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Glossary of terms
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A-B | C-D | E-F | G-H | I-K | L-N | O-Q | R-T | U-Z | [Sources]

gabbro: any of a group of fine- to coarse-grained plutonic igneous rocks, greenish-gray to black, containig pyroxene or olivine and calcic plagioclase feldspar. [AHDOS]

galena: a metallic, gray mineral containing lead and sulfur. It is the most common lead mineral and the chief source of lead. [AHDOS]

gangue: the constituents of a mineral deposit that are not of economic value, in contrast to the ore mineral to be mined. [AHDOS]

garnet: any of a group of hard silicate minerals occurring in many varieties. A common, deep-red, translucent variety is used as a gem and as an abrasive. [AHDOS]

geanticlinal: or or having to do with geanticline. [AHDOS]

geanticline: a general upward flexure of the earth's crust; a major upfold of subcontinental dimensions. [AHDOS]

geobotany: the study of plants in relation to their geological or geographic environment. [AHDOS]

geochemistry: the science dealing with the chemical composition and character of the earth, or of other planets or planet-like celestial bodies; geochemical composition or characteristics. [AHDOS]

geochronology: the science of determining the age of geological formations. [AHDOS]

geochronometry: the measurement of geologic time by means of geochronology. [AHDOS]

geode: a cavity, usually in a sedimentary rock, lined with crystals or other mineral matter; a rock containing such a cavity. [AHDOS]

geohydrology: the branch of geology dealing with water formed or running underground. [AHDOS]

geoid: a geometric figure of the earth which coincides twith the mean sea level over the oceans and extends continuously through the continents. A geoid has the surface at every point perpendicular to the direction of gravity. [AHDOS]

geoisotherm: an imaginary line or surface passing through points in the interios of the earth that have the same mean temperature. Also called isotherm. [AHDOS]

geologic or geological: of or having to do with geology. [AHDOS]

geologic time: the time of existence of the earth before the ages of human history, as studied by historical geology. [AHDOS]

geology: the science that deals with the earth, the layers of which it is composed, and their history. [AHDOS]

geomorphology: the study of the surface features of the earth or of a celestial body, with reference to their arrangement, origin, development, etc. [AHDOS]

geopressured: under great pressure from geologic forces. [AHDOS]

geoscience: any science dealing with the earth, as geology, geophysics, and geochemistry. [AHDOS]

geosyncline: a broad, elongated, downward curve or flexure of the earth's crust. [AHDOS]

geotechtonic: of or having to do with the structure or the arrangment of the rock strata composing the crust of the earth. [AHDOS]

geothermal gradient: the increase in the temperature of the earth from its surface downward to the core, estimated to be 1 degree F per 60 feet. [AHDOS]

geyser: a natural spring that spouts a column of hot water and steam into the air at frequent and irregular intervals. [AHDOS]

geyserite: a variety of opaline silica deposited about the openings of hot springs and geysers. [AHDOS]

glacial: of ice or glaciers; having much ice or many glaciers; of or having to do with a glacial period or epoch. [AHDOS]

glacial drift: clay, gravel, sand, or other rock material transported and deposited by a glacier. [AHDOS]

glacial epoch: any of the times when much of the earth was covered with glaciers; an ice age; the most recent time when much of the Northern Hemisphere was covered with glaciers; the ice Age or Pleistocene. [AHDOS]

glacial period: the period that includes the glacial epochs.

glaciate: to cover with ice or glaciers; to act on by ice or glaciers. [AHDOS]

glacier: a large mass of ice moving very slowly down a mountain or along a valley, or spreading slowly over a large area of land. Glaciers are formed over many years from show wherever winter snowfall exceeds summer melting. [AHDOS]

glaciolacustrine: of or produced by glaciers and their related lakes. [AHDOS]

glaciology: the science that deals with glaciers and glacial action. [AHDOS]

gneiss: any of various regional metamorphic rocks consisting of coarse layers of quartz and feldspar alternating with layers of mafic minerals. [AHDOS]

Gondwana or Gondwanaland: the protocontinent of the Southern Hemisphere, a hypothetical land mass comprising india, Australia, Antarctica, Africa, and South America, which according to the theory of plate techtonics began to break up into the present continents at the end of the Mesozoic era. [AHDOS]

gorge: a deep, narrow passage between mountains, usually steep and rocky; a deep ravine or canyon, usually a river or stream flowing though it. [AHDOS]

gossan: an exposed, oxidized part of a mineral vein, especially a reddish-colored outcrop of iron ore consisting largely of limonite.

graben: a narrow, low-lying trough of land formed between two nearly parallel faults by the depression of a block of the earth's crust. [AHDOS]

gradation: the process by which the land is leveled off through the action of wind, water, etc. [AHDOS]

grade: a level or sloping surface. [AHDOS]

granite: a plutonic igneous rock consisting chiefly of quartz and feldspar, and usually also containing biotite. [AHDOS]

granule: a rounded rock fragment larger than a grain of sand but smaller than a pebble. [AHDOS]

graphite: a soft, black mineral with a metallic luster, consisting of carbon. Graphite is used for lead in pencils and for lubricating machinery. [AHDOS]

gravel: pebbles and rock fragments coarser than sand. [AHDOS]

graywacke: a sedimentary rock, a coarse sandstone or fine conglomerate consisting of rounded fragments of rocks and minerals bound in a fine-grained matrix. [AHDOS]

greisen: an altered granitic rock with a coarse granular texture, consisting chiefly of quartz and mica. [AHDOS]

grit: very fine bits of gravel or sand in sharply angular particles; a coarse sandstone. [AHDOS]

groundmass: the crystalline, granular, or glassybase of a porphyry, in which the more prominent crystals are embedded. [AHDOS]

ground truth: geological data obtained by direct examination of features on the ground in order to verify information gathered by satellite or other airborne means. [AHDOS]

ground water: water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper level of this saturated zone is called the water table. [AHDOS]

gulch: a deep, narrow ravine with steep sides, especially one marking the course of a stream or torrent. [AHDOS]

gumbo: a fine-grained soil that contains much clay and becomes sticky and plastic when wet. It is found in the southerm central and western parts of the United Staets. [AHDOS]

gummite: a reddish-yellow misture of hydrous oxides of uranium, thorium and lead, usually formed by the alteration of uraninite. [AHDOS]

guyot: a seamount with a platform top. [AHDOS]

gypsiferous: containig or yielding gypsum. [AHDOS]

gypsum: a mineral used to make fertilizer and plaster of Paris; hydrous calcium sulfate. Alabaster is one form of gypsum. [AHDOS]

hanging wall: the side of rock that lies above a fault, vein, lode, etc. [AHDOS]

hardpan: compact, impervious, often clayey subsoil through whihc roots cannot gow. [AHDOS]

hematite: a widely distributed mineral which is an important iron ore, occurring in crystalline, massive, or granular form, and reddish-brown when powdered. [AHDOS]

hemimorphite: a mineral, a silicate of zinc, occurring in orthorhombiccrystals. [AHDOS]

Hercynian: of or having to do with a series of mountain-building fractures and folds of the late Paleozoic, especially in southern Europe and northern Africa. [AHDOS]

heulandite: a zeolite containing calcium, found in crystals of various colors with pearly luster. [AHDOS]

hiddenite: a transparent, emerald-green variety of spodumene, sometimes used as a gem. [AHDOS]

highland: land that is higher or hillier than the neighboring land; region high above sea level; plateau. [AHDOS]

hill: a raised part of the earth's surface, smaller than a mountain, usually having an elevation of less than 300 meters. [AHDOS]

hinterland: the land or region behind a coast.

hogback: a low, sharp ridge with steep sides of nearly equal inclination, formed from the edge of an upturned layer of rock resistant to erosion. [AHDOS]

Holocene: the present epoch of the Cenozoic era, after the Pleistocene epoch; the series of rocks formed during this epoch. [AHDOS]

holoclastic: composed of fragments of sedimentary rock. [AHDOS]

horizontal fault: a fault that has no vertical displacement or dip. [AHDOS]

hornblende: a common black, dark-green or brown silicate mineral containing aluminum and varying proportions of calcium, magnesium and iron. Hornblende is a species of amphibole which is found in many igneous and metamorphic rocks. [AHDOS]

hornfels: a compact, fine-grained metamorphic rock composed of quartz, feldspar, mica and other minerals, formed by the contact action of intrusive rock upon shale or other sedimentary rock. [AHDOS]

horst: an elongate, relatively uplifted part of the earth's crust limited by faults and standing out against its surroundings. [AHDOS]

hot spot: and of various regions of the earth where a plume of molten material in the earth's mantle is carried upwards, heating the crust above. [AHDOS]

hot spring: a spring producing warm water, usually at a temperature above tht of the human body. [AHDOS]

humite: one of a group of basic magnesium silicate minerals resembling each other in physical properties and chemical composition. [AHDOS]

humus: a dark-brown or black part of soil formed from decayed leaves and other vegetable matter, containing valuable plant foods. [AHDOS]

hyaline: glassy, as certain volcanic rocks. [AHDOS]

hyalite: a colorless, transparent or translucent variety of opal, occurring in globular forms resembling drops of melted glass. [AHDOS]

hydrosphere: the water portion of the earth. [AHDOS]

hydrothermal: of or having to do with heated water, especially with the action of heated water in dissolving and redopositing minerals. [AHDOS]

hydrous: having water as a constituent. [AHDOS]

hydroxylapatite: a mineral composed of calcium phosphate hydroxide that is the basic inorganic consituent of bone and tooth enamel. [AHDOS]

hypabyssal: of or having to do with rocks that have crystallized under conditions intermediate between the plutonic and extrusive environments. [AHDOS]

hypersthene: a grayish, greenish-black or brown silicate of iron and magnesium; an orthorhombic pyroxene, isomorphous with enstatite. [AHDOS]

hypogene: formed beneath the surface of the earth. [AHDOS]


Ed resources
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Geologic time scale

Geology museums

Glossary of terms

Mineral identification

MOHS hardness scale

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US State Parks

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