The Info List - Geology

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GEOLOGY (from the Ancient Greek γῆ, _gē_, i.e. "earth" and -λoγία, _-logia_, i.e. "study of, discourse" ) is an earth science concerned with the solid Earth , the rocks of which it is composed, and the processes by which they change over time. Geology can also refer generally to the study of the solid features of any terrestrial planet (such as the geology of the Moon or Mars ).

Geology gives insight into the history of the Earth by providing the primary evidence for plate tectonics , the evolutionary history of life , and past climates . Geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources , understanding of natural hazards , the remediation of environmental problems, and for providing insights into past climate change . Geology also plays a role in geotechnical engineering and is a major academic discipline .


* 1 Geologic materials

* 1.1 Rock * 1.2 Unconsolidated material

* 2 Whole- Earth structure

* 2.1 Plate tectonics * 2.2 Earth structure

* 3 Geologic time

* 3.1 Important milestones * 3.2 Brief time scale

* 4 Dating methods

* 4.1 Relative dating * 4.2 Absolute dating

* 5 Geological development of an area

* 6 Methods of geology

* 6.1 Field methods * 6.2 Petrology * 6.3 Structural geology * 6.4 Stratigraphy

* 7 Planetary geology

* 8 Applied geology

* 8.1 Economic geology

* 8.1.1 Mining geology * 8.1.2 Petroleum geology

* 8.2 Engineering geology * 8.3 Hydrology and environmental issues * 8.4 Natural hazards

* 9 History of geology * 10 Fields or related disciplines * 11 Regional geology * 12 See also * 13 References * 14 External links


The majority of geological data comes from research on solid Earth materials. These typically fall into one of two categories: rock and unconsolidated material.


This schematic diagram of the rock cycle shows the relationship between magma and sedimentary, metamorphic, and igneous rock Main articles: Rock (geology) and Rock cycle

The majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous , sedimentary , and metamorphic . The rock cycle is an important concept in geology which illustrates the relationships between these three types of rock, and magma. When a rock crystallizes from melt (magma and/or lava ), it is an igneous rock. Igneous rock can be weathered and eroded , and then redeposited and lithified into a sedimentary rock, or be turned into a metamorphic rock due to heat and pressure that change the mineral content of the rock which gives it a characteristic fabric . The sedimentary rock can then be subsequently turned into a metamorphic rock due to heat and pressure and is then weathered, eroded, deposited, and lithified, ultimately becoming a sedimentary rock. Sedimentary rock may also be re-eroded and redeposited, and metamorphic rock may also undergo additional metamorphism. All three types of rocks may be re-melted; when this happens, a new magma is formed, from which an igneous rock may once again crystallize. Minerals

To study these three types of rocks, geologists evaluate the minerals that make up the rock. All the colors you see in rocks are the different minerals crystallized within. Each mineral has distinct physical properties and there are multiple tests that a geologist can do to determine each one. The Mineral Identification tests are:

* Luster: Measurement of the amount of light reflected from the surface. Luster is broken up into Metallic and Non-metallic. * Color: Minerals are grouped by their color. Mostly diagnostic but impurities can change a mineral’s color. * Streak: Performed by taking a porcelain plate and scratching the mineral on it. The color of the streak can help you name the mineral. * Hardness: The resistance of a mineral to scratch. * Breakage Pattern: A mineral can either show fracture or cleavage. Fracture being breakage of uneven surfaces and cleavage being breakage along closely spaced parallel planes. * Specific Gravity: the weight of a specific volume of a mineral. * Effervescence: Involves dripping HCL on a mineral to test for fizzing. * Magnetism: Involves using a magnet to test for magnetism. * Taste: Minerals can have a distinctive taste like Halite (tastes like salt). * Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs.


Geologists also study unlithified material (commonly referred to as _drift_) which typically comes from more recent deposits. These materials are superficial deposits which lie above the bedrock . Because of this, the study of such material is often known as Quaternary geology , after the recent Quaternary Period . This includes the study of sediment and soils , including studies in geomorphology , sedimentology , and paleoclimatology .



Main article: Plate tectonics Oceanic-continental convergence resulting in subduction and volcanic arcs illustrates one effect of plate tectonics .

In the 1960s, it was discovered that the Earth's lithosphere , including the crust and rigid uppermost portion of the upper mantle , is separated into tectonic plates that move across the plastically deforming, solid, upper mantle, which is called the asthenosphere . This theory is supported by several types of observations, including seafloor spreading, and the global distribution of mountain terrain and seismicity.

There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle : oceanic plate motions and mantle convection currents always move in the same direction, because the oceanic lithosphere is the rigid upper thermal boundary layer of the convecting mantle. This coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics. On this diagram, subducting slabs are in blue, and continental margins and a few plate boundaries are in red. The blue blob in the cutaway section is the seismically imaged Farallon Plate , which is subducting beneath North America. The remnants of this plate on the Surface of the Earth are the Juan de Fuca Plate and Explorer plate in the Northwestern USA / Southwestern Canada, and the Cocos Plate on the west coast of Mexico.

The development of plate tectonics provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features could be explained as plate boundaries. Mid-ocean ridges , high regions on the seafloor where hydrothermal vents and volcanoes exist, were explained as divergent boundaries , where two plates move apart. Arcs of volcanoes and earthquakes were explained as convergent boundaries , where one plate subducts under another. Transform boundaries , such as the San Andreas Fault system, resulted in widespread powerful earthquakes. Plate tectonics also provided a mechanism for Alfred Wegener 's theory of continental drift , in which the continents move across the surface of the Earth over geologic 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.


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

Advances in seismology , computer modeling , and mineralogy and crystallography at high temperatures and pressures give insights into the internal composition and structure of the Earth.

Seismologists can use the arrival times of seismic waves in reverse to image the interior of the Earth. Early advances in this field showed the existence of a liquid outer core (where shear waves were not able to propagate) and a dense solid inner core . These advances led to the development of a layered model of the Earth, with a crust and lithosphere on top, the mantle below (separated within itself by seismic discontinuities at 410 and 660 kilometers), and the outer core and inner core below that. More recently, seismologists have been able to create detailed images of wave speeds inside the earth in the same way a doctor images a body in a CT scan. These images have led to a much more detailed view of the interior of the Earth, and have replaced the simplified layered model with a much more dynamic model.

Mineralogists have been able to use the pressure and temperature data from the seismic and modelling studies alongside knowledge of the elemental composition of the Earth to reproduce these conditions in experimental settings and measure changes in 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.


Geological time put in a diagram called a geological clock , showing 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 the Earth. It is bracketed at the old end by the dates of the earliest Solar System material at 4.567 Ga , (gigaannum: billion years ago) and the age of the Earth at 4.54 Ga at the beginning of the informally recognized Hadean eon . At the young end of the scale, it is bracketed by the present day in the Holocene epoch .


* 4.567 Ga: Solar system formation * 4.54 Ga: Accretion of Earth * c. 4 Ga: End of Late Heavy Bombardment , first life * c. 3.5 Ga: Start of photosynthesis * c. 2.3 Ga: Oxygenated atmosphere , first snowball Earth * 730–635 Ma (megaannum: million years ago): second snowball Earth * 542 ± 0.3 Ma: Cambrian explosion – vast multiplication 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 all land animals die; end of Paleozoic and beginning of Mesozoic * 66 Ma: Cretaceous– Paleogene extinction – Dinosaurs die; end of Mesozoic and beginning of Cenozoic * c. 7 Ma: First hominins appear * 3.9 Ma: First Australopithecus , direct ancestor to modern Homo sapiens , appear * 200 ka (kiloannum: thousand years ago): First modern Homo sapiens appear in East Africa


The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this compresses the most recent eon. Therefore, the second scale shows the most recent eon with an expanded scale. The second scale compresses the most recent era, so the most recent era is expanded in the third scale. The third scale compresses the most recent period, so the most recent period is expanded in the fourth scale. Millions of Years



Main article: Relative dating Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures. Explanations: A – folded rock strata cut by a thrust fault ; B – large intrusion (cutting through A); C – erosional angular unconformity (cutting off A D – volcanic dyke (cutting through A, B E – even younger rock strata (overlying C F – normal fault (cutting through A, B, C accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault .

THE PRINCIPLE OF INCLUSIONS AND COMPONENTS states that, with sedimentary rocks, if inclusions (or _clasts _) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them. The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is an example of both original horizontality and the law of superposition. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park . From top to bottom: Rounded tan domes of the Navajo Sandstone , layered red Kayenta Formation , cliff-forming, vertically jointed, red Wingate Sandstone , slope-forming, purplish Chinle Formation , layered, lighter-red Moenkopi Formation , and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area , Utah.

THE PRINCIPLE OF ORIGINAL HORIZONTALITY states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).

THE PRINCIPLE OF SUPERPOSITION states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

THE PRINCIPLE OF FAUNAL SUCCESSION is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin 's theory of evolution , the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.


Main articles: Absolute dating , radiometric dating , and geochronology

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.

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

For many geologic 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 particular closure temperature , the point at which different radiometric isotopes stop diffusing into and out of the crystal lattice . These are used in geochronologic and thermochronologic studies. Common methods include uranium-lead dating , potassium-argon dating , argon-argon dating and uranium-thorium dating . These methods are used for a variety of applications. Dating of lava and volcanic ash layers found within a stratigraphic sequence can provide absolute age data for sedimentary rock units which 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. Optically stimulated luminescence and cosmogenic radionuclide dating are used to date surfaces and/or erosion rates. Dendrochronology can also be used for the dating of landscapes. Radiocarbon dating is used for geologically young materials containing organic carbon .


An originally horizontal sequence of sedimentary rocks (in shades of tan) are affected by igneous activity. Deep below the surface are a magma chamber and large associated igneous bodies. The magma chamber feeds the volcano , and sends offshoots of magma that will later crystallize into dikes and sills. Magma also advances upwards to form intrusive igneous bodies . The diagram illustrates both a cinder cone volcano, which releases ash, and a composite volcano , which releases both lava and ash. An illustration of the three types of faults. Strike-slip faults occur when rock units slide past one another, normal faults occur when rocks are undergoing horizontal extension, and reverse (or thrust) faults occur when rocks are undergoing horizontal shortening.

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 . 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.

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

Even higher pressures and temperatures during horizontal shortening can cause both folding and metamorphism of the rocks. This metamorphism causes changes in the mineral 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 being placed below older units. Stretching of units can result in their thinning; in fact, there is a location within the Maria Fold and Thrust Belt in which the entire sedimentary sequence of the Grand Canyon can be seen 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. Geologic cross section of Kittatinny Mountain . This cross section shows metamorphic rocks, overlain by younger sediments deposited after the metamorphic event. These rock units were later folded and faulted during the uplift of the mountain.

The addition of new rock units, both depositionally and intrusively, often occurs during deformation. Faulting and other deformational processes result in the creation of topographic gradients, causing material on the rock unit that is increasing in elevation to be eroded by hillslopes and channels. These sediments are deposited on the rock unit that is going down. Continual motion along the fault maintains the topographic gradient in spite of the movement of sediment, and continues to create accommodation space for the material to deposit. Deformational events are often also associated with volcanism and igneous activity. Volcanic ashes and lavas accumulate on the surface, and igneous intrusions enter from below. 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). 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.


A standard Brunton Pocket Transit , commonly used by geologists for mapping and surveying. A typical USGS field mapping camp in the 1950s Today, handheld computers with GPS and geographic information systems software are often used in geological field work (digital geologic mapping ).

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

* Geological mapping

* Structural mapping: identifying the locations of major rock units and the faults and folds that led to their placement there. * Stratigraphic mapping: pinpointing the locations of sedimentary facies (lithofacies and biofacies ) or the mapping of isopachs of equal thickness of sedimentary rock * Surficial mapping: recording the locations of soils and surficial deposits

* Surveying of topographic features

* compilation of topographic maps

* Work to understand change across landscapes, including:

* Patterns of erosion and deposition * River-channel change through migration and avulsion * Hillslope processes

* Subsurface mapping through geophysical methods

* 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 sections on the surface * Well drilling and logging

* Biogeochemistry and geomicrobiology

* 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 it functioned and metabolized * find important compounds for use in pharmaceuticals

* Paleontology : excavation of fossil material

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

* Collection of samples for geochronology and thermochronology * Glaciology : measurement of characteristics of glaciers and their motion


Main article: Petrology A petrographic microscope - an optical microscope fitted with cross-polarizing lenses, a conoscopic lens , and compensators (plates of anisotropic materials; gypsum plates and quartz wedges are common), for crystallographic analysis.

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 . In the electron microprobe, individual locations are analyzed for their exact chemical compositions and variation in composition within individual crystals. Stable and radioactive isotope studies provide insight into the geochemical evolution of rock units.

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. This work can also help to explain processes that occur within the Earth, such as subduction and magma chamber evolution.


Main article: Structural geology A diagram of an orogenic wedge. The wedge grows through faulting in the interior and along the main basal fault, called the décollement . It builds its shape into a critical taper , in which the angles within the wedge remain the same as failures inside the material balance failures along the décollement. It is analogous to a bulldozer pushing a pile of dirt, where the bulldozer is the overriding plate.

Structural geologists use microscopic analysis of oriented thin sections of 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. In the analog versions of these experiments, horizontal layers of sand are pulled along a lower surface into a back stop, which results in realistic-looking patterns of faulting and the growth of a critically tapered (all angles remain the same) orogenic wedge. 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. 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.


Main article: Stratigraphy

In the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those from drill cores . Stratigraphers also analyze data from geophysical surveys that show the locations of stratigraphic units in the subsurface. Geophysical data and well logs can be combined to produce a better view of the subsurface, and stratigraphers often use computer programs to do this in three dimensions. Stratigraphers can then use these data to reconstruct ancient processes occurring on the surface of the Earth, 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. These fossils help scientists to date the core and to understand the depositional environment in which the rock units formed. Geochronologists precisely date rocks within the stratigraphic section to provide better absolute bounds on the timing and rates of deposition. Magnetic stratigraphers look for signs of magnetic reversals in igneous rock units within the drill cores. Other scientists perform stable-isotope studies on the rocks to gain information about past climate.


Surface of Mars as photographed by the Viking 2 lander December 9, 1977. Main articles: Planetary geology and Geology of solar terrestrial planets

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., are also in use.

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



Main article: Economic geology

Economic geology is an important branch of geology which deals with different aspects of economic minerals being used by humankind to fulfill its various needs. The economic minerals are those which can be extracted profitably. 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

Main article: Mining

Mining geology consists of the extractions of mineral resources from the Earth. Some resources of economic interests include gemstones , metals , 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 Geology

Mud log in process, a common way to study the lithology when drilling oil wells. Main article: Petroleum geology

Petroleum geologists study the locations of the subsurface of the Earth which 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.


Main articles: Engineering geology , Soil mechanics , and Geotechnical engineering

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.


Main article: Hydrogeology

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.


Main article: Natural hazard

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. Examples of important natural hazards that are pertinent to geology (as opposed those that are mainly or only pertinent to meteorology) are: Rockfall in the Grand Canyon

* Avalanches * Earthquakes * Floods * Landslides and debris flows * River channel migration and avulsion * Liquefaction * Sinkholes * Subsidence * Tsunamis * Volcanoes


Main articles: History of geology and Timeline of geology William Smith 's geologic map of England , Wales , and southern Scotland . Completed in 1815, it was the second national-scale geologic map, and by far the most accurate of its time.

The study of the physical material of the Earth dates back at least to ancient Greece when Theophrastus (372–287 BCE) wrote the work _Peri Lithon_ (_On Stones_). During the Roman period, Pliny the Elder wrote in detail of the many minerals and metals then in practical use – even correctly noting the origin of amber .

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. Abu al-Rayhan al-Biruni (973–1048 CE) was one of the earliest Persian geologists, whose works included the earliest writings on the geology of India , hypothesizing that the Indian subcontinent was once a sea. Drawing from Greek and Indian scientific literature that were not destroyed by the Muslim conquests , the Persian scholar Ibn Sina (Avicenna, 981–1037) proposed detailed explanations for the formation of mountains, the origin of earthquakes, and other topics central to modern geology, which provided an essential foundation for the later development of the science. In China, the polymath Shen Kuo (1031–1095) formulated a hypothesis for the process of land formation: based on his observation of fossil animal shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by deposition of silt .

Nicolas Steno (1638–1686) is credited with the law of superposition , the principle of original horizontality , and the principle of lateral continuity : three defining principles of stratigraphy .

The word _geology_ was first used by Ulisse Aldrovandi in 1603, then by Jean-André Deluc in 1778 and introduced as a fixed term by Horace-Bénédict de Saussure in 1779. The word is derived from the Greek γῆ, _gê_, meaning "earth" and λόγος, _logos _, meaning "speech". But according to another source, the word "geology" comes from a Norwegian, Mikkel Pedersøn Escholt (1600–1699), who was a priest and scholar. Escholt first used the definition in his book titled, _Geologia Norvegica_ (1657).

William Smith (1769–1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them.

James Hutton is often viewed as the first modern geologist . In 1785 he presented a paper entitled _Theory of the Earth_ to the Royal Society of Edinburgh . In his paper, he explained his theory that the Earth must be much older than had previously been supposed to allow enough time for mountains to be eroded and for 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 1795 (Vol. 1, Vol. 2). Scotsman James Hutton, father of modern geology

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.S. was produced in 1809 by William Maclure . In 1807, Maclure commenced the self-imposed task of making a geological survey of the United States. Almost every state in the Union was traversed and mapped by him, the Allegheny Mountains being crossed and recrossed some 50 times. The results of his unaided labours were submitted to the American Philosophical Society in a memoir entitled _Observations on the Geology of the United States explanatory of a Geological Map_, and published in the _Society's Transactions_, together with the nation's first geological map. This antedates William Smith 's geological map of England by six years, although it was constructed using a different classification of rocks.

Sir Charles Lyell first published his famous book, _Principles of Geology _, in 1830. This book, which influenced the thought of Charles Darwin , successfully promoted the doctrine of uniformitarianism . This theory states that slow geological processes have occurred throughout the Earth\'s history and are still occurring today. In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time.

Much of 19th-century geology revolved around the question of the Earth\'s exact age . Estimates varied from a few hundred thousand to billions of years. By the early 20th century, radiometric dating allowed the Earth's age to be estimated at two billion years. The awareness of this vast amount of time opened the door to new theories about the processes that shaped the planet.

Some of the most significant advances in 20th-century geology have been the development of the theory of plate tectonics in the 1960s and the refinement of estimates of the planet's age. Plate tectonics theory arose from two separate geological observations: seafloor spreading and continental drift . The theory revolutionized the Earth sciences . Today the Earth is known to be approximately 4.5 billion years old.


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ISBN 978-88-7381-056-8 . * ^ Deluc, Jean André de, _Lettres physiques et morales sur les montagnes et sur l'histoire de la terre et de l'homme. …_ , vol. 1 (Paris, France: V. Duchesne, 1779), pp. 4, 5, and 7. From p. 4: _"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 ; … ) From p. 5: _"Je vis que je faisais un Traité, et non une equisse de_ Géologie_."_ (I see that I wrote a treatise, and not a sketch of geology.) From the footnote on p. 7: _"Je répète ici, ce que j'avois dit dans ma première_ Préface_, sur la substitution de mot_ Cosmologie _à celui de_ Géologie_, quoiqu'il ne s'agisse pas de l'Univers, mais seulement de la_ Terre_: … "_ (I repeat here what I said in my first preface about the substitution of the word "cosmology" for that of "geology", although it is not a matter of the universe but only of the Earth: … ) * ^ Saussure, Horace-Bénédict de, _Voyages dans les Alpes_, … (Neuchatel, (Switzerland): Samuel Fauche, 1779). From pp. i–ii: _"La science qui rassemble les faits, qui seuls peuvent servir de base à la Théorie de la Terre ou à la_ Géologie_, c'est la Géographie physique, ou la description de notre Globe; … "_ (The science that assembles the facts which alone can serve as the basis of the theory of the Earth or of "geology", is physical geography, or the description of our globe; … )

* ^ 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 Co., 1905), p. 186. * Eastman, Charles Rochester (12 August 1904) Letter to the Editor: "Variæ Auctoritatis", _Science_, 2nd series, 20 (502) : 215–217 ; see p. 216. * Emmons, Samuel Franklin (21 October 1904) Letter to the Editor: "Variæ Auctoritatis", _Science_, 2nd series, 20 (512) : 537. * Eastman, C. R. (25 November 1904) Letter to the Editor: "Notes on the History of Scientific Nomenclature," _Science_, 2nd series, 20 (517) : 727–730 ; see p. 728. * Emmons, S. F. (23 December 1904) Letter to the Editor: "The term \'geology\' ", _Science_, 2nd series, 20 (521) : 886–887. * Eastman, C. R. (20 January 1905) Letter to the Editor: "Deluc\'s \'Geological Letters\'", _Science_, 2nd series, 21 (525): 111. * Emmons, S. F. (17 February 1905) Letter to the Editor: "Deluc versus de Saussure", _Science_, 2nd series, 21 (529) : 274–275.

* ^ Winchester, Simon (2001). _ The Map that Changed the World _. HarperCollins Publishers. p. 25. ISBN 0-06-093180-9 .

* ^ See:

* Escholt, Michel Pedersøn, _Geologia Norvegica : det er, En kort undervisning om det vitt-begrebne jordskelff som her udi Norge skeedemesten ofuer alt Syndenfields den 24. aprilis udi nærværende aar 1657: sampt physiske, historiske oc theologiske fundament oc grundelige beretning om jordskellfs aarsager oc betydninger_ the 24th of April in the present year 1657: together with physical, historical, and theological bases and a basic account of earthquakes' causes and meanings] (Christiania (now: Oslo), (Norway): Mickel Thomesøn, 1657). (in Danish) * Reprinted in English as: Escholt, Michel Pedersøn with Daniel Collins, trans., _Geologia Norvegica_ … (London, England: S. Thomson, 1663).

* ^ Kermit H., (2003) _Niels Stensen, 1638–1686: the scientist who was beatified_. Gracewing Publishing. p. 127. * ^ James Hutton: The Founder of Modern Geology American Museum of Natural History * ^ Maclure, William (1817). _Observations on the Geology of the United States of America: With Some Remarks on the Effect Produced on the Nature and Fertility of Soils, by the Decomposition of the Different Classes of Rocks; and an Application to the Fertility of Every State in the Union, in Reference to the Accompanying Geological Map ..._ author. * ^ Observations on the Geology of the United States (Maclure). David Rumsey map collection * ^ 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 . * ^ Maclure\'s 1809 Geological Map. davidrumsey.com * ^ Lyell, Charles (1991). _Principles of geology_. Chicago: University of Chicago Press. ISBN 978-0-226-49797-6 . * ^ England, Philip; Molnar, Peter; Richter, Frank (2007). "John Perry's neglected critique of Kelvin's age for the Earth: A missed opportunity in geodynamics". _GSA Today_. 17: 4. doi :10.1130/GSAT01701A.1 . * ^ Dalrymple, G.B. (1991). _The Age of the Earth_. California: Stanford University Press. ISBN 0-8047-1569-6 .


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