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Gaussian coefficients
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Coefficients of a spherical harmonic expansion of the potential function of the geomagnetic field.
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Glomar Challenger
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A research vessel designed specifically for marine geology studies, including the collection of drill-core samples from the deep ocean floor. In 1968, the vessel embarked on a year-long scientific expedition, criss-crossing the Mid-Atlantic Ridge between South America and Africa and drilling core samples at specific locations. When the age of the samples were determined by paleontologic and isotopic dating studies, they provided the clinching evidence that proved the seafloor spreading hypothesis.
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Gondwana
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A continent formed in the Southern Hemisphere during the Late Paleozoic. It included most of South America, Africa, India, Austrailia, and Antarctica.
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gabbro
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A dark, coarse-grained intrusive igneous rock. Gabbro is made of calcium-rich plagioclase, with amphibole and/or pyroxene, and is chemically equivalent to basalt.
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galvanic interaction
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Redox reaction between two mineral entities with different rest potential, where the mineral with lower rest potential acts as reductant and the mineral with the higher rest potential acts as oxidant/reductant in conveying electron to oxygen or some other suitable acceptor.
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garnet
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Family of silicate mineral containing varying amounts of aluminum, iron, magnesium, and calcium. Schist and gneiss often have tiny, glassy red garnet dodecahedrons.
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gas-to-particle reaction
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The formation of particles by the rapid condensation of a gas.
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geochemistry
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A branch of geology that focuses on the chemical composition of Earth materials.
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geochronology
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The science of absolute dating and relative dating of geologic formation and events, primarily through the measurement of daughter element produced by radioactive decay in mineral.
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geodetic
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Pertaining to measurement of the shape and dimensions of the earth.
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geodimeter
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A surveying instrument to measure the distance between two points on the Earth's surface.
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geoid
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Level (or equipotential) surface at mean sea level. Surface of constant gravitational potential that is chosen to define the earth's shape. At sea the geoid corresponds to the time-averaged surface of the ocean (to an accuracy of 1-2 m).
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geologic cycle
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The sequence through which rock material passes in going from its sedimentary form, through diastrophism and deformation of sedimentary rock, then through metamorphism and eventual melting and magma formation, then through volcanism and plutonism to igneous rock formation, and finally through erosion to form new sediments.
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geomagnetic field
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Main magnetic field of the earth -- the part generated within the earth.
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geomagnetic polarity time scale
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Reference for the polarity history of the geomagnetic field, including current correlations of polarity intervals to biozonations, geologic stage and numerical ages.
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geomorphic cycle
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An idealized model of erosion wherein a plain is uplifted epeirogenically, then dissected by rapid streams (youth), then rounded by downslope movements into a landscape of steep hill (maturity), and finally reduced to a new peneplain at sea level (old age).
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geomorphology
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The study of present-day landforms, including their classification, description, nature, origin development, and relationships to underlying structure. Also the history of geologic changes as recorded by these surface features. The term is sometimes restricted to features produced only by erosion and deposition.
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geophysical noise
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Fluctuations in a geophysical parameter that are of a quasirandom nature and cannot be attributed to fluctuations in known causative phenomena.
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geopotential model
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Set of coefficients of a series representation of the earth's gravitational field in terms of spherical harmonics.
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geopressured systems
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Hydrothermal systems contained in sedimentary rock in regions of normal geothermal gradient. They represent a special case in which the pore fluid are under pressure exceeding the hydrostatic pressure.
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geosyncline
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A major downwarp in the Earth's crust, usually more than 1000 kilometers in length, in which sediment accumulate to thicknesses of many kilometers. The sediments may eventually be deformed and metamorphosed during a mountain-building episode.
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geotherm
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A curving surface within Earth along which the temperature is constant.
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geothermal power
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Power generated by using the heat energy of the earth.
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geyser
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A hot spring that throws hot water and steam into the air. The heat is thought to result from the contact of groundwater with magma bodies.
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glacial abrasion
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A common mechanical weathering process where rock and debris frozen into the sides and bottom of a glacier act like sandpaper and wear down the bedrock the glacier is mowing across.
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glacial maximum
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The position or time of the greatest advance of a glacier (e.g., the greatest equatorward advance of Pleistocene glaciation).
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glacial rebound
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The isostatic adjustment of previously glaciated areas after glacial retreat (e.g., the uplift of Scandinavia after the most recent glaciation.
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glacial striations
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Scratches left on bedrock and boulder by overriding ice, and showing the direction of motion.
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glacial valley
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A valley occupied or formerly occupied by a glacier, typically with a U-shaped profile.
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glacier
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"A mass of land ice that is formed by the cumulative recrystallization of firn. A glacier flows slowly from an accumulation area to an ablation area. Some well-known glaciers are: the Zermatt, Stechelberg, Grinelwald, and Les Diablerets in Switzerland; the Nigards, Gaupne, Fanarak, and Lom in Norway; the Wright Taylor and Wilson Piedmont glaciers in Antarctica; Grinnell glacier in Glacier National Park, Montana, U.S., and the Teton glacier in Teton National Park, Wyoming, U.S."
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glacier surge
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A period of unusually rapid movement of one glacier, sometimes lasting more than a year.
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glaclolacustrine
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Used to describe sediment deposited in a lake in contact with or receiving most of its water and sediments from a glacier, and also landforms resulting from such deposition.
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glass
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A rock formed when magma is too rapidly cooled (quenched) to allow crystal growth.
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glassiness
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The content of extent of glass in an igneous rock.
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gneiss
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A coarse-grained, foliated metamorphic rock that commonly has alternating bands of light and dark-colored mineral.
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gneissic layering
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The tecture typical of gneisses. The foliation is more widely spaced, has less marking, and often more discontinuous than the schistosc texture.
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graben
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An elongate block of rock down-dropped along roughly parallel fault.
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graded bedding
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A bed in which the coarsest particles are concentrated at the bottom and grade gradually upward into fine silt, the whole bed having been deposited by a waning current.
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graded stream
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A stream whose smooth profile is unbroken by resistant ledges, lakes, or waterfalls, and which maintains exactly the velocity required to carry the sediment provided to it.
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gradient
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The down valley slope of a steam bed.
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granite
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A coarse-grained intrusive igneous rock with at least 65% silica. Quartz, plagioclase feldspar and potassium feldspar make up most of the rock and give it a fairly light color. Granite has more potassium feldspar than plagioclase feldspar. Usually with biotite, but also may have hornblende.
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granitic
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"A general term for intrusive igneous rocks that look similar to granite but may range in composition from quartzdiorite to granite. All granitic rocks are light colored; feldspar and quartz are visible in hand specimen. "
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granitization
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The formation of metamorphic granite from other rocks by recrystallization with or without complete melting.
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granodiorite
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An intrusive igneous rock imilar to granite, but contains more plagioclase than potassium feldspar.
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granular snow
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Snow that has been metamorphosed into small granules of ice.
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granulite
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A metamorphic rock with coarse interlocking grain and little or no foliation.
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gravel
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All sedimentary particles larger than two millimeters is called gravel. Gravel is subdivided into pebble, cobble, and boulder.
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gravity
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The force of attraction between to masses pulling them towards the center of each other. The attractive force becomes less with distance. A mountain can attract and deflect a plumb bob from true vertica with the Earth's center.
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gravity anomaly
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The value of gravity left after subtracting from a gravity measurement the reference value based on latitude, and possibly the free-air and Bouguer corrections.
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gravity survey
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The measurement of gravity at regularly-spaced grid points with repetitions to control instrument drift.
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great earthquake
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An earthquake having a magnitude of 8 or greater on the Richter scale.
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greenhouse effect
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The heating of the atmosphere by the absorption of infrared energy re-emitted by the Earth as it receives light energy in the visible band from the Sun.
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greenhouse or icehouse
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Useful polarized concepts of the state of the planet. Greenhouse collects together global warmth, flattened thermal gradient, increased humidity, high sea level and reduced vigor of oceanic circulation with a tendency toward enhanced anoxia in some parts of the ocean. Icehouse is essentially the reverse state in that assemblage of generalizations. It is important to add not-so-icecap-prone and icecap-prone, respectively, without implying necessarily that the world was ever ice-free.
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greenschist
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A metamorphic schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism.
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greenstone
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A metamorphic rock derived from basalt or chemically equivalent rock such as gabbro. Greenstones contain sodium-rich plagioclase feldspar, chlorite, and epidote, as well as quartz. The chlorite and epidote make greenstone green.
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grenz
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A soil horizon, frequently marked by a bed of clay that results from a temporary halt in the accumulation of vegetal material.
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ground moraine
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A glacial deposit of till with no marked relief, interpreted as having been transported at the base of the ice.
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groundwater
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The supply of fresh water found beneath the surface of the Earth (usually in aquifer) that often supplies well and spring.
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grus
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Coarse sand and gravel that form from weathering of granitic rocks
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gully
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A very small channel formed by running water. Gullies hold water for brief periods of time after a rain storm or snow melt.
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guyot
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A flat-topped submerged mountain or sea-mount found in the ocean.
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gyre
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The circular rotation of the waters of each major sea, driven by prevailing winds and the Coriolis effect.
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