Geological materialsThe majority of geological data comes from research on solid Earth materials. These typically fall into one of two categories: rock and unlithified material. Meteorites and other extra-terrestrial natural materials are also studied by geological methods.
MineralMinerals are natural occurring elements and compounds with a definite homogeneous chemical composition and ordered atomic composition.
RockA rock is any naturally occurring solid mass or aggregate of minerals or s. 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: , , and . The illustrates the relationships among them (see diagram). When a rock or from melt ( or ), it is an igneous rock. This rock can be and , then and into a sedimentary rock. It can then be turned into a by heat and pressure that change its content, resulting in a . All three types may melt again, and when this happens, new magma is formed, from which an igneous rock may once more solidify. Organic matter, such as coal, bitumen, oil and natural gas, is linked mainly to organic-rich sedimentary rocks.
TestsTo study all three types of rock, geologists evaluate the minerals of which they are composed. Each mineral has distinct physical properties, and there are many tests to determine each of them. The specimens can be tested for: * Luster: Quality of light reflected from the surface of a mineral. Examples are metallic, pearly, waxy, dull. * Color: Minerals are grouped by their color. Mostly diagnostic but impurities can change a mineral's color. * Streak: Performed by scratching the sample on a plate. The color of the streak can help name the mineral. * Hardness: The resistance of a mineral to scratching. * Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces, and the latter a breakage along closely spaced parallel planes. * Specific gravity: the weight of a specific volume of a mineral. * Effervescence: Involves dripping on the mineral to test for fizzing. * Magnetism: Involves using a magnet to test for . * Taste: Minerals can have a distinctive taste, such as (which tastes like ). * Smell: Minerals can have a distinctive odor. For example, smells like rotten eggs.
Unlithified materialGeologists also study unlithified materials (referred to as '), which typically come from more recent deposits. These materials are that lie above the . This study is often known as , after the of geologic history.
MagmaUnlithified material does not only include . Magma is the original unlithified source of all . The active flow of molten rock is closely studied in , and aims to determine the history of igneous rocks from their final crystallization to their original molten source.
Plate tectonicsIn the 1960s, it was discovered that the Earth's , which includes the and rigid uppermost portion of the , is separated into s that move across the deforming, solid, upper mantle, which is called the . 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 (that is, the heat transfer caused by the bulk movement of molecules within fluids). Thus, oceanic plates and the adjoining mantle always move in the same direction – because the oceanic lithosphere is actually the rigid upper thermal of the convecting mantle. This coupling between rigid plates moving on the surface of the Earth and the convecting is called plate tectonics. The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geological features are explained as plate boundaries. For example: * s, high regions on the seafloor where s and volcanoes exist, are seen as , where two plates move apart. * Arcs of volcanoes and earthquakes are theorized as , where one plate , or moves, under another. , such as the system, resulted in widespread powerful earthquakes. Plate tectonics also has provided a mechanism for 's theory of , in which the 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.
Earth structureAdvances in , , and and at high temperatures and pressures give insights into the internal composition and structure of the Earth. Seismologists can use the arrival times of s in reverse to image the interior of the Earth. Early advances in this field showed the existence of a liquid (where were not able to propagate) and a dense solid . These advances led to the development of a layered model of the Earth, with a and on top, the below (separated within itself by 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 modeling 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 timeThe geological time scale encompasses the history of the Earth. It is bracketed at the earliest by the dates of the first material at 4.567 (or 4.567 billion years ago) and the formation of the Earth at 4.54 Ga (4.54 billion years), which is the beginning of the informally recognized a division of geological time. At the later end of the scale, it is marked by the present day (in the ).
Timescale of the Earth
Important milestones on Earth* 4.567 (gigaannum: billion years ago): * 4.54 Ga: , of Earth * c. 4 Ga: End of , the first life * c. 3.5 Ga: Start of * c. 2.3 Ga: Oxygenated , first * 730–635 (megaannum: million years ago): second snowball Earth * 541 ± 0.3 Ma: – vast multiplication of hard-bodied life; first abundant s; start of the * c. 380 Ma: First land animals * 250 Ma: – 90% of all land animals die; end of Paleozoic and beginning of * 66 Ma: – s die; end of Mesozoic and beginning of * c. 7 Ma: First s appear * 3.9 Ma: First , direct ancestor to modern , appear * 200 (kiloannum: thousand years ago): First modern Homo sapiens appear in East Africa
Timescale of the Moon
Timescale of Mars
Relative datingMethods for were developed when geology first emerged as a . Geologists still use the following principles today as a means to provide information about geological history and the timing of geological events. The 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. A fundamental principle of geology advanced by the 18th-century Scottish physician and geologist 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." concerns crosscutting intrusions. In geology, when an intrusion cuts across a formation of , it can be determined that the igneous intrusion is younger than the sedimentary rock. Different types of intrusions include stocks, s, s, and . The pertains to the formation of 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 a or a . The states that, with sedimentary rocks, if inclusions (or ') 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 s are found. These foreign bodies are picked up as or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock that contains them. The 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 is inclined, the overall orientation of cross-bedded units is horizontal). The states that a sedimentary rock layer in a 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. The 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 of 's theory of , 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 ( change in sedimentary strata), and that not all fossils formed globally at the same time.
Absolute datingGeologists 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 geological events using s 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 assign 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, s of radioactive elements are measured in minerals that give the amount of time that has passed since a rock passed through its particular , the point at which different radiometric isotopes stop diffusing into and out of the . These are used in and studies. Common methods include , , and . These methods are used for a variety of applications. Dating of and 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 emplacement. Thermochemical techniques can be used to determine temperature profiles within the crust, the uplift of mountain ranges, and paleo-topography. Fractionation of the elements is used to compute ages since rocks were removed from the mantle. Other methods are used for more recent events. and dating are used to date surfaces and/or erosion rates. can also be used for the dating of landscapes. is used for geologically young materials containing .
Geological development of an areaThe 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 . Deposition can occur when sediments settle onto the surface of the Earth and later into sedimentary rock, or when as such as or s blanket the surface. s such as s, s, , and , 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 and/or . Deformation typically occurs as a result of horizontal shortening, , or side-to-side () motion. These structural regimes broadly relate to , , and transform boundaries, respectively, between tectonic plates. When rock units are placed under horizontal , they shorten and become thicker. Because rock units, other than muds, , this is accomplished in two primary ways: through and . In the shallow crust, where 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 the , 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 "s", or where it buckles downwards, creating "s". If the tops of the rock units within the folds remain pointing upwards, they are called s and s, 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 of the rocks. This metamorphism causes changes in the of the rocks; creates a , or planar surface, that is related to mineral growth under stress. This can remove signs of the original textures of the rocks, such as in sedimentary rocks, flow features of s, and crystal patterns in s. Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through ing 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 , the entire sedimentary sequence of the 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 ', after the French word for "sausage" because of their visual similarity. Where rock units slide past one another, s develop in shallow regions, and become s 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 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. , 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 s, such as those that are observable across the Canadian shield, or rings of dikes around the 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 , for example, consist almost entirely of layered ic lava flows. The sedimentary sequences of the mid-continental United States and the in the southwestern United States contain almost-undeformed stacks of sedimentary rocks that have remained in place since 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 of the in northwestern , the 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 the of an area.
Methods of geologyGeologists use a number of fields, laboratory, and numerical modeling methods to decipher Earth history and to understand the processes that occur on and inside the Earth. In typical geological investigations, geologists use primary information related to (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, s, s, and s; investigate past and current life and pathways, and use to investigate the subsurface. Sub-specialities of geology may distinguish endogenous and exogenous geology.
Field methodsGeological varies depending on the task at hand. Typical fieldwork could consist of: * ping ** 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 ( and ) or the mapping of s of equal thickness of sedimentary rock ** Surficial mapping: recording the locations of soils and surficial deposits * Surveying of topographic features ** compilation of s ** Work to understand change across landscapes, including: *** Patterns of and *** River-channel change through and *** Hillslope processes * Subsurface mapping through ** These methods include: *** Shallow surveys *** *** s *** ** They aid in: *** *** Finding *** * High-resolution stratigraphy ** Measuring and describing stratigraphic sections on the surface ** and * and ** Collecting samples to: *** determine s *** identify new of organisms *** identify new s ** and to use these discoveries to: *** understand early life on Earth and how it functioned and metabolized *** find important compounds for use in pharmaceuticals * : excavation of material ** For research into past life and ** For s and education * Collection of samples for and * : measurement of characteristics of glaciers and their motion
PetrologyIn addition to identifying rocks in the field (), petrologists identify rock samples in the laboratory. Two of the primary methods for identifying rocks in the laboratory are through and by using an . In an analysis, petrologists analyze s of rock samples using a , where the minerals can be identified through their different properties in plane-polarized and cross-polarized light, including their , , , and interference properties with a . In the electron microprobe, individual locations are analyzed for their exact chemical compositions and variation in composition within individual crystals. and studies provide insight into the evolution of rock units. Petrologists can also use 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 and evolution.
Structural geologyStructural geologists use microscopic analysis of oriented thin sections of geological samples to observe the 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 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 . A stereonet is a stereographic projection of a sphere onto a plane, in which planes are projected as lines and lines are projected as points. These can be used to find the locations of fold axes, relationships between faults, and relationships between other geological structures. Among the most well-known experiments in structural geology are those involving , which are zones in which s are built along 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 (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.
StratigraphyIn the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those from s. Stratigraphers also analyze data from geophysical surveys that show the locations of stratigraphic units in the subsurface. Geophysical data and s 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, 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 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.
Planetary geologyWith the advent of in the twentieth century, geologists have begun to look at other planetary bodies in the same ways that have been developed to study the . This new field of study is called (sometimes known as astrogeology) and relies on known geological principles to study other bodies of the solar system. Although the Greek-language-origin prefix ' 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 " and "". Specialized 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 , which analyzed polar soil for water, chemical, and mineralogical constituents related to biological processes.
Economic geologyEconomic 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's s, such as petroleum and coal, as well as mineral resources, which include metals such as iron, copper, and uranium.
Mining geologyconsists of the extractions of mineral resources from the Earth. Some resources of economic interests include s, s such as and , and many minerals such as , , , s, , , , , and , as well as elements such as , , and .
Petroleum geologys study the locations of the subsurface of the Earth that can contain extractable hydrocarbons, especially and . Because many of these reservoirs are found in s, 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 geologyEngineering 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 from , particularly in North America. In the field of , 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.
HydrologyGeology and geological principles can be applied to various environmental problems such as , the restoration of , and the understanding of the interaction between and the geological environment. Groundwater hydrology, or , 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.
PaleoclimatologyGeologists also obtain data through stratigraphy, , s, and s. Ice cores and sediment cores are used for paleoclimate reconstructions, which tell geologists about past and present temperature, precipitation, and across the globe. These datasets are our primary source of information on outside of instrumental data.
Natural hazardsGeologists and geophysicists study natural hazards in order to enact safe s 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:
HistoryThe study of the physical material of the Earth dates back at least to when (372–287 BCE) wrote the work ''Peri Lithon'' (''On Stones''). During the period, wrote in detail of the many minerals and metals, then in practical use – even correctly noting the origin of . Additionally, in the 4th century BCE Aristotle 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. Some modern scholars, such as , are of the opinion that the origin of the science of geology can be traced to after the had come to an end. (973–1048 CE) was one of the earliest geologists, whose works included the earliest writings on the , hypothesizing that the was once a sea. Drawing from Greek and Indian scientific literature that were not destroyed by the , the Persian scholar (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 (1031–1095) formulated a hypothesis for the process of land formation: based on his observation of fossil animal shells in a geological in a mountain hundreds of miles from the ocean, he inferred that the land was formed by the erosion of the mountains and by of . (1638–1686) is credited with the , the , and the : three defining principles of . The word ''geology'' was first used by in 1603, then by in 1778 and introduced as a fixed term by in 1779. The word is derived from the γῆ, ''gê'', meaning "earth" and λόγος, ', 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). (1769–1839) drew some of the first geological maps and began the process of ordering (layers) by examining the fossils contained in them. In 1763, published his treatise ''On the Strata of Earth''. 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. (1726-1797) is often viewed as the first modern geologist. In 1785 he presented a paper entitled ''Theory of the Earth'' to the . 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 s 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. Followers of Hutton were known as ' because they believed that some rocks were formed by ''vulcanism'', which is the deposition of lava from volcanoes, as opposed to the ', led by , who believed that all rocks had settled out of a large ocean whose level gradually dropped over time. The first was produced in 1809 by . 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 being crossed and recrossed some 50 times. The results of his unaided labours were submitted to the 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 's geological map of England by six years, although it was constructed using a different classification of rocks. (1797-1875) first published his famous book, ', in 1830. This book, which influenced the thought of , successfully promoted the doctrine of . This theory states that slow geological processes have occurred throughout the and are still occurring today. In contrast, 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 . Estimates varied from a few hundred thousand to billions of years. By the early 20th century, 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 in the 1960s and the refinement of estimates of the planet's age. Plate tectonics theory arose from two separate geological observations: and . The theory revolutionized the . Today the Earth is known to be approximately 4.5 billion years old.
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