett, MPF, Isam, Nichalp, Nadavspi, SwissMoomin, MSGJ, Everyking, Maha ts, Niteowlneils, DO'Neil, Cam, Wmahan,Pucicu, Andycjp, Slowking Man, Pcarbonn, Antandrus, The Singing Badger, RockBandit, Karol Langner, APH, Roisterer, Dragonfly-Sixtyseven, Maximaximax, Zfr, Karl-Henner, Sam Hocevar, Thparkth, Canterbury Tail, Freakofnurture, Stepp-Wulf, DanielCD, RichFarmbrough, Guanabot, Cacycle, Hidaspal, Vsmith, Izogi, Mani1, Bender235, RJHall, CanisRufus, Ben Webber, El C, PhilHibbs, Si-etse Snel, Art LaPella, RoyBoy, DavidRader, Bobo192, Shenme, Viriditas, Bryankennedy, Maurreen, WoKrKmFK3lwz8BKvaB94, Manvyi, Mdd, Jumbuck, RedWinged Duck, Danski14, Alansohn, Gary, JYolkowski, Paleorthid, Plumbago, Lephafta~enwiki, TaintedMustard,Knowledge Seeker, Andy F, Evil Monkey, Tsuba~enwiki, Stemonitis, Bacteria, Mel Etitis, Woohookitty, GVOLTT, Camw, MiaowMiaow,WadeSimMiser, Duncan.france, Eleassar777, Tactel, GeorgeShaw, Pufferfish101, Sengkang, Kralizec!, Phlebas, Kmontgom, Dysepsion,Ashmoo, Graham87, Marskell, Sjö, Drbogdan, Sjakkalle, Rjwilmsi, Quiddity, BlueMoonlet, Daniel Collins, Kalogeropoulos, The wub,MarnetteD, Yamamoto Ichiro, Dracontes, JanSuchy, Vuong Ngan Ha, FayssalF, Titoxd, FlaBot, Windchaser, RexNL, Gurch, Akhen-aten0, Chobot, DVdm, Gwernol, ErilLanin, The Rambling Man, Wavelength, RussBot, Fabartus, Pigman, Madraschap, Van der Hoorn,Stephenb, TheGrappler, MosheA, Wiki alf, UCaetano, Ragesoss, Bobbo, Moe Epsilon, BOT-Superzerocool, Stefeyboy, Martinwilke1980,Closedmouth, Spacebirdy, Glynwj, Arthur Rubin, BorgQueen, Jay Gregg, JLaTondre, ArielGold, Curpsbot-unicodify, Mawa, Smack-Bot, Enlil Ninlil, KnowledgeOfSelf, Hydrogen Iodide, Unyoyega, Goldfishbutt, KocjoBot~enwiki, Jagged 85, Clpo13, Hardyplants, Half-Shadow, Yamaguchi , Gilliam, Ohnoitsjamie, Saros136, Chris the speller, Keegan, MalafayaBot, SchfiftyThree, RayAYang, Neo-Jay,DHN-bot~enwiki, Can't sleep, clown will eat me, Jahiegel, Abyssal, Bekom, Berland, Valich, Addshore, SundarBot, Decltype, Rolina-tor, Apologist7, Paul H., Mini-Geek, Gump Stump, Mwtoews, RichAromas, Ultraexactzz, RJBurkhart, -Marcus-, Jóna Þórunn, Nov ial-iste, SashatoBot, ArglebargleIV, Rory096, J 1982, Kipala, Lazylaces, Robert Stevens, Bjankuloski06en~enwiki, Peterlewis, IronGargoyle,Melody Concerto, Geologyguy, Qyd, Ryulong, Dr.K., Citicat, Jose77, Rickington, Levineps, Pegasus1138, Sam Li, Adambiswanger1,Tawkerbot2, MarylandArtLover, Poolkris, Tanthalas39, Errebe, Woudloper, TheMightyOrb, Some P. 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18 15 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

15.2 Images• File:Active_Margin.svg Source: https://upload.wikimedia.org/wikipedia/commons/2/29/Active_Margin.svg License: Public domainContributors: http://upload.wikimedia.org/wikipedia/en/b/b7/Oceanic-continental_convergence_Fig21oceancont.svg Original artist: en:User:Booyabazooka

• File:Antecline_(PSF).png Source: https://upload.wikimedia.org/wikipedia/commons/e/e8/Antecline_%28PSF%29.png License: Publicdomain Contributors: Archives of Pearson Scott Foresman, donated to the Wikimedia Foundation Original artist: Pearson Scott Foresman

• File:Brunton.JPG Source: https://upload.wikimedia.org/wikipedia/commons/e/e1/Brunton.JPG License: CC BY-SA 3.0 Contributors: I(Qfl247 (talk)) created this work entirely by myself. (Original uploaded on en.wikipedia) Original artist: Qfl247 (talk) (Transferred byCitypeek/Original uploaded by Qfl247)

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• File:Earth_Western_Hemisphere.jpg Source: https://upload.wikimedia.org/wikipedia/commons/7/7b/Earth_Western_Hemisphere.jpg License: Public domain Contributors: http://visibleearth.nasa.gov/view.php?id=57723 Original artist:

• Reto Stöckli (land surface, shallow water, clouds)• Robert Simmon (enhancements: ocean color, compositing, 3D globes, animation)• Data and technical support: MODIS Land Group; MODIS Science Data Support Team; MODIS Atmosphere Group; MODIS Ocean

Group• Additional data: USGS EROS Data Center (topography); USGS Terrestrial Remote Sensing Flagstaff Field Center (Antarctica);

Defense Meteorological Satellite Program (city lights).

• File:Earthquake_wave_paths.svg Source: https://upload.wikimedia.org/wikipedia/commons/c/c8/Earthquake_wave_paths.svg License:Public domain Contributors: http://pubs.usgs.gov/gip/interior/fig2.gif ; original upload in english wikipedia, 15 April 2005 by SEWilcoOriginal artist: SEWilco

• File:Farallon_Plate.jpg Source: https://upload.wikimedia.org/wikipedia/commons/a/a0/Farallon_Plate.jpg License: Public domain Con-tributors: http://svs.gsfc.nasa.gov/vis/a000000/a002400/a002410/ Original artist: NASA

• File:Fault_types.png Source: https://upload.wikimedia.org/wikipedia/commons/0/0e/Fault_types.png License: Public domain Contribu-tors: earthquake.usgs.gov Original artist: USGS

• File:Folder_Hexagonal_Icon.svg Source: https://upload.wikimedia.org/wikipedia/en/4/48/Folder_Hexagonal_Icon.svg License: Cc-by-sa-3.0 Contributors: ? Original artist: ?

• File:GCRockfall.JPG Source: https://upload.wikimedia.org/wikipedia/commons/9/96/GCRockfall.JPG License: CC BY-SA 3.0 Con-tributors: I (Qfl247 (talk)) created this work entirely by myself. (Original uploaded on en.wikipedia) Original artist: Qfl247 (talk) (Trans-ferred by Citypeek/Original uploaded by Qfl247)

• File:Geologic_Clock_with_events_and_periods.svg Source: https://upload.wikimedia.org/wikipedia/commons/7/77/Geologic_Clock_with_events_and_periods.svg License: Public domain Contributors: File:Geologic_clock.jpg Original artist:

• Woudloper• File:Geological_map_Britain_William_Smith_1815.jpg Source: https://upload.wikimedia.org/wikipedia/commons/9/98/Geological_

map_Britain_William_Smith_1815.jpg License: Public domain Contributors: LiveScience Image Gallery Scan by the Library Foundation,Buffalo and Erie County Public Library. Image now at http://www.livescience.com/449-map-changed-world.html Original artist: WilliamSmith (1769-1839)

• File:Hutton_James_portrait_Raeburn.jpg Source: https://upload.wikimedia.org/wikipedia/commons/0/0e/Hutton_James_portrait_Raeburn.jpg License: Public domain Contributors: Scottish National Portrait Gallery Original artist: Henry Raeburn

• File:Jordens_inre-numbers.svg Source: https://upload.wikimedia.org/wikipedia/commons/0/02/Jordens_inre-numbers.svg Li-cense: CC-BY-SA-3.0 Contributors: <a href='//commons.wikimedia.org/wiki/File:Jordens_inre.jpg' class='image'><img alt='Jordensinre.jpg' src='https://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Jordens_inre.jpg/150px-Jordens_inre.jpg' width='150'height='156' srcset='https://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Jordens_inre.jpg/225px-Jordens_inre.jpg 1.5x,https://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Jordens_inre.jpg/300px-Jordens_inre.jpg 2x' data-file-width='800'data-file-height='833' /></a> Original artist: Original Mats Halldin Vectorization: Chabacano

• File:Kittatinny_Mountain_Cross_Section.jpg Source: https://upload.wikimedia.org/wikipedia/commons/0/06/Kittatinny_Mountain_Cross_Section.jpg License: Public domain Contributors: United States Geological Survey - 32. Delaware Water Gap National RecreationArea: http://3dparks.wr.usgs.gov/nyc/parks/loc32.htm Original artist: United States Geological Survey

• File:Leica_DMRX.jpg Source: https://upload.wikimedia.org/wikipedia/commons/d/d2/Leica_DMRX.jpg License: Public domain Con-tributors: http://energy.er.usgs.gov/images/organic_petrology/leica_dmrx.jpg Original artist: United States Geological Survey

• File:Mars_Viking_21i093.png Source: https://upload.wikimedia.org/wikipedia/commons/7/7a/Mars_Viking_21i093.png License: Pub-lic domain Contributors: Own work based on images in the NASA Viking image archiveOriginal artist: “Roel van der Hoorn (Van der Hoorn)"

• File:Mudlogging.JPG Source: https://upload.wikimedia.org/wikipedia/commons/8/87/Mudlogging.JPG License: CC BY-SA 3.0 Con-tributors: (Original uploaded on en.wikipedia) Original artist: Qfl247 (talk) (Transferred by Citypeek/Original uploaded by Qfl247)

• File:Orogenic_wedge.jpg Source: https://upload.wikimedia.org/wikipedia/commons/8/87/Orogenic_wedge.jpg License: Public domainContributors: U.S. Geological Survey Professional Paper 1624 - Online version 1.0: Migration of the Acadian Orogen and Foreland BasinAcross the Northern Appalachians of Maine and Adjacent Areas: http://pubs.usgs.gov/pp/pp1624/ Original artist: Dwight C. Bradley,Robert D. Tucker, Daniel R. Lux, Anita G. Harris, and D. Colin McGregor

15.3 Content license 19

• File:PDA_Mapping.jpg Source: https://upload.wikimedia.org/wikipedia/commons/a/a0/PDA_Mapping.jpg License: Public domainContributors: Advanced GIS-Based Field Mapping Techniques for Multi-Disciplinary Research: http://geography.wr.usgs.gov/science/mapping.html Original artist: US Geological Survey: Nathan Wood

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• File:Rock_cycle.gif Source: https://upload.wikimedia.org/wikipedia/commons/6/6b/Rock_cycle.gif License: CC BY 3.0 Contributors:• Kreislauf_der_gesteine.png Original artist: Kreislauf_der_gesteine.png: The original uploader was Chd at German Wikipedia• File:SEUtahStrat.JPG Source: https://upload.wikimedia.org/wikipedia/commons/6/66/SEUtahStrat.JPG License: CC BY-SA 3.0 Con-tributors: I (Qfl247 (talk)) created this work entirely by myself. (Original uploaded on en.wikipedia) Original artist: Qfl247 (talk) (Trans-ferred by Citypeek/Original uploaded by Qfl247)

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• File:The_Earth_seen_from_Apollo_17_with_transparent_background.png Source: https://upload.wikimedia.org/wikipedia/commons/4/43/The_Earth_seen_from_Apollo_17_with_transparent_background.png License: Public domain Contributors:http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/a17_h_148_22727.html Original artist: NASA

• File:USGS_1950s_mapping_field_camp.jpg Source: https://upload.wikimedia.org/wikipedia/commons/f/f3/USGS_1950s_mapping_field_camp.jpg License: Public domain Contributors: USGS Historical Photographs: Mapping Field Camp: http://online.wr.usgs.gov/outreach/historicPhotos/enlarged/mapping_camp_1950.html Original artist: US Geological Survey

• File:Volcanosed.svg Source: https://upload.wikimedia.org/wikipedia/commons/9/95/Volcanosed.svg License: Public domain Contribu-tors:

• United States Geological Survey - Geologic History of Lake Mead National Recreation Area: http://3dparks.wr.usgs.gov/2005/lakemead/html/lame_history.htm Original artist:

• derivative work: Hardwigg• File:Wikibooks-logo-en-noslogan.svg Source: https://upload.wikimedia.org/wikipedia/commons/d/df/Wikibooks-logo-en-noslogan.

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