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 are naturally occurringelements andcompounds 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:[5]
Color: Minerals are grouped by their color. Mostly diagnostic but impurities can change a mineral's color.
Streak: Performed by scratching the sample on aporcelain 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 orcleavage, the former being breakage of uneven surfaces, and the latter a breakage along closely spaced parallel planes.
Luster: Quality of light reflected from the surface of a mineral. Examples are metallic, pearly, waxy, dull.
A rock is any naturally occurring solid mass or aggregate of minerals ormineraloids. 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, andmetamorphic. Therock cycleillustrates the relationships among them (see diagram).
When a rocksolidifies orcrystallizes from melt (magma orlava), it is anigneous rock. This rock can beweathered anderoded, thenredeposited andlithified 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).[6] Igneous and sedimentary rocks can then be turned intometamorphic rocks by heat and pressure that change itsmineral content, resulting in acharacteristic 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 astexture andfabric.
Magma is the original unlithified source of alligneous rocks. The active flow of molten rock is closely studied involcanology, andigneous petrology aims to determine the history ofigneous rocks from their original molten source to their final crystallization.
In the 1960s, it was discovered that the Earth'slithosphere, which includes thecrust and rigid uppermost portion of theupper mantle, is separated intotectonic plates that move across theplastically deforming, solid, upper mantle, which is called theasthenosphere. This theory is supported by several types of observations, including seafloor spreading[9][10] and the global distribution of mountain terrain and seismicity.
There is an intimate coupling between the movement of the plates on the surface and theconvection of the mantle (that is, theheat transfer caused by the slow movement of ductile mantle rock). Thus, oceanic parts of plates and the adjoining mantleconvection currents always move in the same direction – because the oceanic lithosphere is actually the rigid upper thermalboundary layer of the convecting mantle. This coupling between rigid plates moving on the surface of the Earth and the convectingmantle is called platetectonics.
The development of plate tectonics has provided a physical basis for many observations of the solidEarth. Long linear regions of geological features are explained as plate boundaries:[11]
Plate tectonics has provided a mechanism forAlfred Wegener's theory ofcontinental drift,[12] in which thecontinents 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.
TheEarth's layered structure. (1) inner core; (2) outer core; (3) lower mantle; (4) upper mantle; (5) lithosphere; (6) crust (uppermost part of the lithosphere)Earth layered structure. Typical wave paths from earthquakes like these gave early seismologists insights into the layered structure of the Earth.
Seismologists can use the arrival times ofseismic waves to image the interior of the Earth. Early advances in this field showed the existence of a liquidouter core (whereshear waves were not able to propagate) and a dense solidinner core. These advances led to the development of a layered model of the Earth, with alithosphere (including crust) on top, themantle below (separated within itself byseismic 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 aCT 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.
The geological time scale encompasses the history of the Earth.[13] It is bracketed at the earliest by the dates of the firstSolar System material at 4.567Ga[14] (or 4.567 billion years ago) and the formation of the Earth at4.54 Ga[15][16](4.54 billion years), which is the beginning of theHadean eon – a division of geological time. At the later end of the scale, it is marked by the present day (in theHolocene epoch).
The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.
Methods forrelative dating were developed when geology first emerged as anatural science. Geologists still use the following principles today as a means to provide information about geological history and the timing of geological events.
Theprinciple 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.[17] A fundamental principle of geology advanced by the 18th-century Scottish physician and geologistJames 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."[18]
Theprinciple of cross-cutting relationships pertains to the formation offaults 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 anormal fault or athrust fault.[19]
Theprinciple of inclusions and components states that, with sedimentary rocks, if inclusions (orclasts) 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 whenxenoliths are found. These foreign bodies are picked up asmagma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock that contains them.
Theprinciple 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 (althoughcross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).[19]
Theprinciple of superposition states that a sedimentary rock layer in atectonically 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.[19]
Theprinciple 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 ofCharles Darwin's theory ofevolution, 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.[20]
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.[21]
At the beginning of the 20th century, advancement in geological science was facilitated by the ability to obtain accurate absolute dates to geological events usingradioactive isotopes 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 assignabsolute 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 ratios of radioactive elements are measured in minerals that give the amount of time that has passed since a rock passed through its particularclosure temperature, the point at which different radiometric isotopes stop diffusing into and out of thecrystal lattice.[22][23] These are used ingeochronologic andthermochronologic studies. Common methods includeuranium–lead dating,potassium–argon dating,argon–argon dating anduranium–thorium dating. These methods are used for a variety of applications. Dating oflava andvolcanic 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 ofpluton emplacement.Thermochemical techniques can be used to determine temperature profiles within the crust, the uplift of mountain ranges, and paleo-topography.
Fractionation of thelanthanide series elements is used to compute ages since rocks were removed from the mantle.
An originally horizontal sequence of sedimentary rocks (in shades of tan) are affected byigneous activity. Deep below the surface is amagma chamber and large associated igneous bodies. The magma chamber feeds thevolcano, and sends offshoots ofmagma that will later crystallize into dikes and sills. Magma also advances upwards to formintrusive igneous bodies. The diagram illustrates both acinder cone volcano, which releases ash, and acomposite volcano, which releases both lava and ash. An illustration of the three types of faults. A. Strike-slip faults occur when rock units slide past one another. B. Normal faults occur when rocks are undergoing horizontal extension. C. Reverse (or thrust) faults occur when rocks are undergoing horizontal shortening.TheSan Andreas Fault inCalifornia
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 theoverlying rock. Deposition can occur when sediments settle onto the surface of the Earth and laterlithify into sedimentary rock, or when asvolcanic material such asvolcanic ash orlava flows blanket the surface.Igneous intrusions such asbatholiths,laccoliths,dikes, andsills, push upwards into the overlying rock, and crystallize as they intrude.
After the initial sequence of rocks has been deposited, the rock units can bedeformed and/ormetamorphosed. Deformation typically occurs as a result of horizontal shortening,horizontal extension, or side-to-side (strike-slip) motion. These structural regimes broadly relate toconvergent boundaries,divergent boundaries, and transform boundaries, respectively, between tectonic plates.
When rock units are placed under horizontalcompression, they shorten and become thicker. Because rock units, other than muds,do not significantly change in volume, this is accomplished in two primary ways: throughfaulting andfolding. In the shallow crust, wherebrittle 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 theprinciple 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 "antiforms", or where it buckles downwards, creating "synforms". If the tops of the rock units within the folds remain pointing upwards, they are calledanticlines andsynclines, 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.
Even higher pressures and temperatures during horizontal shortening can cause both folding andmetamorphism of the rocks. This metamorphism causes changes in themineral composition of the rocks; creates afoliation, or planar surface, that is related to mineral growth under stress. This can remove signs of the original textures of the rocks, such asbedding in sedimentary rocks, flow features oflavas, and crystal patterns incrystalline rocks.
Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished throughnormal faulting 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 theMaria Fold and Thrust Belt, the entire sedimentary sequence of theGrand 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 asboudins, after the French word for "sausage" because of their visual similarity.
Where rock units slide past one another,strike-slip faults develop in shallow regions, and becomeshear zones at deeper depths where the rocks deform ductilely.
Geologicalcross section ofKittatinny 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 createaccommodation 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.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 ofdike swarms, such as those that are observable across the Canadian shield, or rings of dikes around thelava 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. TheHawaiian Islands, for example, consist almost entirely of layeredbasaltic lava flows. The sedimentary sequences of the mid-continental United States and theGrand Canyon in the southwestern United States contain almost-undeformed stacks of sedimentary rocks that have remained in place sinceCambrian 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 theAcasta gneiss of theSlave craton in northwesternCanada, theoldest 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 thegeological history of an area.
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 topetrology (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,rivers,landscapes, andglaciers; investigate past and current life andbiogeochemical pathways, and usegeophysical methods to investigate the subsurface. Sub-specialities of geology may distinguishendogenous andexogenous geology.[24]
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 throughoptical microscopy and by using anelectron microprobe. In anoptical mineralogy analysis, petrologists analyzethin sections of rock samples using apetrographic microscope, where the minerals can be identified through their different properties in plane-polarized and cross-polarized light, including theirbirefringence,pleochroism,twinning, and interference properties with aconoscopic lens.[31] In the electron microprobe, individual locations are analyzed for their exact chemical compositions and variation in composition within individual crystals.[32]Stable[33] andradioactive isotope[34] studies provide insight into thegeochemical evolution of rock units.
Petrologists can also usefluid inclusion data[35] and perform high temperature and pressure physical experiments[36] to understand the temperatures and pressures at which different mineral phases appear, and how they change through igneous[37] and metamorphic processes. This research can be extrapolated to the field to understand metamorphic processes and the conditions of crystallization of igneous rocks.[38] This work can also help to explain processes that occur within the Earth, such assubduction andmagma chamber evolution.[39]
A diagram of an orogenic wedge. The wedge grows through faulting in the interior and along the main basal fault, called thedécollement. It builds its shape into acritical 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 thefabric 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 performanalog 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 ontostereonets. 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 involvingorogenic wedges, which are zones in whichmountains are built alongconvergent tectonic plate boundaries.[40] 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 acritically tapered (all angles remain the same) orogenic wedge.[41] 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.[42] 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.[43]
In the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those fromdrill cores.[44] Stratigraphers also analyze data from geophysical surveys that show the locations of stratigraphic units in the subsurface.[45] Geophysical data andwell logs can be combined to produce a better view of the subsurface, and stratigraphers often use computer programs to do this in three dimensions.[46] Stratigraphers can then use these data to reconstruct ancient processes occurring on the surface of the Earth,[47] interpret past environments, and locate areas for water, coal, and hydrocarbon extraction.
In the laboratory,biostratigraphers analyze rock samples from outcrop and drill cores for the fossils found in them.[44] These fossils help scientists to date the core and to understand thedepositional 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.[48]Magnetic stratigraphers look for signs of magnetic reversals in igneous rock units within the drill cores.[44] Other scientists perform stable-isotope studies on the rocks to gain information about past climate.[44]
With the advent ofspace exploration in the twentieth century, geologists have begun to look at other planetary bodies in the same ways that have been developed to study theEarth. This new field of study is calledplanetary geology (sometimes known as astrogeology) and relies on known geological principles to study other bodies of the solar system. This is a major aspect ofplanetary science, and largely focuses on theterrestrial planets,icy moons,asteroids,comets, andmeteorites. However, some planetary geophysicists study thegiant planets andexoplanets.[49]
Although the Greek-language-origin prefixgeo 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 "thegeology of Mars" and "Lunar geology". Specialized terms such asselenology (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 thePhoenix lander, which analyzedMartian polar soil for water, chemical, and mineralogical constituents related to biological processes.
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'snatural resources, such as petroleum and coal, as well as mineral resources, which include metals such as iron, copper, and uranium.
Petroleum geologists study the locations of the subsurface of the Earth that can contain extractable hydrocarbons, especiallypetroleum andnatural gas. Because many of these reservoirs are found insedimentary basins,[50] 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 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 fromgeological engineering, particularly in North America.
A child drinks water from awell built as part of a hydrogeological humanitarian project inKenya.
In the field ofcivil 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.[51]
Geology and geological principles can be applied to various environmental problems such asstream restoration, the restoration ofbrownfields, and the understanding of the interaction betweennatural habitat and the geological environment. Groundwater hydrology, orhydrogeology, is used to locate groundwater,[52] which can often provide a ready supply of uncontaminated water and is especially important in arid regions,[53] and to monitor the spread of contaminants in groundwater wells.[52][54]
Geologists also obtain data through stratigraphy,boreholes,core samples, andice cores. Ice cores[55] and sediment cores[56] are used for paleoclimate reconstructions, which tell geologists about past and present temperature, precipitation, andsea level across the globe. These datasets are our primary source of information onglobal climate change outside of instrumental data.[57]
Geologists and geophysicists study natural hazards in order to enact safebuilding codes and warning systems that are used to prevent loss of property and life.[58] Examples of important natural hazards that are pertinent to geology (as opposed those that are mainly or only pertinent to meteorology) are:
The study of the physical material of the Earth dates back at least toancient Greece whenTheophrastus (372–287 BCE) wrote the workPeri Lithon (On Stones). During theRoman period,Pliny the Elder wrote in detail of the many minerals and metals, then in practical use – even correctly noting the origin ofamber. Additionally, in the 4th century BCEAristotle 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.[60][61]
Abu al-Rayhan al-Biruni (973–1048 CE) was one of the earliestPersian geologists, whose works included the earliest writings on thegeology of India, hypothesizing that theIndian subcontinent was once a sea.[62] Drawing from Greek and Indian scientific literature that were not destroyed by theMuslim conquests, the Persian scholarIbn 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.[63][64] In China, thepolymathShen Kuo (1031–1095) formulated a hypothesis for the process of land formation: based on his observation of fossil animal shells in a geologicalstratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by the erosion of the mountains and bydeposition ofsilt.[65]
Georgius Agricola (1494–1555) published his groundbreaking workDe Natura Fossilium in 1546 and is seen as the founder of geology as a scientific discipline.[66]
The wordgeology was first used byUlisse Aldrovandi in 1603,[67][68] then byJean-André Deluc in 1778[69] and introduced as a fixed term byHorace-Bénédict de Saussure in 1779.[70][71] The word is derived from theGreek γῆ,gê, meaning "earth" and λόγος,logos, meaning "speech".[72] 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).[73][74]
William Smith (1769–1839) drew some of the first geological maps and began the process of orderingrock strata (layers) by examining the fossils contained in them.[59]
In 1763,Mikhail Lomonosov published his treatiseOn the Strata of Earth.[75] 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.[76]
James Hutton (1726–1797) is often viewed as the first modern geologist.[77] In 1785 he presented a paper entitledTheory of the Earth to theRoyal 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 forsediments 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.[78]
Followers of Hutton were known asPlutonists because they believed that some rocks were formed byvulcanism, which is the deposition of lava from volcanoes, as opposed to theNeptunists, led byAbraham Werner, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.
The firstgeological map of the U.S. was produced in 1809 byWilliam Maclure.[79] 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, theAllegheny Mountains being crossed and recrossed some 50 times.[80] The results of his unaided labours were submitted to theAmerican Philosophical Society in a memoir entitledObservations on the Geology of the United States explanatory of a Geological Map, and published in theSociety's Transactions, together with the nation's first geological map.[81] This antedatesWilliam 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,[82] in 1830. This book, which influenced the thought ofCharles Darwin, successfully promoted the doctrine ofuniformitarianism. This theory states that slow geological processes have occurred throughout theEarth'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 theEarth's exact age. Estimates varied from a few hundred thousand to billions of years.[83] 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 ofplate 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 andcontinental drift. The theory revolutionized theEarth sciences. Today the Earth is known to be approximately 4.5 billion years old.[16]
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^Condomines, M.; Tanguy, J.; Michaud, V. (1995). "Magma dynamics at Mt Etna: Constraints from U-Th-Ra-Pb radioactive disequilibria and Sr isotopes in historical lavas".Earth and Planetary Science Letters.132 (1):25–41.Bibcode:1995E&PSL.132...25C.doi:10.1016/0012-821X(95)00052-E.
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^Spear, Frank S. (1995).Metamorphic phase equilibria and pressure-temperature-time paths. Washington, DC: Mineralogical Soc. of America.ISBN978-0-939950-34-8.
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^Bally, A. W., ed. (1987).Atlas of seismic stratigraphy. Tulsa, Oklahoma: American Association of Petroleum Geologists.ISBN978-0-89181-033-9.
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^Toulmin, S., and Goodfield, J. (1965)The Ancestry of science: The Discovery of Time, Hutchinson & Company, London, England, p. 64.
^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).From p. 81:Archived 2017-02-16 at theWayback Machine " …& anco la Giologia, ovvero de Fossilibus; … " ( … and likewise geology, or [the study] of things dug from the earth; … )
^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.From p. 4:Archived 2018-11-22 at theWayback Machine"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]; … )From p. 5:Archived 2018-11-22 at theWayback Machine"Je vis que je faisais un Traité, et non une equisse deGéologie." (I see that I wrote a treatise, and not a sketch of geology.)From the footnote on p. 7:Archived 2018-11-22 at theWayback Machine"Je répète ici, ce que j'avois dit dans ma premièrePréface, sur la substitution de motCosmologie à celui deGéologie, quoiqu'il ne s'agisse pas de l'Univers, mais seulement de laTerre: … " (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.]
^Saussure, Horace-Bénédict de,Voyages dans les Alpes, … (Neuchatel, (Switzerland): Samuel Fauche, 1779).From pp. i–ii:Archived 2017-02-06 at theWayback Machine"La science qui rassemble les faits, qui seuls peuvent servir de base à la Théorie de la Terre ou à laGéologie, 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; … )
^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),p. 76.
Geikie, Archibald,The Founders of Geology, 2nd ed. (London, England: Macmillan and Company, 1905),p. 186..Archived 2017-02-16 at theWayback Machine.
Reprinted in English as: Escholt, Michel Pedersøn with Daniel Collins, trans.,Geologia Norvegica ….Archived 2017-02-16 at theWayback Machine. (London, England: S. Thomson, 1663).
^Lomonosov, Mikhail (2012).On the Strata of the Earth. Translation and commentary by S.M. Rowland and S. Korolev. The Geological Society of America, Special Paper 485.ISBN978-0-8137-2485-0.Archived from the original on 2021-06-24. Retrieved2021-06-19.
^Vernadsky, V. (1911). Pamyati M.V. Lomonosova. Zaprosy zhizni, 5: 257–262 (in Russian) [In memory of M.V. Lomonosov].
^Greene, J. C.; Burke, J. G. (1978). "The Science of Minerals in the Age of Jefferson".Transactions of the American Philosophical Society. New Series.68 (4): 1–113 [39].doi:10.2307/1006294.JSTOR1006294.
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