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Professor Anne Hofmeister loading a rock sample into a laser-flash apparatus to measure the (credit: Professor Randy Korotev in Department of Earth and Planetary Sciences, Washington U., St. Louis MO.)

Scientific study of the Earth, including its composition, structure, physical properties, and history. Geology is commonly divided into subdisciplines concerned with the chemical makeup of the Earth, including the study of minerals (mineralogy) and rocks (petrology); the structure of the Earth (structural geology) and volcanic phenomena (volcanology); landforms and the processes that produce them (geomorphology and glaciology); geologic history, including the study of fossils (paleontology), the development of sedimentary strata (stratigraphy), and the evolution of planetary bodies and their satellites (astrogeology); and economic geology and its various branches, such as mining geology and petroleum geology. Some major fields closely allied to geology are geodesy, geophysics, and geochemistry. environmental geology.

For more information on geology, visit Britannica.com.

The science of the Earth. The study of the Earth's materials and of the processes that shape them is known as physical geology. Historical geology is the record of past events. See also Earth; Earth sciences.

Geology is an interdisciplinary subject that overlaps and depends on other scientific disciplines. Physical geology is concerned primarily with the Earth's materials (minerals, rocks, soils, water, ice, and so forth) and the processes of their origin and alteration. Chemistry and physics are the two scientific disciplines most closely related—study of the chemistry of the Earth's materials is geochemistry, and study of the physical properties of the Earth is geophysics. See also Geochemistry; Geophysics; Structural geology.

Historical geology is based on two complementary disciplines, stratigraphy and paleontology. Stratigraphy is the systematic study of stratified rocks through geologic time. The stratigraphic record reveals the sequence of events that have affected the Earth through eons of time. Absolute dates for the stratigraphic record are provided from geochemical studies of naturally occurring radioactive isotopes. Paleontology is the study of fossilized plants and animals with regard to their distribution in space and time. Paleontology is closely related to biology. The distinctions between physical and historical geology are more matters of convenience than substance, because it is increasingly clear, within the framework of plate tectonics, that all aspects of geology are interrelated. See also Paleontology; Plate tectonics; Stratigraphy.

Mineralogy concerns the study of natural inorganic substances (minerals), the basic building blocks of rocks. About 3600 minerals have been identified, but fewer than 50 are common constituents in the types of rocks that are abundant in the Earth. The most common minerals in the crust are feldspars, quartz, micas, amphiboles, pyroxenes, olivine, and calcite. Modern laboratories have effective devices for resolving the mineral content of rock materials; even the ultramicroscopic particles in clays are clearly defined under the electron microscope. See also Crystal structure; Electron microscope; Mineral; Mineralogy.

Petrology is the study of rocks, their physical and chemical properties, and their modes of origin. The primary families are igneous rocks, which have solidified from molten matter (magma); sedimentary rocks, made of fragments derived by weathering of preexisting rocks, of chemical precipitates from sea or lake water, and of organic remains; and metamorphic rocks derived from igneous or sedimentary rocks under conditions that brought about changes in mineral composition, texture, and internal structure (fabric). The secondary rock families are pyroclastic rocks, which are partly igneous and partly sedimentary rocks because they are composed largely or entirely of fragments of igneous matter erupted explosively from a volcano; diagenetic rocks are transitional between sedimentary and metamorphic rocks because their textures or compositions were affected by low-temperature, postsedimentation processes below conditions of metamorphism; migmatites are transitional between metamorphic and igneous rocks because they form when metamorphic rocks are raised to temperatures and pressures so that small localized fractions of the rock start to melt but the melting is insufficient for a large body of magma to develop. See also Petrography; Petrology; Rock.

A general knowledge of geology has many practical applications, and large numbers of geologists receive special training for service in solving problems met in the mining of metals and nonmetals, in discovering and producing petroleum and natural gas, and in engineering projects of many kinds. Human use of materials has become so great that waste materials are influencing natural geological processes. As a result, a new discipline, environmental geology, is starting to emerge. See also Engineering geology; Petroleum geology.

Although often sharing ground and affiliated with other natural sciences, geology at its core is the study of the earth's crust considered with respect to its rock and mineral content, its layered structure, and its dynamic transformations over time. In the United States, the science first emerged as a popular, organized pursuit around 1820, and about half a century later began to look something like the highly technical, professional discipline it is today. The crucial transition in its evolution occurred at the time of the Civil War (1861–1865), which thus serves as a boundary between the two major periods into which the history of American geology may conveniently be divided. In the first—an organizing, professionalizing stage—geologists were primarily engaged in identifying, naming, and classifying rock strata and in gathering information on mineral locations. Geology was greatly appreciated by the public for its educational value, its health benefits (claimed for the exercise and fresh air of field excursions), and its economic utility. It also attracted wide attention because of its unorthodox religious implications. In the second phase, when geology became the preserve of an enlarged corps of trained specialists, it tended to slip from public view even while it was charting paths for western expansion, unearthing the ores and energy resources needed to sustain a burgeoning economy, and performing other useful services. While geology became increasingly technical and inaccessible to the public, the plate tectonics revolution of the 1960s and 1970s made geology momentarily newsworthy, and thereafter there have been signs of the reentry of the science into the public arena as concern over global warming, water shortages, and other environmental problems has grown.

Geology in America Before the Civil War

In the early nineteenth century, geology was a fledgling science. Its practitioners were committed to avoiding the groundless speculations that had marred earlier "theories of the earth" and dedicated themselves to erecting geology on a solid foundation of observational data and well-ascertained facts. At the same time, making a pitch for public support of their endeavors, they gave assurances that geology was exceedingly useful. It could illuminate the earth's structure and chronicle its history, furnish valuable technical advice bearing on the progress of agriculture, mining, and manufacturing, and perhaps even lend confirmation to the biblical accounts of the Creation and the Flood. Receptive to these claims, the American public held geology in high esteem. In fact, during the first half of the nineteenth century it was the most popular of all the sciences. It won a place in the college curriculum, and textbooks setting forth its basic principles appeared. It was the subject of popular works and of primers, of articles in the quarterlies and the newspapers. Geology was a frequent topic for lyceum courses and public lectures, some like the Lowell Lectures in Boston, attracting thousands of auditors. In part, this high level of interest was stimulated by the ethos of "self-improvement" that Americans had adopted. Undergoing rapid growth, geology was making new discoveries in abundance, and it was the part of the educated person to keep abreast of these noteworthy advances in scientific knowledge.

But interest in geology was also stimulated by its bearing on religion. When geologists considered the rates at which geological processes like denudation and sedimentation take place, they were forced to conclude that the earth was millions of years old. How could this finding be reconciled with widely credited inferences from Old Testament history putting the age of the world at 6,000 years? And then there was the newly uncovered fossil record showing that vast stretches of time separated the first appearance on earth of the major types of plants and animals and that most of the ancient forms had become extinct before humans appeared. How could these facts be harmonized with the doctrine of the divine creation of the world in six days set forth in the book of Genesis? Benjamin Silliman Sr. and Edward Hitchcock, among other antebellum geologists, believed in the inspiration and authority of the Bible, but they were also stout champions of geology and did not want to see it succumb to biblical censorship. Certain that Genesis and geology must ultimately agree, they reconciled the two by adopting nonliteral interpretations of Scripture; Silliman, for example, subscribed to the "day-age" view, whereby the days of the Bible were interpreted as geological periods. This kind of harmonizing exegesis had an appeal for a while, but by the middle of the century it had begun to appear less convincing.

Noah's Flood was even easier to reconcile with geology, since it had long been invoked to explain the sculpting the earth's crust had undergone. As suggestive as this idea was, it did not stand up to close scrutiny, and by the mid-1830s the Flood had been abandoned as a universal, geological agency by the leading geologists. Among nonspecialists, of course, these issues were not so quickly resolved, and as long as geology appeared to bear in critical ways on the truth of Scripture, it continued to interest the public.

The perception that geology might be a source of substantial economic benefits also contributed to its popularity. To realize these benefits and join the national campaign for "internal improvements," virtually all the state legislatures authorized geological or natural history surveys. The first of these was instituted in North Carolina in 1823. By 1865, only Oregon and Louisiana had not initiated surveys. From state to state the surveys varied in scope and emphasis, but ordinarily they included a cataloging of the state's mineral deposits, an analysis of its soils for the benefit of farmers, and topographical reconnaissance to determine routes for turnpikes, railroads, and canals. Supported by annual appropriations that the geologists were obliged to justify in their annual reports, surveys would typically last a few years and conclude with the publication of "final reports." Aside from the evidence they presented of the industriousness and scientific acumen of the state geologists and their assistants, these state-funded documents served as valuable publicity for geology and gave evidence of its far-reaching utility. The public was appreciative, although occasionally there were complaints by those who did not want to hear that geological examinations of the state excluded the possibility of finding within its boundaries deposits of desirable mineral substances (coal in New York, for example).

The geologists employed in these surveys were far less specialized and professional than geologists would subsequently become. Among the approximately 500 individuals who published on earth science topics in antebellum America, few cultivated geology exclusively. Typically, their publications extended to other areas of science, notably natural history or chemistry. But whether specialists or not, they had only limited opportunities for making a living in science. Generally it was an avocational pursuit for those who found their principal work in medicine, the church, or business. Nonetheless, a living could be made in geology, and more readily than in most areas of science, since geologists could find employment in government surveys and in private consulting, as well as in college teaching. To be sure, combining the pay from two or more jobs in these different sectors might be necessary to ensure an adequate annual income.

Since as yet there were no graduate programs providing research training (these were inaugurated after the Civil War), there was no standard educational stepladder giving entry to a geological career. Apprenticing and on the-job training as assistants in government surveys gave the best preparation. Providing not only an introduction to the practicalities of fieldwork that were essential to geology, survey work also made available through the many reports it generated a publication outlet for the aspiring geologist. The experience of the state surveys was also important in developing a collective esprit de corps among geologists and spurring them to organize on a national level. The year 1840 saw the founding of the Association of American Geologists, which shortly was to become the Association of American Geologists and Naturalists, and then in 1848 the American Association for the Advancement of Science. These developments bear witness to the fact that the professionalization and institutionalization of science in America was spearheaded by geologists.

As the Civil War approached, American geologists could feel they were part of a flourishing enterprise. The esteem in which geology was held by the public, the career opportunities it afforded, and the still-limited professionalism it practiced were all on the rise. There was pride in the surveying and mapping that had been accomplished in the preceding fifty years and a zest for continuing the exploration of the trans-Mississippi West. Geology was one facet of culture in which Americans no longer needed to feel they were inferior to Europeans. Textbooks now illustrated geological principles with American material, and one fundamental concept adopted by geologists everywhere, that of the geosyncline (a trough-like downwarp of the earth's crust supposed to be foundational in mountain building), had its origins in America.

Geology in America Since the Civil War

Although during the war years geological activity ground nearly to a halt, the end of the conflict launched a new and vibrant era in the cultivation of the earth sciences. Compared with its antebellum history, geology was now much more national in framework and in closer partner-ship with the federal government. The most expensive and highly publicized of the new projects were the federal surveys of the West. Unlike the U. S. government surveys undertaken before the Civil War, they were not primarily military in purpose nor under army direction. They were multifaceted exploring enterprises conducted by such ambitious civilian "entrepreneurs" as F. V. Hayden, Clarence King, and John Wesley Powell. The cost, competitiveness, and overlap of these surveys led in 1879 to their replacement by a consolidated bureau under the Department of the Interior, the United States Geological Survey (USGS). Initially, the USGS was especially concerned with serving the western mining industry, but subsequently its purposes broadened to include mapping the country, studying water resources, researching marine geology, and much else. In the world wars of the twentieth century it gave priority to the provision of strategic materials.

The creation of a consolidated, national framework for geological research was paralleled by the establishment in 1888 of a new association of national scope (or supranational, as it took in all of North America) dedicated to the professional growth of earth scientists. Still active in the twenty-first century and boasting a global membership in excess of 16,000, the Geological Society of America (GSA) holds an annual meeting and sponsors six regional sections that conduct their own yearly meetings. It further serves its members by publishing research papers and monographs, distributing research grants, recognizing outstanding achievements with medals and other honorific awards, and operating an employment clearinghouse. Twenty percent of GSA's members are students, and a wider participation of women in geology is encouraged by the activities of an associated society, the Association for Women Geoscientists. In seeking to achieve its aim of advancing the geosciences, the GSA, shaped by the modern culture of professionalism, has concentrated heavily on the practitioners, on the geoscientists themselves, their recruitment, development, and rewards.

This inner-directed orientation of geology's leaders, in combination with the growing technicality and inaccessibility of the science to outsiders, has opened up a gap between geology and the public that did not exist in the antebellum period. Few people are now drawn to geology because of cosmic or religious implications it is supposed to have. Nor does probing the relations between Genesis and geology currently have any cultural urgency. The GSA, to be sure, has issued a position paper (pro) on the theory of evolution, and more generally it has characterized the organization's vision as "applying geoscience knowledge and insight to human needs and aspirations and stewardship of the Earth." But getting this idealistic message to be taken seriously by an indifferent public has been difficult.

There have been signs that public awareness of geology may once again be stirring. The theory of plate tectonics established in the 1960s and 1970s has been so revolutionary and consequential that some word of it has reached almost everyone. It was a legacy of nineteenth-century geological thinking that throughout the history of the earth, continents and ocean basins have been permanently fixed (save for occasional motions upward or downward). When, starting in 1912, the German meteorologist Alfred Wegener challenged this fixist theory, arguing that continents have drifted laterally, collided, and separated, he made hardly any converts. By the late 1960s, however, continental drift had been incorporated into a new, synthetic theory that supposed the earth's crust to consist of a dozen or so rigid plates that move horizontally and interact with one another in response to heat convection patterns in the mantle. Turning back all challenges, the theory has revolutionized geology, giving it a remarkable unity and coherence and raising its explanatory power many times.

Just as the plate tectonics revolution was occurring, James Lovelock was publicizing his Gaia Hypothesis (in its biosphere the Earth functions as a single, self-regulating superorganism), the science of ecology was gaining broad recognition, and environmental alarms were registering in the public consciousness. One upshot of these developments has been a new and earnest regard for the planet, incorporating the knowledge and perspective of many fields—geology, biology, oceanography, atmospheric sciences, climatology, and so forth. If the idea is to understand how we depend on the environment and how we can keep it in balance, then help from all these sciences and others may be required. The processes to be understood are complex. They function as "systems" that only a multidisciplinary approach can unravel. Enough is at stake to suggest that geology, which is already a multi-disciplinary field, will once again gain public attention.

Bibliography

Aldrich, Michele L. New York Natural History Survey, 1836– 1845: A Chapter in the History of American Science. Ithaca, N. Y. : Paleontological Research Institution, 2000.

Goetzmann, William H. Exploration and Empire: the Explorer and the Scientist in the Winning of the American West. New York: Knopf, 1966.

Kohlstedt, Sally Gregory. "The Geologists' Model for National Science. 1840–1847," Proceedings of the American Philosophical Society, 1974, 118: 179–195.

Manning, Thomas G. Government in Science: The U. S. Geological Survey, 1867–1894. Lexington: University of Kentucky Press, 1967.

Merrill, George P. The First One Hundred Years of American Geology. New Haven, Conn. : Yale University Press, 1924.

Newell, Julie Renee, "American Geologists and Their Geology: The Formation of the American Geological Community, 1780–1865," Ph. D. Diss., Univ. of Wisconsin-Madison, 1993.

Oreskes, Naomi. The Rejection of Continental Drift: Theory and Method in American Earth Science. New York and Oxford: Oxford University Press, 1999.

Schneer, Cecil J., ed. Two Hundred Years of Geology in America: Proceedings of the New Hampshire Bicentennial Conference on the History of Geology. Hanover, N. H. : University of New Hampshire, 1979.

—Robert H. Silliman

Geology was only in the process of becoming a recognized science near the close of the eighteenth century. Tracing geology's root sources, during the several centuries prior to its emergence as a distinct science, requires attention to varied forms of activity and knowledge, including (1) practical activities such as quarrying, mining, surveying, and the metallurgical arts; (2) descriptive and classificatory inquiries in fields of natural history such as mineralogy and physical geography; (3) philosophical explorations of the causes of the formation of minerals, stones, and crystals; (4) history proper, which is to say chronological and antiquarian research; and (5) efforts to construct a theory of the earth, a genre that began to flourish especially after the middle of the seventeenth century.

Varied Modes of Pursuit of Earth Science

Growing confidence in the practical value of systematic knowledge lay behind efforts to survey mineral resources and promote their exploitation. The writings of the German mining physician Georgius Agricola (1494–1555) are representative of increasingly acute descriptions and rationalizations of technical procedures for extracting and treating those resources. By the seventeenth century, under state ownership or patronage of mining authorities in several Continental countries, formalized institutes were being founded as centers for instruction and analysis in the extraction industries. The leading eighteenth-century example was the Saxon Bergakademie (Mining Academy) at Freiberg, where Abraham Gottlob Werner (1749–1817) achieved fame as both teacher and theoretician. Similar practical and economic motives lay behind royal support for a French mineralogical survey launched in the 1760s.

Until well into the eighteenth century, the term fossil referred comprehensively to things found in or dug out of the ground. Renaissance naturalists such as the Swiss physician Conrad Gessner (1516–1565) undertook to codify knowledge of fossils, through both observation of specimens and study of texts from Greco-Roman antiquity. Such efforts at literary compilation were echoed by the enthusiasm of collectors (such as the Dane Ole Worm [1588–1654] and the Jesuit polymath Athanasius Kircher [1601?–1680]) for assembling displays of stones, gems, and other "natural antiquities." How stones form, and the possible causative roles played by water or generative seeds in that process, was a central question of early modern natural philosophy. It was perhaps most prominently posed in chemical cosmogonies from Jean Baptiste van Helmont (1579–1644) to Georg Ernst Stahl (1660–1734), and physicians regularly addressed it when explaining the formation of bladder stones. Of obvious relevance was assaying of mineral waters, one of the most frequently treated topics of geological investigation during the sixteenth and seventeenth centuries.

Related to these problems was the prolonged debate concerning the origins of "figured stones," or fossil bodies of regular form. One group of theories attributed these bodies to generative powers or seeds indigenous to the earth—the mineral domain being considered capable of engendering "intrinsic fossils" through its own specific powers, analogous to those of plants or animals. Such theories were effectively modified toward the end of the seventeenth century, particularly by the Danish anatomist Niels Stensen (Nicolaus Steno, 1638–1686) and some contemporaries. While employed at the Tuscan court, Steno recognized that the fossils known as glossopetrae resembled sharks' teeth. In his examination of "solid bodies contained naturally within solids," Steno developed a lucid analysis of the processes of sedimentation and petrifaction whereby an actual tooth or other durable organic part might become preserved within solid rock, thus making it an "extrinsic" fossil object. Extrinsic fossils were treated by many naturalists as relics of the biblical Flood, but such "diluvial" interpretations came under broad attack during the eighteenth century as difficulties multiplied for those viewing fossils as remnants of a single event within the time constraints of orthodox biblical chronology.

Advances in antiquarian scholarship during the seventeenth century, meanwhile, provided new standards for authenticating, dating, and interpreting historical relics and records, whether sacred, civil, or natural (terms such as monument or inscription were commonly applied to both human and natural productions). Thus, increasingly rigorous and critical analytical procedures used to study the human past—often with the aim of confirming historical knowledge found in the Bible—were applied simultaneously to comprehension of the earth's history, extending backward in time from the reconstructed physical geography of the classical era. Finally, as European scholars took Chinese historical records and New World inhabitants into consideration, comparisons of biblical chronology with archaeological and historical discoveries about non-Western peoples yielded doubts about the sufficiency of classical texts, including the Bible, as sources of historical information applicable to all of humanity. Such developments promoted lines of investigation that eventually led to a separation of natural history from civil history, and conviction grew that nature has had a long prehuman history.

Notwithstanding various challenges posed by geological activities and thinking to traditional religious doctrines, pursuit of geological questions up through 1800 proceeded with wide acceptance—often with hearty endorsement—of the presumed consistency of natural knowledge with revealed knowledge. It remained unusual for geological writers to dispute the compatibility of their scientific endeavor with religiously sanctioned belief in the divine superintendence of nature; few geological authors distanced themselves very far from a vision of nature laden with moral meaning.

Eighteenth-Century Developments

While much early modern study of minerals and fossils consisted of examining specimens in the cabinet or museum, an ethos grew emphasizing travel and field observation, especially during the eighteenth century. Notable among the results were sustained efforts to discern the configurations of mountains and the patterns of distribution in their constituent rock masses. Around 1750 a consensus began to develop, distinguishing relatively unstructured and nonfossiliferous "primary" rocks, often found in the core districts of mountain ranges, from the stratified and frequently fossiliferous "secondary" rocks. Whether systematic distinctions between these types of rocks might promise access to a satisfactorily inclusive account of the earth's history since its inception was contested; some thought the evidence indicated a series of changes ("revolutions") of perhaps indeterminate number and scope. In general, a broadly shared sense of satisfaction with real progress in precise description of geological phenomena was not matched with agreement about which phenomena mattered most, or about their proper causal explanation. A strong preference existed for explaining the origins and transformations of most geological features through the agency of water ("Neptunism"), although field investigations were gradually yielding information warranting expanded roles for "fire" or heat. Aqueous agency tended to be seen as ordered and constructive (the organized strata of the earth's crust were, after all, mainly sedimentary), whereas fire was commonly viewed as a cause of disorder and disfigurement. The eighteenth century also witnessed a widening adoption of interpretive attitudes that have in retrospect been called "actualistic": this entailed the presumption that causal explanations should rely only on natural agents of types empirically known to operate. ("Actualism" thus differed from nineteenth-century uniformitarianism, which in addition to presuming continuity of kinds or types of cause also assumed continuity in the rate or intensity of their operation.)

Notwithstanding nineteenth-century attacks on the intellectual consequences of theories of the earth—Charles Lyell argued in Principles of Geology (vol. 1, 1830) that they promoted intellectual indolence—in their post-Cartesian heyday such syntheses or systems tended to serve geological investigation as both motivators for and receptacles of new information and drew attention to geological problems. Whether comprehensive theories constituted good science became increasingly controversial in the second half of the eighteenth century, especially in debates over the merits of theories published by Georges Louis Leclerc Buffon (1707–1788). Late Enlightenment skepticism about geological "systems" helps explain the generally inhospitable reception given the Theory of the Earth (1788, 1795) offered by the deistic Scottish philosopher James Hutton (1726–1797). His was a synthetic perspective on the maintenance of geological conditions propitious for support of life on the earth's surface, through a dynamic equilibrium between internal processes of heat-driven rock consolidation and elevation on one hand and external processes of erosion and deposition on the other (the original expression of what has since come to be known as the geostrophic cycle).

In the last quarter of the eighteenth century the science of geognosy (German Geognosie) made a bid for recognition as the leading means of analyzing mineral phenomena on a local and by extension even a global scale. Geognosy was a method or doctrine taught by Werner, at Freiberg, to an international cadre of students, most of whom were preparing for careers in their respective mining establishments. It elaborated on the litho-stratigraphic insights traceable back to Steno (since adapted and extended by other naturalists), and on skills in mineral identification, to develop recognition of how distinct rock masses relate to one another in subterranean space. Wernerian geognosy produced a key new geological concept, the "formation," defined essentially as a rock mass distinguishable in its lithological character and evident mode of origin, and thus as presumably formed at a given point in time. The formation, as a time-specific rock entity, became the focus of research on the relative positions of differentiated geological elements in the earth's crust (stratigraphy), and thus on their relative ages.

Geology'semergenceasadistinctsciencearound 1800 marked a momentous transformation in the history of Western science: an unprecedentedly definitive investment in nature with a sense of historical development. The classic aim of natural philosophy, prior to this shift of conception, had been confined mainly to the delineation of a presumably fixed order of nature, acting through processes usually believed not to have generated substantially altered configurations in the natural framework or in the objects furnishing it. With the advent of historical geology, the sciences added to their agenda the objective of tracing nature's successive changes. A portentous outcome of this new kind of research was the dawning cognizance, at the end of the eighteenth century, of the reality of biological extinction.

Historiography

The complications of disciplinary history apply with special force to geology in the early modern period; during most of this time no geological discipline existed. At least until recently, histories of geology have most often been written as retrospective accounts of the science's ancestry. Leading historical interpretations, founded by nineteenth- and twentieth-century geologists wishing to understand how their science came to take its modern form (or to use history as a tool to advance their particular conception of the science), tended to yield Whiggish historical accounts assigning credit or blame in accord with the degree to which various figures or scientific approaches contributed to, or obstructed, geology's progress. This kind of history thus tended also to obscure the motivations and intentions of many of the relevant actors, since few of them (at least until the late eighteenth century) conceived of the establishment of geology as their purpose. Genuinely historical recovery of geology's antecedents requires consultation of research literatures addressing the diverse fields in which, looking back, geological topics are seen to have been treated. Some of the better modern historical research—carried out largely within the "retrospective" tradition, but in calculated avoidance of Whig history—has called into question a long-standing Anglophone tendency to honor British over Continental strands in early geology's development, and to redress heavy emphasis on the physical and historical features of certain theories of the earth as preludes to geology, in favor of greater roles for descriptive and chemical-mineralogical enterprises (cf. Laudan). Modern scholarship has also tended to draw back from an earlier inclination to identify a single founder or "father" of geology—Hutton was long a British favorite, Werner a Continental one—and to see in geology, instead, a creature of multiple parentage.

Bibliography

Ellenberger, François. Histoire de la géologie. 2 vols. Paris, 1988–1994.

Gohau, Gabriel. Les sciences de la terre aux XVIIe et XVIIIe siècles: Naissance de la géologie. Paris, 1990.

Jardine, N., J. A. Secord, and E. C. Spary, eds. Cultures of Natural History. Cambridge, U.K., and New York, 1996.

Laudan, Rachel. From Mineralogy to Geology: The Foundations of a Science, 1650–1830. Chicago, 1987.

Oldroyd, David R. Thinking about the Earth: A History of Ideas in Geology. Cambridge, Mass., 1996.

Porter, Roy. The Making of Geology: Earth Science in Britain, 1660–1815. Cambridge, U.K., and New York, 1977.

Rappaport, Rhoda. "The Earth Sciences." In The Cambridge History of Science. Vol. 4: Eighteenth-Century Science, edited by Roy Porter, pp. 417–435. Cambridge, U.K., and New York, 2003.

——. When Geologists Were Historians, 1665–1750. Ithaca, N.Y., 1997.

Rossi, Paolo. The Dark Abyss of Time: The History of the Earth and the History of Nations from Hooke to Vico. Chicago, 1984. Translation of I segni del tempo (1979) by Lydia G. Cochrane.

Rudwick, Martin J. S. The Meaning of Fossils: Episodes in the History of Palaeontology. 2nd ed. Chicago, 1985.

Schneer, Cecil J., ed. Toward a History of Geology. Cambridge, Mass., 1969.

—KENNETH L. TAYLOR, KERRY V. MAGRUDER

n.

The science of the earth's crust -- to which, doubtless, will be added that of its interior whenever a man shall come up garrulous out of a well. The geological formations of the globe already noted are catalogued thus: The Primary, or lower one, consists of rocks, bones or mired mules, gas-pipes, miners' tools, antique statues minus the nose, Spanish doubloons and ancestors. The Secondary is largely made up of red worms and moles. The Tertiary comprises railway tracks, patent pavements, grass, snakes, mouldy boots, beer bottles, tomato cans, intoxicated citizens, garbage, anarchists, snap-dogs and fools.

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The science devoted to the study of the Earth, particularly the solid Earth and the rocks that compose it.

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Key topics on
Geology
Overview Outline of geology
Dating methods Relative dating · Absolute dating Radiometric dating · Geochronology
Сomposition and structure Geochemistry · Crystallography · Mineralogy · Petrography · Petrology
Historical Geology Overview Stratigraphy · Paleontology · Paleoclimatology
Motion Structural geology · Geodynamics · Plate tectonics · Geomorphology
Hydrogeology Overview Engineering geology · Glaciology · Oceanography
Geophysics Overview Geodesy · Geomagnetism · Geophysical surveying · Mathematical geophysics · Seismology · Tectonophysics
History of Geologic science History of geology · Timeline of geology
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v t e

For the scientific journal, see Geology (journal).

Students examining the Wasatch Fault near Salt Lake City, Utah.

Geology (from the Greek γῆ, gê, "earth" and λόγος, logos, "study") is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates. In modern times, geology is commercially important for mineral and hydrocarbon exploration and for evaluating water resources; is publicly important for the prediction and understanding of natural hazards, the remediation of environmental problems, and for providing insights into past climate change; plays a role in geotechnical engineering; and is a major academic discipline.

Contents 1 History 2 Geologic time 2.1 Important milestones 2.2 Brief time scale 2.3 Relative and absolute dating 2.3.1 Relative dating 2.3.2 Absolute dating 3 Geologic materials 3.1 Rock 3.2 Unconsolidated material 4 Whole-Earth structure 4.1 Plate tectonics 4.2 Earth structure 5 Geological development of an area 6 Methods of geology 6.1 Field methods 6.2 Laboratory methods 6.2.1 Petrology 6.2.2 Structural geology 6.2.3 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 Fields or related disciplines 10 Regional geology 10.1 By mountain range 10.2 By nations 10.3 By planet 11 See also 12 Notes 13 External links

History

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 first national-scale geologic map, and by far the most accurate of its time.^[1]

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). In the Roman period, Pliny the Elder wrote in detail of the many minerals and metals then in practical use, and correctly noted the origin of amber.

Some modern scholars, such as Fielding H. Garrison, are of the opinion that modern geology began in the medieval Islamic world.^[2] Abu al-Rayhan al-Biruni (973–1048 CE) was one of the earliest Muslim geologists, whose works included the earliest writings on the geology of India, hypothesizing that the Indian subcontinent was once a sea.^[3] Islamic 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.^{[4][5][unreliable source?]} 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.^[6]

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,^[7] 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".^[8] But according to another source, the word "Geology" comes from the Norwegian, Mikkel Pedersøn Escholt (1600–1699), who was a priest and scholar. Escholt was first used the definition in his book titled, Geologica Norvegica (1657).^[9]

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.^[1]

James Hutton is often viewed as the first modern geologist.^[10] 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 in order 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).

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.

Sir Charles Lyell first published his famous book, Principles of Geology,^[11] in 1830. The 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 100,000 to billions of years.^[12] 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.

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 tectonic theory arose out of 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.^[13]

Geologic time

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 the Earth and Geologic time scale

The geologic time scale encompasses the history of the Earth.^[14] It is bracketed at the old end by the dates of the earliest solar system material at 4.567 Ga,^[15] (gigaannum: billion years ago) and the age of the Earth at 4.54 Ga^[16][17] 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.

Important milestones

• 4.567 Ga: Solar system formation^[15]
• 4.54 Ga: Accretion of Earth^[16][17]
• 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): two snowball Earths
• 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
• 65 Ma: Cretaceous–Paleogene extinction – Dinosaurs die; end of Mesozoic and beginning of Cenozoic
• c. 7 Ma – Present: Hominins
  • 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

Brief time scale

The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. The Holocene (the latest epoch) is too small to be shown clearly on this timeline.

Millions of Years

Relative and absolute dating

Geological events can be given a precise date at a point in time, or they can be related to other events that came before and after them. Geologists use a variety of methods to give both relative and absolute dates to geological events. They then use these dates to find the rates at which processes occur.

Relative dating

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 & B) on which rock strata were deposited; D - volcanic dyke (cutting through A, B & C); E - even younger rock strata (overlying C & D); F - normal fault (cutting through A, B, C & E).

Methods for relative dating were developed when geology first emerged as a formal science. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.

The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.^[18] A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."^[19]

The principle of intrusive relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.

The principle of cross-cutting relationships pertains to the formation of faults 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 normal fault or a thrust fault.^[20]

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 a great 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).^[20]

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.^[20]

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.^[21]

Absolute dating

Main articles: Absolute dating, Radiometric dating, and Geochronology

Geologists can also give precise absolute dates to geologic events. These dates are useful on their own, and can also be used in conjunction with relative dating methods or to calibrate relative dating methods.^[22]

A large advance in geology in the advent of the 20th century was the ability to give precise absolute dates to geologic events through radioactive isotopes and other methods. The advent of radiometric dating changed the understanding of geologic time. Before, geologists could only use fossils to date sections of rock relative to one another. With isotopic dates, absolute dating became possible, and these absolute dates could be applied fossil sequences in which there was datable material, converting the old relative ages into new absolute ages.

For many geologic applications, isotope ratios 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.^[23][24] These are used in geochronologic and thermochronologic studies. Common methods include uranium-lead dating, potassium-argon dating and argon-argon dating, and uranium-thorium dating. These methods are used for a variety of applications. Dating of lavas and ash layers can help to date stratigraphy 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 proiles 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 radionucleide 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 young organic material.

Geologic materials

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

Rock

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

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. This 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 and give it a characteristic fabric. The sedimentary rock can then be subsequently turned into a metamorphic rock due to heat and pressure, and the metamorphic rock can be weathered, eroded, deposited, and lithified, 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.

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.

Unconsolidated material

Geologists also study unlithified material, which typically comes from more recent deposits. 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, and is important to some (or many) studies in geomorphology, sedimentology, and paleoclimatology.

Whole-Earth structure

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

Plate tectonics

Main article: 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.

In the 1960s, a series of discoveries, the most important of which was seafloor spreading,^[25][26] showed that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into a number of tectonic plates that move across the plastically deforming, solid, upper mantle, which is called the asthenosphere. 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.

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.^[27] 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,^[28] 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.

Earth structure

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

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 at depth 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 development of an area

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 off shoots 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 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 cause 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 the rock layers are dragged along, forming drag folds, as slip occurs are 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; and 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.

Methods of geology

Geologists use a number of field, laboratory, and numerical modeling methods to decipher Earth history and understand the processes that occur on and in 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.

Washington State Land Forms

Field methods

A standard Brunton Geo compass, used commonly by geologists in 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^[29]
  • Structural mapping: the locations of the major rock units and the faults and folds that led to their placement there.
  • Stratigraphic mapping: the locations of sedimentary facies (lithofacies and biofacies) or the mapping of isopachs of equal thickness of sedimentary rock
  • Surficial mapping: the locations of soils and surficial deposits
• Surveying of topographic features
  • Creation of topographic maps^[30]
  • 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^[31]
  • These methods include:
    • Shallow seismic surveys
    • Ground-penetrating radar
    • Electrical resistivity tomography
  • They are used for:
    • 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^[32]
  • Collecting samples to:
    • Determine biochemical pathways
    • Identify new species of organisms. These organisms may help to show:
    • 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^[33]
• Glaciology: measurement of characteristics of glaciers and their motion^[34]

Laboratory methods

A petrographic microscope, which is a 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.

Petrology

Main article: Petrology

In addition to the field identification of rocks, 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, thin sections of rock samples are analyzed through 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.^[35] In the electron microprobe, individual locations are analyzed for their exact chemical compositions and variation in composition within individual crystals.^[36] Stable^[37] and radioactive isotope^[38] studies provide insight into the geochemical evolution of rock units.

Petrologists use fluid inclusion data^[39] and perform high temperature and pressure physical experiments^[40] to understand the temperatures and pressures at which different mineral phases appear, and how they change through igneous^[41] and metamorphic processes. This research can be extrapolated to the field to understand metamorphic processes and the conditions of crystallization of igneous rocks.^[42] This work can also help to explain processes that occur within the Earth, such as subduction and magma chamber evolution.

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.

Main article: Structural geology

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 crystal structure of the rocks. They also plot and combine measurements of geological structures in order to better understand the orientations of faults and folds in order 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 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 several 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.^[43] 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.^[44] 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.^[45] This helps to show the relationship between erosion and the shape of the mountain range. These studies can also give useful information about pathways for metamorphism through pressure, temperature, space, and time.^[46]

Stratigraphy

Main article: Stratigraphy

Exploration geologists examining a freshly recovered drill core. Chile, 1994

In the laboratory, stratigraphers analyze samples of stratigraphic sections that can be returned from the field, such as those from drill cores.^[47] Stratigraphers also analyze data from geophysical surveys that show the locations of stratigraphic units in the subsurface.^[48] 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.^[49] Stratigraphers can then use these data to reconstruct ancient processes occurring on the surface of the Earth,^[50] 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.^[47] 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 in order to provide better absolute bounds on the timing and rates of deposition.^[51] Magnetic stratigraphers look for signs of magnetic reversals in igneous rock units within the drill cores.^[47] Other scientists perform stable isotope studies on the rocks to gain information about past climate.^[47]

Planetary geology

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 way as the Earth. This led to the establishment of the field of planetary geology, sometimes known as astrogeology, in which geologic principles are applied to 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 all aspects of the planets, a significant focus is in the search for past or present life on other worlds. This has led to many missions whose purpose (or one of their purposes) is to examine planetary bodies for evidence of life. One of these is the Phoenix lander, which analyzed Martian polar soil for water and chemical and mineralogical constituents related to biological processes.

Applied geology

Economic geology

Main article: Economic geology

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 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,^[52] 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

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.^[53]

Hydrology and environmental issues

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 interactions between natural habitat and the geologic environment. Groundwater hydrology, or hydrogeology, is used to locate groundwater,^[54] which can often provide a ready supply of uncontaminated water and is especially important in arid regions,^[55] and to monitor the spread of contaminants in groundwater wells.^[54][56]

Geologists also obtain data through stratigraphy, boreholes, core samples, and ice cores. Ice cores^[57] and sediment cores^[58] are used to for paleoclimate reconstructions, which tell geologists about past and present temperature, precipitation, and sea level across the globe. These data are our primary source of information on global climate change outside of instrumental data.^[59]

Natural hazards

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.^[60] 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

Fields or related disciplines

• Earth science
• Economic geology
  • Mining geology
  • Petroleum geology
• Engineering geology
• Environmental geology
• Geoarchaeology
• Geochemistry
  • Biogeochemistry
  • Isotope geochemistry
• Geochronology
• Geodetics
• Geography
• Geological modelling
• Geometallurgy
• Geomicrobiology
• Geomorphology
• Geomythology
• Geophysics
• Glaciology
• Historical geology
• Hydrogeology
• Meteorology
• Mineralogy
• Oceanography
  • Marine geology
• Paleoclimatology
• Paleontology
  • Micropaleontology
  • Palynology
• Petrology
• Petrophysics
• Plate tectonics
• Sedimentology
• Seismology
• Soil science
  • Pedology (soil study)
• Speleology
• Stratigraphy
  • Biostratigraphy
  • Chronostratigraphy
  • Lithostratigraphy
• Structural geology
• Volcanology

Regional geology

By mountain range

• Geology of the Alps
• Geology of the Andes
• Geology of the Appalachians
• Geology of the Himalaya
• Geology of the Rocky Mountains

By nations

• Geology of Australia
  • Geology of the Australian Capital Territory
  • Geology of Tasmania
  • Geology of Victoria
  • Geology of the Yilgarn Craton
• Geology of China
• Geology of Hong Kong
• Geology of Europe
  • Geology of Iberia
  • Geology of Iceland
  • Geology of the Netherlands
  • Geology of Norway
  • Geology of Sweden
    • Geology of Gotland
  • Geology of the United Kingdom
    • Geology of England
      • Geology of Cheshire
      • Geology of Cornwall
        • Geology of Lizard, Cornwall
      • Geology of Dorset
      • Geology of Gloucestershire
      • Geology of Hampshire
      • Geology of East Sussex
      • Geology of Hertfordshire
      • Geology of Shropshire
      • Geology of Somerset
      • Geology of Yorkshire
    • Geology of Scotland
      • Geology of Orkney
    • Geology of Wales
    • Geology of Jersey
    • Geology of Guernsey
• Geology of South America
  • Geology of Bolivia
  • Geology of Chile
  • Geology of Colombia
  • Geology of the Falkland Islands
• Geology of India
  • Geology of Sikkim
• Geology of Japan
• Geology of the Philippines
• Geology of New Zealand
• Geology of Vietnam
• Geology of the United States of America
  • US geology by state:
    • Geology of Alabama
    • Geology of Connecticut
    • Geology of Delaware
    • Geology of Florida
    • Geology of Georgia
    • Geology of Idaho
    • Geology of Illinois
    • Geology of Iowa
    • Geology of Kansas
    • Geology of Massachusetts
    • Geology of Minnesota
    • Geology of New Jersey
    • Geology of Oklahoma
    • Geology of Oregon
    • Geology of Pennsylvania
    • Geology of Tennessee
    • Geology of Texas
    • Geology of West Virginia
  • US Geology by region or feature:
    • Geology of the Appalachians
    • Geology of the Pacific Northwest
    • Geology of the Bryce Canyon area(Utah)
    • Geology of the Canyonlands area (Utah)
    • Geology of the Capitol Reef area (Utah)
    • Geology of the Death Valley area (California)
    • Geology of the Grand Canyon area (Arizona)
    • Geology of the Grand Teton area (Wyoming)
    • Geology of the Lassen area (California)
    • Geology of Mount Adams (Washington)
    • Geology of Mount Shasta (California)
    • Geology of the Yosemite area (California)
    • Geology of the Zion and Kolob canyons area (Utah)
    • Glacial geology of the Genesee River (New York, Pennsylvania)

By planet

• Geology of Mars
• Geology of Mercury
• Geology of the Moon
• Geology of Venus

See also

	Environment portal
	Ecology portal
	Earth sciences portal
	Energy portal

Main article: Outline of geology

• Agrogeology
• Digital geologic mapping
• Geologic modeling
• Geoprofessions
• Glossary of geology terms
• International Union of Geological Sciences (IUGS)
• List of fossil sites (with link directory)
• List of geology topics
• List of Russian geologists
• List of important publications in geology
• List of minerals
• List of rock textures
• List of rock types
• List of soil topics
• Paleorrota
• Timeline of geology

Notes

1. ^ ^{a b} Simon Winchester ; (2002). The map that changed the world: William Smith and the birth of modern geology. New York, NY: Perennial. ISBN 0-06-093180-9. 
2. ^ "The Saracens themselves were the originators not only of algebra, chemistry, and geology, but of many of the so-called improvements or refinements of civilization, such as street lamps, window-panes, fireworks, stringed instruments, cultivated fruits, perfumes, spices, etc." (Fielding H. Garrison, An introduction to the history of medicine, W.B. Saunders, 1921, p. 116)
3. ^ Asimov, M. S.; Bosworth, Clifford Edmund, eds. The Age of Achievement: A.D. 750 to the End of the Fifteenth Century : The Achievements. History of civilizations of Central Asia. pp. 211–214. ISBN 978-92-3-102719-2. 
4. ^ Toulmin, S. and Goodfield, J. (1965), ’The Ancestry of science: The Discovery of Time’, Hutchinson & Co., London, p. 64
5. ^ Munin M. Al-Rawi (November 2002) (pdf). The Ancestry of Science: The Discovery of Time (Report). Manchester, UK: Foundation for Science Technology and Civilisation. Publication 4039. http://www.muslimheritage.com/uploads/ibnsina.pdf. Retrieved April 2012. 
6. ^ Needham, Joseph (1986). Science and Civilization in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth. Taipei: Caves Books, Ltd.. pp. 603–604. 
7. ^ Four centuries of the word geology: Ulisse Aldrovandi 1603 in Bologna
8. ^ Winchester, Simon (2001). The Map that Changed the World. HarperCollins Publishers. p. 25. ISBN 0-06-093180-9. 
9. ^ Kermit H., (2003). Niels Stensen, 1638-1686: the scientist who was beatified. Gracewing Publishing. p. 127.
10. ^ James Hutton: The Founder of Modern Geology, American Museum of Natural History
11. ^ Charles Lyell. (1991). Principles of geology. Chicago: University of Chicago Press. ISBN 978-0-226-49797-6. 
12. ^ 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. 
13. ^ Dalrymple, G.B. (1991). The Age of the Earth. California: Stanford University Press. ISBN 0-8047-1569-6. 
14. ^ International Commission on Stratigraphy
15. ^ ^{a b} Amelin, Y; Krot, An; Hutcheon, Id; Ulyanov, Aa (Sep 2002). "Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions.". Science 297 (5587): 1678–83. Bibcode 2002Sci...297.1678A. doi:10.1126/science.1073950. ISSN 0036-8075. PMID 12215641. 
16. ^ ^{a b} Patterson, C., 1956. “Age of Meteorites and the Earth.” Geochimica et Cosmochimica Acta 10: p. 230-237.
17. ^ ^{a b} G. Brent Dalrymple (1994). The age of the earth. Stanford, Calif.: Stanford Univ. Press. ISBN 0-8047-2331-1. 
18. ^ Reijer Hooykaas, Natural Law and Divine Miracle: The Principle of Uniformity in Geology, Biology, and Theology, Leiden: EJ Brill, 1963.
19. ^ Levin, Harold L. (2010). The earth through time (9th ed.). Hoboken, N.J.: J. Wiley. p. 18. ISBN 978-0-470-38774-0. 
20. ^ ^{a b c} Olsen, Paul E. (2001). "Steno's Principles of Stratigraphy". Dinosaurs and the History of Life. Columbia University. http://rainbow.ldeo.columbia.edu/courses/v1001/steno.html. Retrieved 2009-03-14. 
21. ^ As recounted in Simon Winchester, The Map that Changed the World (New York: HarperCollins, 2001), pp. 59–91.
22. ^ Tucker, R. D.; Bradley, D. C.; Ver Straeten, C. A.; Harris, A. G.; Ebert, J. R.; McCutcheon, S. R. (1998). "New U–Pb zircon ages and the duration and division of Devonian time". Earth and Planetary Science Letters 158 (3–4): 175. Bibcode 1998E&PSL.158..175T. doi:10.1016/S0012-821X(98)00050-8.  edit
23. ^ Hugh R. Rollinson (1996). Using geochemical data evaluation, presentation, interpretation. Harlow: Longman. ISBN 978-0-582-06701-1. 
24. ^ Gunter Faure. (1998). Principles and applications of geochemistry : a comprehensive textbook for geology students. Upper Saddle River, NJ: Prentice-Hall. ISBN 978-0-02-336450-1. 
25. ^ H. H. Hess, "History Of Ocean Basins" (November 1, 1962). IN: Petrologic studies: a volume in honor of A. F. Buddington. A. E. J. Engel, Harold L. James, and B. F. Leonard, editors. [New York?]: Geological Society of America, 1962. pp. 599–620.
26. ^ Kious, Jacquelyne; Tilling, Robert I. (February 1996). "Developing the Theory". This Dynamic Earth: The Story of Plate Tectonics. Kiger, Martha, Russel, Jane (Online ed.). Reston, Virginia, USA: United States Geological Survey. ISBN 0-16-048220-8. http://pubs.usgs.gov/gip/dynamic/understanding.html. Retrieved 13 March 2009. 
27. ^ Kious, Jacquelyne; Tilling, Robert I. (February 1996). "Understanding Plate Motions". This Dynamic Earth: The Story of Plate Tectonics. Kiger, Martha, Russel, Jane (Online ed.). Reston, Virginia, USA: United States Geological Survey. ISBN 0-16-048220-8. http://pubs.usgs.gov/gip/dynamic/understanding.html. Retrieved 13 March 2009. 
28. ^ Origin of continents and oceans. S.l.: Dover Pub. 1999. ISBN 0-486-61708-4. 
29. ^ Robert R. Compton. (1985). Geology in the field. New York: Wiley. ISBN 0-471-82902-1. 
30. ^ "USGS Topographic Maps". United States Geological Survey. http://topomaps.usgs.gov/. Retrieved 2009-04-11. 
31. ^ H. Robert Burger, Anne F. Sheehan, Craig H. Jones. (2006). Introduction to applied geophysics : exploring the shallow subsurface. New York: W.W. Norton. ISBN 0-393-92637-0. 
32. ^ ed. by Wolfgang E. Krumbein (1978). Environmental biogeochemistry and geomicrobiology. Ann Arbor, Mich.: Ann Arbor Science Publ.. ISBN 0-250-40218-1. 
33. ^ Ian McDougall, T. Mark Harrison. (1999). Geochronology and thermochronology by the ♯°Ar/©Ar method. New York: Oxford University Press. ISBN 0-19-510920-1. 
34. ^ Bryn Hubbard, Neil Glasser. (2005). Field techniques in glaciology and glacial geomorphology. Chichester, England: J. Wiley. ISBN 0-470-84426-4. 
35. ^ William D. Nesse. (1991). Introduction to optical mineralogy. New York: Oxford University Press. ISBN 0-19-506024-5. 
36. ^ Morton, ANDREW C. (1985). "A new approach to provenance studies: electron microprobe analysis of detrital garnets from Middle Jurassic sandstones of the northern North Sea". Sedimentology 32 (4): 553. Bibcode 1985Sedim..32..553M. doi:10.1111/j.1365-3091.1985.tb00470.x. 
37. ^ Zheng, Y; Fu, Bin; Gong, Bing; Li, Long (2003). "Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime". Earth-Science Reviews 62: 105. Bibcode 2003ESRv...62..105Z. doi:10.1016/S0012-8252(02)00133-2. 
38. ^ Condomines, M; Tanguy, J; Michaud, V (1995). "Magma dynamics at Mt Etna: Constraints from U-Th-Ra-Pb radioactive disequilibria and Sr isotopes in historical lavas". Earth and Planetary Science Letters 132: 25. Bibcode 1995E&PSL.132...25C. doi:10.1016/0012-821X(95)00052-E. 
39. ^ T.J. Shepherd, A.H. Rankin, D.H.M. Alderton. (1985). A practical guide to fluid inclusion studies. Glasgow: Blackie. ISBN 0-412-00601-4. 
40. ^ Sack, Richard O.; Walker, David; Carmichael, Ian S. E. (1987). "Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids". Contributions to Mineralogy and Petrology 96: 1. Bibcode 1987CoMP...96....1S. doi:10.1007/BF00375521. 
41. ^ Alexander R. McBirney. (2007). Igneous petrology. Boston: Jones and Bartlett Publishers. ISBN 978-0-7637-3448-0. 
42. ^ Frank S. Spear (1995). Metamorphic phase equilibria and pressure-temperature-time paths. Washington, DC: Mineralogical Soc. of America. ISBN 978-0-939950-34-8. 
43. ^ Dahlen, F A (1990). "Critical Taper Model of Fold-And-Thrust Belts and Accretionary Wedges". Annual Review of Earth and Planetary Sciences 18: 55. Bibcode 1990AREPS..18...55D. doi:10.1146/annurev.ea.18.050190.000415. 
44. ^ Gutscher, M (1998). "Material transfer in accretionary wedges from analysis of a systematic series of analog experiments". Journal of Structural Geology 20 (4): 407. Bibcode 1998JSG....20..407G. doi:10.1016/S0191-8141(97)00096-5. 
45. ^ Koons, P O (1995). "Modeling the Topographic Evolution of Collisional Belts". Annual Review of Earth and Planetary Sciences 23: 375. Bibcode 1995AREPS..23..375K. doi:10.1146/annurev.ea.23.050195.002111. 
46. ^ Dahlen, F. A.; Suppe, J.; Davis, D. (1984). "Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory". J. Geophys. Res. 89 (B12): 10087–10101. doi:10.1029/JB089iB12p10087. 
47. ^ ^{a b c d} Hodell, David A.; Benson, Richard H.; Kent, Dennis V.; Boersma, Anne; Rakic-El Bied, Kruna (1994). "Magnetostratigraphic, Biostratigraphic, and Stable Isotope Stratigraphy of an Upper Miocene Drill Core from the Salé Briqueterie (Northwestern Morocco): A High-Resolution Chronology for the Messinian Stage". Paleoceanography 9 (6): 835. Bibcode 1994PalOc...9..835H. doi:10.1029/94PA01838. 
48. ^ edited by A.W. Bally. (1987). Atlas of seismic stratigraphy. Tulsa, Okla., U.S.A.: American Association of Petroleum Geologists. ISBN 0-89181-033-1. 
49. ^ Fernández, O.; Muñoz, J. A.; Arbués, P.; Falivene, O.; Marzo, M. (2004). "Three-dimensional reconstruction of geological surfaces: An example of growth strata and turbidite systems from the Ainsa basin (Pyrenees, Spain)". AAPG Bulletin 88 (8): 1049. doi:10.1306/02260403062. 
50. ^ Poulsen, Chris J.; Flemings, Peter B.; Robinson, Ruth A. J.; Metzger, John M. (1998). "Three-dimensional stratigraphic evolution of the Miocene Baltimore Canyon region: Implications for eustatic interpretations and the systems tract model". Geological Society of America Bulletin 110 (9): 1105. doi:10.1130/0016-7606(1998)110<1105:TDSEOT>2.3.CO;2. 
51. ^ Toscano, M; Lundberg, Joyce (1999). "Submerged Late Pleistocene reefs on the tectonically-stable S.E. Florida margin: high-precision geochronology, stratigraphy, resolution of Substage 5a sea-level elevation, and orbital forcing". Quaternary Science Reviews 18 (6): 753. Bibcode 1999QSRv...18..753T. doi:10.1016/S0277-3791(98)00077-8. 
52. ^ Richard C. Selley. (1998). Elements of petroleum geology. San Diego: Academic Press. ISBN 0-12-636370-6. 
53. ^ Braja M. Das. (2006). Principles of geotechnical engineering. England: THOMSON LEARNING (KY). ISBN 0-534-55144-0. 
54. ^ ^{a b} Hamilton, Pixie A.; Helsel, Dennis R. (1995). "Effects of Agriculture on Ground-Water Quality in Five Regions of the United States". Ground Water 33 (2): 217. doi:10.1111/j.1745-6584.1995.tb00276.x. 
55. ^ Seckler, David; Barker, Randolph; Amarasinghe, Upali (1999). "Water Scarcity in the Twenty-first Century". International Journal of Water Resources Development 15: 29. doi:10.1080/07900629948916. 
56. ^ Welch, Alan H.; Lico, Michael S.; Hughes, Jennifer L. (1988). "Arsenic in Ground Water of the Western United States". Ground Water 26 (3): 333. doi:10.1111/j.1745-6584.1988.tb00397.x. 
57. ^ Barnola, J. M.; Raynaud, D.; Korotkevich, Y. S.; Lorius, C. (1987). "Vostok ice core provides 160,000-year record of atmospheric CO2". Nature 329 (6138): 408. Bibcode 1987Natur.329..408B. doi:10.1038/329408a0. 
58. ^ Colman, S.M.; Jones, G.A.; Forester, R.M.; Foster, D.S. (1990). "Holocene paleoclimatic evidence and sedimentation rates from a core in southwestern Lake Michigan". Journal of Paleolimnology 4 (3). doi:10.1007/BF00239699. 
59. ^ Jones, P. D. (2004). "Climate over past millennia". Reviews of Geophysics 42 (2): RG2002. Bibcode 2004RvGeo..42.2002J. doi:10.1029/2003RG000143. 
60. ^ USGS Natural Hazards Gateway

External links

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• GeologyForum.org an online community for fans of everything Geologic!
• James Hutton's Theory of the Earth
• Charles Lyell's Elements of Geology
• Charles Lyell's Principles of Geology, or the Modern Changes of the Earth and its Inhabitants, Considered as Illustrative of Geology
• American Geophysical Union
• European Geosciences Union
• Geological Society of America
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• Geological Society of London

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This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Layers of the Earth

The interior of the Earth has three distinct compositional layers. The thin outermost layer, called the crust, varies in thickness between averages of 5 kilometers (under the oceans) and 31 kilometers (under the continents). It is composed of various kinds of solid rock and sits atop the mantle, the Earth’s middle layer. The mantle is about 2,900 kilometers thick. It is made up primarily of silicate rock at temperatures that vary between 1000°C nearest the crust and 5000°C in the deepest regions. At these temperatures, the rock has little strength and can flow very slowly (centimeters per year) like a very viscous liquid. The innermost layer, the core, has a radius of about 3,480 kilometers, and is made up primarily of iron and nickel. It has two distinct layers as well: the outer one is liquid, and the inner is a rigid mass of superhot iron that spins faster than the rest of the Earth around its axis. It is believed that whirlpool-like currents in the outer core give rise to the Earth’s magnetic field.

The Earth’s outer layer is not a solid shell, but is broken up into many irregular segments known as plates, which extend through the top 100 km. of the earth’s surface, known as the lithosphere. The nine largest plates are the Pacific, North American, South American, Eurasian, Indo-Australian, Antarctic, African, Nazca, and Cocos. The convection movements of the Earth’s mantle appear to create local areas of circulation in the upper mantle called convection cells, which cause a boundary layer of hot, malleable rock called the asthenosphere to flow slowly. The asthenosphere carries the plates of the lithosphere along in a constant motion. These pull apart in some places, collide in others, and in some slide past each other. This movement is known as plate tectonics, and gives rise to many geological phenomena such as earthquakes (caused by sudden movement of plates past each other), mountain ranges (caused by the buckling up of one plate as another pushes under it), and mid-ocean ridges, underwater mountain ranges formed where plates pull apart and hot magma (highly heated rock) wells up from the mantle.

The movement of plate tectonics means that the continents carried on the plates have not always been arranged as we know them. Some 250 million years ago, Earth’s landmasses were all joined into the supercontinent known as Pangea. In the Triassic period, about 200 million years ago, Pangea had divided into the northern mass of Laurasia and southern Gondwanaland. The component plates of these two supercontinents slowly continued to drift apart until the continents reached today’s configuration. This drifting continues.

There are three main types of rock: igneous, sedimentary, and metamorphic. Igneous rock is formed when molten rock (magma) rises through the crust and cools into solid form; it may cool beneath the crust’s surface (intrusive) or above the surface (extrusive). Fine-grained extrusive rock such as basalt results when hot mantle rocks are brought close to the crust and a partial melting occurs. This cools too quickly for large crystals to form. Intrusive rock such as granite forms when pockets of magma cool slowly while trapped underground, giving crystals of quartz, feldspar, and other minerals a chance to grow large.

Sedimentary rock is formed when particles of rock weathered from granite and other rocky masses accumulate and eventually are compacted and cemented together into a new rocky mass. Some examples of sedimentary rock are sandstone, breccia, shale, and limestone.

When sedimentary or igneous rock is subjected to extreme pressure, high temperature, and hot water underground, its mineral constituents are changed, and it transforms into what is known as metamorphic rock. Some examples of metamorphic rock are slate, schist, marble, and gneiss.

Chernicoff, Stanley. Essentials of Geology, Second Edition. Boston: Houghton Mifflin, 2000.
Encyclopedia of Geology. London: Fitzroy Dearborn, 1999.
Kious, Jacqueline, and Robert I. Tilling. This Dynamic Earth: The Story of PlateTectonics. Washington, D.C.: U.S. Government Printing Office, 1996. (also online at http://pubs.usgs.gov/publications/text/dynamic.html)

Dansk (Danish)
n. - geologi

Nederlands (Dutch)
geologie, geologische kenmerken van een regio, geologische verhandeling

Français (French)
n. - géologie

Deutsch (German)
n. - Geologie

Ελληνική (Greek)
n. - γεωλογία

Italiano (Italian)
geologia

Português (Portuguese)
n. - geologia (f)

Русский (Russian)
геология

Español (Spanish)
n. - geología

Svenska (Swedish)
n. - geologi

中文（简体）(Chinese (Simplified))
地质学, 地质概况

中文（繁體）(Chinese (Traditional))
n. - 地質學, 地質概況

한국어 (Korean)
n. - 지질학

日本語 (Japanese)
n. - 地質学, 地質

العربيه (Arabic)
‏(الاسم) علم الجيولوجيا‏

עברית (Hebrew)
n. - ‮גיאולוגיה‬

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