   #copyright

Geology

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   World geologic provinces Oceanic crust ██ 0-20 Ma ██ 20-65 Ma ██ >65 Ma
   Geologic provinces ██ Shield ██ Platform ██ Orogen ██ Basin ██ Large
   igneous province ██ Extended crust
   Enlarge
   World geologic provinces
   Oceanic crust ██ 0-20 Ma ██ 20-65 Ma ██ >65 Ma Geologic provinces ██
   Shield ██  Platform ██  Orogen ██  Basin ██ Large igneous province
   ██ Extended crust

   Geology (from Greek γη- (ge-, "the earth") and λογος ( logos, "word",
   "reason")) is the science and study of the solid matter of a celestial
   body, its composition, structure, physical properties, history and the
   processes that shape it. It is one of the Earth sciences. Geologists
   have helped establish the age of the Earth at about 4.6 billion
   (4.6x10^9) years, and have determined that the Earth's lithosphere,
   which includes the crust, is fragmented into tectonic plates that move
   over a rheic upper mantle ( asthenosphere) via processes that are
   collectively referred to as plate tectonics. Geologists help locate and
   manage the earth's natural resources, such as petroleum and coal, as
   well as metals such as iron, copper, and uranium. Additional economic
   interests include gemstones and many minerals such as asbestos,
   perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica,
   as well as elements such as sulphur, chlorine, and helium.

   Planetary geology (sometimes known as Astrogeology) refers to the
   application of geologic principles to other bodies of the solar system.
   However, specialised terms such as selenology (studies of the Moon),
   areology (of Mars), etc., are also in use.

   The word "geology" was first used by Jean-André Deluc in the year 1778
   and introduced as a fixed term by Horace-Bénédict de Saussure in the
   year 1779. The science was not included in Encyclopædia Britannica's
   third edition completed in 1797, but had a lengthy entry in the fourth
   edition completed by 1809. An older meaning of the word was first used
   by Richard de Bury to distinguish between earthly and theological
   jurisprudence.

History

   In China, the polymath Shen Kua (1031 - 1095) formulated a hypothesis
   for the process of land formation: based on his observation of fossil
   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.

   The work Peri lithon (On Stones) by Theophrastus (372 - 287 BC), a
   student of Aristotle, remained authoritative for millennia. Its
   interpretation of fossils was not overturned until after the Scientific
   Revolution. It was translated into Latin and the other languages of
   Europe such as French.

   Georg Agricola ( 1494- 1555)), a physician, wrote the first systematic
   treatise about mining and smelting works, De re metallica libri XII,
   with an appendix Buch von den Lebewesen unter Tage (Book of the
   Creatures Beneath the Earth). He covered subjects like wind energy,
   hydrodynamic power, melting cookers, transport of ores, extraction of
   soda, sulfur and alum, and administrative issues. The book was
   published in 1556.

   Nicolaus 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.

   By the 1700s Jean-Etienne Guettard and Nicolas Desmarest hiked central
   France and recorded their observations on geological maps; Guettard
   recorded the first observation of the volcanic origins of this part of
   France.

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

   James Hutton is often viewed as the first modern geologist. In 1785 he
   presented a paper entitled Theory of the Earth to the Royal Society of
   Edinburgh. In his paper, he explained his theory that the Earth must be
   much older than had previously been supposed in order to allow enough
   time for mountains to be eroded and for sediment 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).
   The geologist, 19th century painting by Carl Spitzweg.
   Enlarge
   The geologist, 19th century painting by Carl Spitzweg.

   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, who believed that all
   rocks had settled out of a large ocean whose level gradually dropped
   over time.

   In 1811 Georges Cuvier and Alexandre Brongniart published their
   explanation of the antiquity of the Earth, inspired by Cuvier's
   discovery of fossil elephant bones in Paris. To prove this, they
   formulated the principle of stratigraphic succession of the layers of
   the earth. They were independently anticipated by William Smith's
   stratigraphic studies on England and Scotland.

   Sir Charles Lyell first published his famous book, Principles of
   Geology, in 1830 and continued to publish new revisions until he died
   in 1875. He 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.
   Plate tectonics - seafloor spreading and continental drift illustrated
   on relief globe of the Field Museum Enlarge
   Plate tectonics - seafloor spreading and continental drift illustrated
   on relief globe of the Field Museum

   By 1827 Charles Lyell's Principles of Geology reiterated Hutton's
   uniformitarianism, which influenced the thought of Charles Darwin.

   19th Century geology revolved around the question of the Earth's exact
   age. Estimates varied from a few 100,000 to billions of years. The most
   significant advance in 20th century geology has been the development of
   the theory of plate tectonics in the 1960s. Plate tectonic theory arose
   out of two separate geological observations: seafloor spreading and
   continental drift. The theory revolutionised the Earth sciences.

   The theory of continental drift was proposed by Alfred Wegener in 1912
   and by Arthur Holmes, but wasn't broadly accepted until the 1960s when
   the theory of plate tectonics was developed.

Important principles of geology

   There are a number of important principles in geology. Many of these
   involve the ability to provide the relative ages of strata or the
   manner in which they were formed.

   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.

   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 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. 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."

   The principle of original horizontality states that the deposition of
   sediments occurs as essentially horizontal beds. Observation of modern
   marine and nonmarine sediments in a wide variety of environments
   supports this generalisation (although cross-bedding is inclined, the
   overall orientation of cross-bedded units is horizontal).

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

   The principle of faunal succession is based on the appearance of
   fossils in sedimentary rocks. As organisms exist at the same time
   period throughout the world, their presence or (sometimes) absence may
   be used to provide a relative age of the formations in which they are
   found. Based on principles laid out by William Smith almost a hundred
   years before the publication of Charles Darwin's theory of evolution,
   the principles of succession were developed independently of
   evolutionary thought. The principle becomes quite complex, however,
   given the uncertainties of fossilisation, the localisation 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.

Fields or related disciplines

   An illustrated depiction of a syncline and anticline commonly studied
   in Structural geology and Geomorphology.
   Enlarge
   An illustrated depiction of a syncline and anticline commonly studied
   in Structural geology and Geomorphology.
     * Earth science
     * Economic geology
          + Mining geology
          + Petroleum geology
     * Engineering geology
     * Environmental geology
     * Geoarchaeology
     * Geochemistry
          + Biogeochemistry
          + Isotope geochemistry
     * Geochronology
     * Geodetics
     * Geomicrobiology
     * Geomorphology
     * Geophysics
     * Glaciology
     * Historical geology
     * Hydrogeology or geohydrology
     * Mineralogy
     * Oceanography
          + Marine geology
     * Paleoclimatology
     * Paleontology
          + Micropaleontology
          + Palynology
     * Petroleum Geology
     * Petrology
     * Petrophysics

   Oceanic-continental convergence resulting in subduction and volcanic
   arcs illustrates one effect of plate techtonics.
   Enlarge
   Oceanic-continental convergence resulting in subduction and volcanic
   arcs illustrates one effect of plate techtonics.
     * Plate tectonics
     * Sedimentology
     * Seismology
     * Soil science
          + Pedology (soil study)
     * Speleology
     * Stratigraphy
          + Biostratigraphy
     * Structural geology
     * Volcanology

   Retrieved from " http://en.wikipedia.org/wiki/Geology"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
