Glossary Database

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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

 Glossary Term: 

Definition:

 Gabbro  A dark-colored, coarse-grained rock composed of Ca-plagioclase, pyroxene, and possibly olivine, but no quartz.
 Gait  a manner of walking, stepping, or running (Stein 1966).
 ganoid scales  Diamond-shaped scales which are hard and shiny and form an armor-like covering.
 Garnet  A family of silicate minerals common in many kinds of igneous and metamorphic rocks, as well as in placer deposits. Most garnets are red.
 Gas  The state of matter in which a substance has neither independent shape nor independent volume. Gases can readily be compressed and tend to expand indefinitely.
 General  A general management program applied to lakes and reservoirs and rivers and streams not suited for wild trout or put-and-take trout management. General management waters may provide warm water, cold water, or mixed fisheries. Cold water fishing is primarily managed through stocking of fry, fingerling, or catchable-size fish which are expected to achieve some growth and contribute to the fishery over an extended time. No special regulations are applied.
 Genus (pl. Genera)  A group of related species. The genus name is the first word in the scientific name of a species. For example, the monarch's scientific name is Danaus plexippus. Danaus is the name of the genus the monarch belongs to.
 Genus (plural genera)  A classification group between the species and the family.
 Geochronometry  Measurement of geologic time by methods, such as radiometric dating.
 Geode  A hollow nodule of rock lined with crystals; when separated from the rock body by weathering, it appears as a hollow, rounded shell partly filled with crystal.
 Geographic Information System (GIS)  Go to the What is GIS? page.
 Geography  The study of all aspects of the earth's surface including its natural and political divisions.
 Geologic column  A diagram representing divisions of geologic time and the rock units formed during each major period.
 Geologic cross section  A diagram showing the structure and arrangement of rocks as they would appear in a vertical plane below Earth's surface.
 Geologic map  A map showing the distribution of rocks at Earth's surface.
 Geologic province  An extensive region characterized throughout by similar geologic history or similar structural, petrographic, or physiographic features.
 geologic time scale  The time scale determined by the geologic column and by radiometric dating of rocks.
 Geology  The study of the planet earth, the materials of which it is made, the processes that act on these materials, the products formed, and the history of the planet and its life forms since its origin.
 Geophysical exploration  The exploration of an area in which geophysical properties and relationships are mapped using a geophysical method, such as ground penetrating radar.
 Geothermal  Pertaining to the heat of the interior of Earth.
 Geothermal energy  Energy useful to human beings that can be extracted from steam and hot water found within Earth's crust.
 Geothermal gradient  The rate at which temperature increases with depth.
 Gestation  refers to the length of time between when a fertilized egg implants in the wall of a female's uterus and birth of the young. In many small mammals, such as various mice, gestation is only about a month, while in larger mammals such as deer, it can be about 7 months.
 Geyser  A thermal spring that intermittently erupts steam and boiling water. Example: Old Faithful at Yellowstone National Park.
 gill arch  The bony structure supporting the gill filaments and the gill arches.
 Gill rakers  1.) Finger-like processes on the gill arches in fishes and larval amphibians (Peters 1964). 2.) The rod-like projections on the inside of the gill arches.
 Glacial drift  Transported by glaciers or icebergs.
 Glacial environment  The sedimentary environment of glaciers and their meltwaters.
 Glacial epoch  Any part of geologic time, from Precambrian onward, in which glaciers covered a much larger total land area than those of the present day.
 Glacial valley  Valley shaped by glacial erosion. They are different than valleys formed by streams because they are u-shaped instead of v-shaped, and the floor of the main valley lies well below the floors of the smaller tributary valleys, called hanging valleys.
 Glaciation  The covering of a large area of land by ice; an ice age.
 Glacier  A mass of ice formed from compacted, recrystallized snow that is thick enough to flow plastically.
 Glass  1. A state of matter in which a substance displays many properties of a solid but lacks crystal structure. 2. An amorphous igneous rock formed from a rapidly cooling magma.
 Glassy texture  The texture of igneous rocks in which the material is in the form of natural glass rather than crystal.
 Glossopteris flora  An assemblage of late Paleozoic fossil plants named for the fossil gymnosperm genus Glossopteris, one of the plants in the assemblage. These flora are widespread in South America, Africa, Australia, India, and Antarctica, and provide important evidence for the theory of continental drift.
 Gneiss  A coarse-grained metamorphic rock with a characteristic type of foliation (gneissic layering), resulting from alternating layers of light-colored and dark-colored minerals. Its composition is generally similar to that of granite.
 Gneissic layering  The type of foliation characterizing gneiss, resulting from alternating layers of the constituent silicic and mafic minerals.
 Gondwanaland  The ancient continental landmass that is thought to have split apart during Mesozoic time to form the present-day continents of South America, Africa, India, Australia, and Antarctica.
 Gorget  a patch on the throat of a bird or other animal, distinguished by color, texture, etc. (Stein 1966)
 Graben  An elongate fault block that has been lowered in relation to the blocks on either side.
 Graded bedding  A type of bedding in which each layer is characterized by a progressive decrease in grain size from the bottom of the bed to the top.
 Graded stream  A stream that has attained a state of equilibrium, or balance, between erosion and deposition, so that the velocity of the water is just great enough to transport the sediment load supplied from the drainage basin, and neither erosion nor deposition occurs.
 Gradient (stream)  The slope of a stream channel measured along the course of the stream.
 Grain  A particle of a mineral or rock, generally lacking well-developed crystal faces.
 Graminoids  grasses
 Granite  A coarse-grained igneous rock composed of K-feldspar, plagioclase, and quartz, with small amounts of mafic minerals.
 Granulite  A high-grade metamorphic rock that typically lacks hydrous minerals like micas and amphibole.
 Gravel  A loose mixture of sediment consisting mainly of rounded particles with a diameter greater than 2 millimeters.
 Gravid  Describing a female that is carrying developing offspring within its body (Morris 1992).
 Gravity  The force on any body of matter at or near the earth's surface due to the attraction by the earth and its rotation.
 Gravity anomaly  An area where gravitational attraction is greater or less than its expected value.
 Graywacke  An impure sandstone consisting of rock fragments and grains of quartz and feldspar in a matrix of clay-size particles.
 Grazer  refers to a mammal that predominantly feeds on non-woody vegetation such as grasses and forbs (non-woody leafy plants). Elk are predominantly grazers except for a short period of time during the winter when they are browsers. Typically grazers do eat some browse, and browsers do eat some grass.
 Greenhouse effect  The warming of a planet's atmosphere caused when certain gases (especially water vapor and carbon dioxide) absorb solar energy reflected off the surface.
 Greenschist facies  Metamorphic conditions typified by low temperature and low pressure.
 Greenstone  A low-grade metamorphic rock that commonly has green minerals such as chlorite and talc.
 Gregarious  Living in herds or flock (Guralnik 1970)
 Ground moraine  Glacial deposits that cover an area formerly occupied by a glacier; they typically produce a landscape of low, gently rolling hills.
 Groundmass  The matrix of relatively fine-grained material between the phenocrysts in a porphyritic rock.
 Groundwater  Water below Earth's surface. It generally occurs in pore spaces of rocks and soil. this water will flow freely into a well. See also: aquifer.
 Guano  a natural manure composed of chiefly of bat excrement
 Gular fold  A transverse fold of skin across the throat (Peters 1964).
 Guttural  Produced in the throat; harsh, rasping, etc.: said of sounds (Guralnik 1970).
 Guyot  A sea mount with a flat top.
 Gypsum  An evaporate mineral composed of calcium sulfate with water (CaSO4*2H 20).