Common methods include uranium-lead dating , potassium-argon dating , argon-argon dating and uranium-thorium dating. These methods are used for a variety of applications. Dating of lava and volcanic ash layers found within a stratigraphic sequence can provide absolute age data for sedimentary rock units that do not contain radioactive isotopes and calibrate relative dating techniques. These methods can also be used to determine ages of pluton emplacement. Thermochemical techniques can be used to determine temperature profiles within the crust, the uplift of mountain ranges, and paleotopography.
Fractionation of the lanthanide series elements is used to compute ages since rocks were removed from the mantle. Other methods are used for more recent events. Dendrochronology can also be used for the dating of landscapes. Radiocarbon dating is used for geologically young materials containing organic carbon.
The geology of an area changes through time as rock units are deposited and inserted, and deformational processes change their shapes and locations. Rock units are first emplaced either by deposition onto the surface or intrusion into the overlying rock.
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Deposition can occur when sediments settle onto the surface of the Earth and later lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows blanket the surface. Igneous intrusions such as batholiths , laccoliths , dikes , and sills , push upwards into the overlying rock, and crystallize as they intrude. Deformation typically occurs as a result of horizontal shortening, horizontal extension , or side-to-side strike-slip motion.
These structural regimes broadly relate to convergent boundaries , divergent boundaries , and transform boundaries, respectively, between tectonic plates. When rock units are placed under horizontal compression , they shorten and become thicker. Because rock units, other than muds, do not significantly change in volume , this is accomplished in two primary ways: through faulting and folding. In the shallow crust, where brittle deformation can occur, thrust faults form, which causes deeper rock to move on top of shallower rock.
Because deeper rock is often older, as noted by the principle of superposition , this can result in older rocks moving on top of younger ones. Movement along faults can result in folding, either because the faults are not planar or because rock layers are dragged along, forming drag folds as slip occurs along the fault. Deeper in the Earth, rocks behave plastically and fold instead of faulting. These folds can either be those where the material in the center of the fold buckles upwards, creating " antiforms ", or where it buckles downwards, creating " synforms ".
If the tops of the rock units within the folds remain pointing upwards, they are called anticlines and synclines , 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 and metamorphism of the rocks.
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This metamorphism causes changes in the mineral composition of the rocks; creates a foliation , or planar surface, that is related to mineral growth under stress. This can remove signs of the original textures of the rocks, such as bedding in sedimentary rocks, flow features of lavas , and crystal patterns in crystalline rocks. Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through normal 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 the Maria Fold and Thrust Belt , the entire sedimentary sequence of the Grand Canyon appears over a length of less than a meter. Rocks at the depth to be ductilely stretched are often also metamorphosed. These stretched rocks can also pinch into lenses, known as boudins , 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 become shear zones at deeper depths where the rocks deform ductilely. The addition of new rock units, both depositionally and intrusively, often occurs during deformation.
Faulting and other deformational processes result in the creation of topographic gradients, causing material on the rock unit that is increasing in elevation to be eroded by hillslopes and channels. These sediments are deposited on the rock unit that is going down. Continual motion along the fault maintains the topographic gradient in spite of the movement of sediment, and continues to create accommodation space for the material to deposit. Deformational events are often also associated with volcanism and igneous activity.
Volcanic ashes and lavas accumulate on the surface, and igneous intrusions enter from below. Dikes , long, planar igneous intrusions, enter along cracks, and therefore often form in large numbers in areas that are being actively deformed. This can result in the emplacement of dike swarms , such as those that are observable across the Canadian shield, or rings of dikes around the lava tube of a volcano.
All of these processes do not necessarily occur in a single environment, and do not necessarily occur in a single order. The Hawaiian Islands , for example, consist almost entirely of layered basaltic lava flows. The sedimentary sequences of the mid-continental United States and the Grand Canyon in the southwestern United States contain almost-undeformed stacks of sedimentary rocks that have remained in place since Cambrian time. Other areas are much more geologically complex. In the southwestern United States, sedimentary, volcanic, and intrusive rocks have been metamorphosed, faulted, foliated, and folded.
Even older rocks, such as the Acasta gneiss of the Slave craton in northwestern Canada , the oldest known rock in the world have been metamorphosed to the point where their origin is undiscernable without laboratory analysis. In addition, these processes can occur in stages.
In many places, the Grand Canyon in the southwestern United States being a very visible example, the lower rock units were metamorphosed and deformed, and then deformation ended and the upper, undeformed units were deposited. Although any amount of rock emplacement and rock deformation can occur, and they can occur any number of times, these concepts provide a guide to understanding the geological history of an area. Geologists use a number of field, laboratory, and numerical modeling methods to decipher Earth history and to understand the processes that occur on and inside the Earth.
In typical geological investigations, geologists use primary information related to petrology the study of rocks , stratigraphy the study of sedimentary layers , and structural geology the study of positions of rock units and their deformation.
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In many cases, geologists also study modern soils, rivers , landscapes , and glaciers ; investigate past and current life and biogeochemical pathways, and use geophysical methods to investigate the subsurface. Sub-specialities of geology may distinguish endogenous and exogenous geology. Geological field work varies depending on the task at hand. Typical fieldwork could consist of:. In addition to identifying rocks in the field lithology , petrologists identify rock samples in the laboratory.
Two of the primary methods for identifying rocks in the laboratory are through optical microscopy and by using an electron microprobe. In an optical mineralogy analysis, petrologists analyze thin sections of rock samples using a petrographic microscope , where the minerals can be identified through their different properties in plane-polarized and cross-polarized light, including their birefringence , pleochroism , twinning , and interference properties with a conoscopic lens. Petrologists can also use fluid inclusion data  and perform high temperature and pressure physical experiments  to understand the temperatures and pressures at which different mineral phases appear, and how they change through igneous  and metamorphic processes.
This research can be extrapolated to the field to understand metamorphic processes and the conditions of crystallization of igneous rocks. Structural geologists use microscopic analysis of oriented thin sections of geologic samples to observe the fabric within the rocks, which gives information about strain within the crystalline structure of the rocks. They also plot and combine measurements of geological structures to better understand the orientations of faults and folds to reconstruct the history of rock deformation in the area.
In addition, they perform analog and numerical experiments of rock deformation in large and small settings. The analysis of structures is often accomplished by plotting the orientations of various features onto stereonets. A stereonet is a stereographic projection of a sphere onto a plane, in which planes are projected as lines and lines are projected as points. These can be used to find the locations of fold axes, relationships between faults, and relationships between other geologic structures.
Among the most well-known experiments in structural geology are those involving orogenic wedges , which are zones in which mountains are built along convergent tectonic plate boundaries. These studies can also give useful information about pathways for metamorphism through pressure, temperature, space, and time. In the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those from drill cores.
In the laboratory, biostratigraphers analyze rock samples from outcrop and drill cores for the fossils found in them. Geochronologists precisely date rocks within the stratigraphic section to provide better absolute bounds on the timing and rates of deposition. With the advent of space exploration in the twentieth century, geologists have begun to look at other planetary bodies in the same ways that have been developed to study the Earth. This new field of study is called planetary geology sometimes known as astrogeology and relies on known geologic principles to study other bodies of the solar system.
Although the Greek-language-origin prefix geo refers to Earth, "geology" is often used in conjunction with the names of other planetary bodies when describing their composition and internal processes: examples are "the geology of Mars " and " Lunar geology ". Specialised terms such as selenology studies of the Moon , areology of Mars , etc.
Although planetary geologists are interested in studying all aspects of other planets, a significant focus is to search for evidence of past or present life on other worlds. This has led to many missions whose primary or ancillary purpose is to examine planetary bodies for evidence of life. One of these is the Phoenix lander , which analyzed Martian 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.
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Economic minerals are those extracted profitably for various practical uses. Economic geologists help locate and manage the Earth's natural resources , such as petroleum and coal, as well as mineral resources, which include metals such as iron, copper, and uranium. Mining geology consists of the extractions of mineral resources from the Earth.
Some resources of economic interests include gemstones , metals such as gold and copper , and many minerals such as asbestos , perlite , mica , phosphates , zeolites , clay , pumice , quartz , and silica , as well as elements such as sulfur , chlorine , and helium. Petroleum geologists study the locations of the subsurface of the Earth that can contain extractable hydrocarbons, especially petroleum and natural gas. Because many of these reservoirs are found in sedimentary basins ,  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 the geologic principles to engineering practice for the purpose of assuring that the geologic factors affecting the location, design, construction, operation, and maintenance of engineering works are properly addressed. In the field of civil engineering , geological principles and analyses are used in order to ascertain the mechanical principles of the material on which structures are built.
This allows tunnels to be built without collapsing, bridges and skyscrapers to be built with sturdy foundations, and buildings to be built that will not settle in clay and mud. Geology and geologic principles can be applied to various environmental problems such as stream restoration , the restoration of brownfields , and the understanding of the interaction between natural habitat and the geologic environment. Groundwater hydrology, or hydrogeology , is used to locate groundwater,  which can often provide a ready supply of uncontaminated water and is especially important in arid regions,  and to monitor the spread of contaminants in groundwater wells.
Geologists also obtain data through stratigraphy, boreholes , core samples , and ice cores. Ice cores  and sediment cores  are used to for paleoclimate reconstructions, which tell geologists about past and present temperature, precipitation, and sea level across the globe. These datasets are our primary source of information on global climate change outside of instrumental data. Geologists and geophysicists study natural hazards in order to enact safe building codes and warning systems that are used to prevent loss of property and life. During the Roman period, Pliny the Elder wrote in detail of the many minerals and metals then in practical use — even correctly noting the origin of amber.
James Hutton , Scottish geologist and father of modern geology. John Tuzo Wilson , Canadian geophysicist and father of plate tectonics. The volcanologist David A.
Johnston 13 hours before his death at the eruption of Mount St. Some modern scholars, such as Fielding H. Garrison , are of the opinion that the origin of the science of geology can be traced to Persia after the Muslim conquests had come to an end. Nicolas Steno — is credited with the law of superposition , the principle of original horizontality , and the principle of lateral continuity : three defining principles of stratigraphy. Escholt first used the definition in his book titled, Geologia Norvegica William Smith — drew some of the first geological maps and began the process of ordering rock strata layers by examining the fossils contained in them.
James Hutton is often viewed as the first modern geologist. In his paper, he explained his theory that the Earth must be much older than had previously been supposed to allow enough time for mountains to be eroded and for sediments 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 Vol. Followers of Hutton were known as Plutonists because they believed that some rocks were formed by vulcanism , which is the deposition of lava from volcanoes, as opposed to the Neptunists , led by Abraham Werner , who believed that all rocks had settled out of a large ocean whose level gradually dropped over time.
The first geological map of the U. Almost every state in the Union was traversed and mapped by him, the Allegheny Mountains being crossed and recrossed some 50 times. Sir Charles Lyell first published his famous book, Principles of Geology ,  in This book, which influenced the thought of Charles Darwin , successfully promoted the doctrine of uniformitarianism. This theory states that slow geological processes have occurred throughout the Earth's history and are still occurring today.
In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time. Much of 19th-century geology revolved around the question of the Earth's exact age. Estimates varied from a few hundred thousand to billions of years. The awareness of this vast amount of time opened the door to new theories about the processes that shaped the planet.
Some of the most significant advances in 20th-century geology have been the development of the theory of plate tectonics in the s and the refinement of estimates of the planet's age. Plate tectonics theory arose from two separate geological observations: seafloor spreading and continental drift. The theory revolutionized the Earth sciences. Today the Earth is known to be approximately 4.
From Wikipedia, the free encyclopedia. For the scientific journal, see Geology journal. The study of the composition, structure, physical properties, and history of Earth's components, and the processes by which they are shaped. Main articles: Rock geology and Rock cycle. Main article: Plate tectonics. Main article: Structure of the Earth. Main articles: History of Earth and Geologic time scale. Main article: Lunar geologic timescale. Main article: Geological history of Mars.
Main article: Relative dating.
Main articles: Absolute dating , radiometric dating , and geochronology. Main article: Petrology. Main article: Structural geology. Main article: Stratigraphy. Main articles: Planetary geology and Geology of solar terrestrial planets. Main article: Economic geology. Main article: Mining. Main article: Petroleum geology.
Main articles: Engineering geology , Soil mechanics , and Geotechnical engineering. Main article: Hydrogeology. Main articles: History of geology and Timeline of geology. Earth sciences portal Geography portal. Online Etymology Dictionary. Radiochimica Acta. Geoman's Mineral ID Tests. Retrieved 17 April November 1, " History Of Ocean Basins ", pp. Engel, Harold L. James, and B. Leonard eds.
Geological Society of America. Kiger, Martha, Russel, Jane Online ed. Reston: United States Geological Survey. Retrieved 13 March Origin of continents and oceans. Courier Corporation. Bibcode : Sci Geochimica et Cosmochimica Acta. Bibcode : GeCoA.. Brent The age of the earth. Stanford, CA: Stanford Univ. The earth through time 9th ed. Hoboken, NJ: J.
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Dinosaurs and the History of Life. Columbia University. Retrieved Earth and Planetary Science Letters. Using geochemical data evaluation, presentation, interpretation. Harlow: Longman. Principles and applications of geochemistry: a comprehensive textbook for geology students.
In Rosenberg, Gary D. The Revolution in Geology from the Renaissance to the Enlightenment. Geological Society of America Memoir. Geology in the field. New York: Wiley. United States Geological Survey. Archived from the original on Robert; Sheehan, Anne F. The book concludes with an extensive glossary of terms; appendices cover basic maths, explain basic solution chemistry, and list the chemical elements and the symbols, units and constants used in the book.
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