Gabbro - An intrusive igneous rock that develops from mafic magma and whose mineral crystals are coarse. Mineralogically, this rock is identical to basalt.
Gaia Hypothesis - The Gaia hypothesis states that the temperature and composition of Earth's surface are actively controlled by life on the planet. It suggests that if changes in the gas composition, temperature, or oxidation state of the Earth’s atmosphere are induced by astronomical, biological, lithological, or other perturbations, life responds to these changes by growth and metabolism.
Galaxy - A celestial body found in the Universe. Consists of a dense grouping of gas, dust, and millions to trillions of stars orbiting a central point in space because of gravity.
Galaxy Group - A gravity-produced spatial clustering of 50 or fewer galaxies somewhere in the Universe. These celestial bodies are often part of large aggregations of galaxy groups called superclusters. The Milky Way Galaxy is part of the Local Group.
Gale - A term used in the Beaufort Wind Scale and by the United States National Weather Service to describe wind speed. The National Weather Service defines a gale as a wind with a sustained speed of 63 to 87 kilometers per hour, 17.5 to 24.2 meters per second, or 39 to 54 miles per hour. The Beaufort Wind Scale defines a gale in the following way: 7: ModerateGale (50 to 61 kilometers per hour or 32 to 38 miles per hour), 8: FreshGale (62 to 74 kilometers per hour or 39 to 46mph), 9: StrongGale (75 to 88 kilometers per hour or 47 to 54mph) and 10: Storm/WholeGale (89 to 102 kilometers per hour or 55 to 63mph).
Galena - Common name for the mineral lead sulfide (PbS). Ore is often used to obtain the chemical element lead (Pb). Galena is one of the most common sulfide minerals and is often associated with the minerals sphalerite, calcite, and fluorite. Some galena deposits contain about 1–2% silver.
Gall-Peters Map Projection - A map projection system that reduces the area distortion found in Mercator map projections.
Gallery Forest - A border of forest found along a stream in a habitat that is usually too dry for tree growth. The higher soil moisture levels found near the stream are required for the trees to grow.
Gamma Diversity - A measure of total species diversity in a landscape as measured by alpha diversity and beta diversity, such that gamma diversity = alpha diversity X beta diversity. This measure was introduced by ecologist R. H. Whittaker. Compare with alpha diversity and beta diversity.
Gamete - A haploid reproductive cell that fuses with a similar type of cell during fertilization to create a diploid embryonic organism.
Gamma Radiation - A type of ionizing, electromagnetic radiation that readily penetrates the body tissues of organisms. This form of radiation has a wavelength of less than 0.03 nanometers.
Gap - A spatial opening in a plant community. A gap can be produced by the natural death or by some other abiotic or biotic disturbance.
Gap Species - A threatened or endangered species that has no level of protection in its geographic range. This lack of protection may occur because this species is not found in the current global network of national parks, sanctuaries, refuges, and other protected areas for biodiversity.
Gas - A state of matter where molecules are free to move in any direction they like. A state of matter where the substance will completely fill any container that it occupies.
Gauging Station - A location on the course of a stream where measurements related to stream flow and stream discharge are made.
Geest - Alluvial sediments laid down by a stream in the distant past that still cover the land surface where they were deposited.
Gelifluction - A form of mass movement in a periglacial environment where a permafrost layer exists. Gelifluction is characterized by the oozing movement of soil material over the permafrost layer and the formation of lobe-shaped deposition features. Also see solifluction.
Gelisols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. This soil is common to high-latitude tundra environments. The main identifying feature of this soil is a permafrost layer within 1 meter of the soil surface.
Gemstone (Gem) - A mineral or rock that is of value to humans. Gemstones are usually classified as precious or semi-precious, with the former commanding a much higher monetary value. Traditionally, in the West, diamonds, rubies, sapphires, and emeralds have been considered precious gemstones.
Gendarmes - Tall pointed rocks occasionally extending upward from a mountain ridge.
Gene(s) - Organic material that allows organisms to pass on the inheritance of adaptations or traits. In most organisms, these adaptations are encoded in the genetic material DNA (Deoxyribonucleic Acid). New adaptations arise through mutations.
Gene Frequency - The proportion of specific variants of genes that exist in the gene pool of a population of a particular species. A higher gene frequency corresponds to greater genetic diversity.
Gene Pool - The sum of all the genes found in the individuals of a population of a particular species.
Genecology - The ecological study of the relationship of genetic processes to the biotic and abiotic components of the environment.
General Circulation Model (GCM) - A computer-based climate model that produces a short-term future forecast (up to 10 days) of weather and climate conditions for regions of the Earth or the complete planet. GCMs use complex mathematical equations and physical relationships to approximate the operation of atmospheric processes in a three-dimensional grid.
Generalist Species - A species that can survive and tolerate a broad range of environmental conditions.
Genetic Adaptation - Imposed changes in the genetic makeup of organisms within a species due to a mutation. The new adaptations produced by the mutation enable the species to reproduce more successfully and gain a competitive advantage under changed environmental conditions.
Genetic Diversity - The genetic variability found in either a population of a species or all the populations of a species. Also see biodiversity, ecosystem diversity, and species diversity.
Genetic Drift - The change in gene frequency over generations caused by random sampling. For any organism, some individuals and their genes may produce more descendants simply by chance, not because they have higher fitness or are better competitors.
Genus - A category in the classification of organisms. Genus is a taxonomic rank above the species level. It consists of all of the species with similar morphological and physiological characteristics and closely related genetics. Similar genera (plural form of genus) are grouped into a family.
Geochemistry - A sub-discipline of Earth Science that employs the knowledge associated with chemistry to study and explain natural phenomena associated with our planet.
Geocoding - The conversion of features found on an analog map into a computer-digital form, often for use in GIS software. In this process, the spatial locations of the various features are referenced to a coordinate system used by the computer's software.
Geodesy - The science that measures the size, shape, surface configuration, gravity, and other phenomena of the Earth and other celestial bodies in our Universe.
Geographical Coordinate System - A mapping system that uses the measures of latitude and longitude to locate points on the spherical surface of the Earth.
Geographic Information System (GIS) - A geographic information system merges spatial information in a computer database with geographical coordinates on a digital map.
Geographic Cycle - A theory developed by William Morris Davis, beginning in the 1880s, that models the formation of stream-eroded landscapes. This theory suggests that landscapes go through three stages of development (youth, maturity, and old age) and argues that landscape rejuvenation arises from tectonic uplift.
Geographic Isolation - See spatial isolation.
Geographic Range - The spatial distribution of a species. The geographic ranges of species often change over time.
Geography - The study of natural and human-constructed phenomena relative to a spatial dimension.
Geoid - The shape of the Earth in three-dimensions. A geoid represents the correct shape of the Earth, which deviates slightly from a perfect sphere because of a slight bulge at the equator.
Geologic Time Scale - (1) Scale used to measure time relative to events of geological significance. (2) Time scale that occurs over millions and billions of years. The subdivision of geologic time uses the following units: Supereon, Eon, Era, Period, Epoch, and Age. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Geology - The field of knowledge that studies the origin, structure, chemical composition, and history of the Earth and other planets.
Geomagnetic Field - The Earth's magnetic field. This magnetic field extends from Earth's interior into space and is vital for deflecting charged solar particles carried by the solar wind away from our planet's surface. The source of the Earth's magnetic field is a dynamo driven by the motion of the liquid outer core around the solid inner core. We use the geomagnetic field to determine directions on the Earth's surface with a compass.
Geomagnetic Poles - The North and South Poles associated with the Earth's magnetic field. These two magnetic poles are found at 78.5° North, 69° West and 78.5° South, 11° East, respectively.
Geomorphic Threshold - The amount of slow accumulated change a landform can take before it suddenly moves into an accelerated rate of change that takes it to a new system state.
Geomorphological Hazard - A hazard that is the result of natural processes that operate to create and modify landforms. Extreme natural events often trigger geomorphological hazards.
Geomorphology - The field of knowledge that investigates the origin of landforms on the Earth and other planets.
Geophysics - A sub-discipline of Earth Science that employs the knowledge associated with Physics to study and explain natural phenomena related to our planet.
Geophyte - A perennial plant that has its budding tissues found below the ground surface. At the end of the growing season, the above-ground part of the plant will die back. In most cases, this budding tissue is found in a storage organ, such as corms, tubers, tuberous roots, bulbs, or rhizomes. Common examples of such plants include tulips, crocus, lilies, and iris.
Geostationary Orbit - A satellite that has an orbit that keeps it always over the same point on the Earth. To do this, the satellite must travel in space at the same angular velocity as the Earth.
Geostrophic Wind - A horizontal wind found in the upper atmosphere that travels parallel to isobars. Geostrophic wind is the result of a balance between pressure gradient force and the Coriolis effect.
Geothermal Energy - a form of heat energy derived from the Earth's interior.
Geosyncline - A very large depression in the Earth's crust that is 100s of kilometers (miles) in diameter and up to 10 kilometers (6 miles) in depth. These depressions accumulate sediments eroded from surrounding higher elevations.
Germination - The beginning of vegetative growth of a plant from a seed.
Geyser - A spring that periodically releases hot water and steam vertically into the atmosphere. Most of these features are located in areas with ongoing volcanic activity. Close proximity to magma causes water deep below Earth's surface to heat to its boiling point and become pressurized. This pressure is released when the hot water and steam reach the surface. Approximately 1,000 geysers are known on our planet, with half located in Yellowstone National Park, Wyoming, USA.
Gibber - A type of stone pavement found in desert areas and having a ground surface covered with pebbles and boulders.
Glacial (Glaciation) - (1) A time during an ice age when many glaciers advanced because of colder temperatures. Compare with interglacial. (2) Something involving glaciers and moving ice, usually pertaining to processes associated with glaciers.
Glacial Advance - (1) The forward downhill movement of a single snout of a glacier. May also refer to the growth and expansion of many glaciers in a defined region. (2) The growth and spatial expansion of the area covered by a continental glacier.
Glacial Deposit - A deposit of sediment that came directly or indirectly from a glacier or ice sheet.
Glacial Drift - A generic term applied to all glacial and glaciofluvial deposits.
Glacial Erosion - Erosion caused by a glacier or ice sheet.
Glacial Ice - A very dense form of frozen water that is much harder than snow, névé, or firn.
Glacial Lake - A natural impoundment of meltwater at the front of a glacier.
Glacial Landform - Is a landform formed from erosional and depositional processes associated with glaciers. Glacial landforms include the following geomorphologic features: moraines, glacial troughs, cirques, horns, hanging valleys, eskers, drumlins, outwash plains, and kames.
Glacial Maximum - At the continental scale, it refers to the greatest spatial extent of ice sheets during a period of glacial advance during the Pleistocene.
Glacial Milk - A term used to describe glacial meltwater, which has a light-colored or cloudy appearance because of clay-sized sediment held in suspension.
Glacial Outburst - A sudden flood caused by the release of ice-dammed water associated with a glacier.
Glacial Polish - The abrasion of bedrock surfaces by materials carried on the bottom of a glacier. This process leaves these surfaces smooth and shiny.
Glacial Retreat - (1) The backwards uphill movement of a single snout of a glacier. May also refer to the retreat and contraction of many glaciers in a defined region. (2) The retreat and spatial contraction of a continental glacier.
Glacial Surge - A rapid forward movement of the snout of a glacier.
Glacial Trough - A deep U-shaped valley with steep valley walls that formed from glacial erosion. At the base of many of these valleys are cirques.
Glacial Uplift - The upward movement of the Earth's crust following isostatic depression from the weight of the continental glaciers.
Glacial Valley - A valley that was influenced by the presence of glaciers. The cross-section of such valleys tends to be U-shaped because of glacial erosion. Similar to a glacial trough.
Glaciation - A period during an ice age when glaciers and ice sheets are growing in size and spatial extent.
Glacier - A large, long-lasting accumulation of snow and ice that develops on land. Most glaciers flow along topographic gradients because of their weight and gravity. Glaciers can exist at several different spatial scales on Earth. Smaller alpine glaciers are found in most mountain ranges on our planet. The largest glaciers, covering 1000s of square kilometers (miles), are found on Greenland and Antarctica. Also see cirque glacier, alpine glacier, continental glacier, ice cap, ice field, and ice sheet.
Glacieret - A very small glacier that forms from a snow patch.
Glacierization - The gradual covering of a landscape by glaciers because of a cooling climate.
Glaciofluvial - A geomorphic feature whose origin is related to the processes associated with glacial meltwater.
Glaciology - The scientific study of glaciers and ice sheets.
Glaciotectonism - The displacement, folding, and faulting of sediments and bedrock because of the weight associated with massive glaciers.
Glacis - A term of French origin meaning gentle slope.
Glass Sand - A type of sand composed almost entirely of silica. Glass sand is used for making glass because of its purity.
Glauconite - A mineral composed of iron potassium phyllosilicate (mica group). Glauconite has a distinctive green color, low weathering resistance, and is very friable. This mineral is believed to form mainly in marine environments.
Glaze - A thin coating of ice that forms when rain descends on a surface with a temperature below freezing.
Gleization - A soil formation process that occurs in waterlogged and poorly drained environments. This soil process results in the development of an extensive layer of soil organic matter on top of a chemically reduced clay layer that turns blue.
Gley (Glei) - A generic name for a soil that develops in locations that are poorly drained or permanently waterlogged. The horizons of this soil show the chemical signs of oxidation and reduction.
Gleysol Soil - Soil order (type) of the Canadian System of Soil Classification. This soil type is found in habitats that are poorly drained or permanently waterlogged. The horizons of this soil show the chemical signs of oxidation and reduction. For more information on this soil type, see the textbook Canadian System of Soil Classification, 3rd Edition, available online - https://sis.agr.gc.ca/cansis/publications/manuals/1998-cssc-ed3/index.html.
Global Circulation - Usually refers to the large-scale flow of air in the troposphere. Also called planetary circulation.
Global Climate Model (GCM) - Computer-based simulation model that produces a long-term future forecast of the state of weather, climate, vegetation cover, snow cover, soil water content, organic carbon storage, sea ice coverage, ocean currents, ocean heat storage, and sea level conditions for regions of the Earth or the entire planet. GCMs use complex mathematical equations and physical relationships to approximate the operation of processes in the atmosphere, biosphere, land surface, cryosphere, and the ocean in a three-dimensional grid.
Global Dimming - The reduction in the intensity of solar insolation passing through the atmosphere caused by the presence of human-released aerosols. These aerosols reduce sunlight intensity by increasing atmospheric absorption and reflection. Sulfate aerosols are believed to be the main cause of global dimming. Research estimates that a 4% reduction in insolation received globally occurred from 1960 to 1990. Aerosols responsible for global dimming have a strong local component, with the greatest reduction being close to the source of the air pollutants.
Global Environmental Change - The perceived and documented changes occurring to environmental systems on our planet at the global scale. This includes altering various components of the biosphere, hydrosphere, and atmosphere due to human socioeconomic activities. Scientists speculate that future increases in the size of the human population and economic well-being will make these environmental problems more common.
Global Ocean Circulation - Large-scale surface and subsurface ocean currents that exist in Earth's ocean basins. These systems move ocean water in mostly closed loops that are often guided by continental boundaries. Global ocean circulation transports significant amounts of heat energy from the low to the mid and high latitudes. Surface ocean currents are mostly wind-driven and can rotate clockwise or counterclockwise due to the Coriolis effect. Subsurface ocean currents are mainly driven by changes in seawater density and temperature.
Global Positioning System (GPS) - A system used to determine latitude, longitude, and elevation anywhere on or above the Earth's surface. This system involves the transmission of radio signals from many specialized satellites to a handheld receiving unit. The receiving unit uses triangulation to calculate elevation and spatial position on the Earth's surface.
Global Warming - The warming of the Earth's average global temperature because of an increase in the concentration of atmospheric greenhouse gases. A higher concentration of greenhouse gases in the atmosphere is believed to enhance the greenhouse effect, thereby increasing the amount of heat energy.
Globular Cluster - A spherical-shaped, gravity-caused, spatial aggregation of many stars commonly found orbiting the center of a galaxy.
Gloger's Rule - States that, in general, the pigmentation of mammals, birds, and some other animal groups tends to get darker as one moves from the poles to the equator.
Glucose - A simple six-carbon sugar produced by autotrophic organisms as a result of photosynthesis. The chemical formula for glucose is C6H12O6.
Gneiss - A common type of metamorphosed coarse-grained igneous rock. One significant defining characteristic of this rock is the recrystallization of the minerals quartz, feldspars, micas, and amphiboles into long bands.
Gnomonic Map Projection - Is a two-dimensional map projection system that projects the Earth onto a flat surface or plane, displaying great circles as straight lines (see graphic centered on the North Pole). This map projection is based on the Azimuthal Map Projection system, but it is altered to allow one to determine the shortest path (not distance) between two points on a map. This type of map does not allow for the calculation of the shortest distance between two points.
GOES (Geostationary Operational Environmental Satellite) - A series of geostationary meteorological satellites launched by the United States starting in 1968. The primary purpose behind these satellites was to use a variety of remote sensing devices for weather forecasting and environmental monitoring.
Gold - A precious metallic mineral found in veins, as an ore in rock, as nuggets, grains in alluvial deposits, and dissolved in seawater. Often occurs in free elemental form. Chemical abbreviation is Au.
Gondwana (Gondwanaland) - A supercontinent that existed mainly in the Southern Hemisphere between 320 and 550 million years ago. The tectonic separation of this landmass created Africa, South America, Antarctica, India, Australia, and Arabia. Gondwana and Laurasia together formed the Pangaea supercontinent during the Carboniferous (299 to 359 million years ago).
Gorge - A narrow stretch of stream valley that has almost vertical slopes. Often found in mountainous locations.
Graben Fault - A fault produced by tectonic tensional stresses resulting in the subsidence of a block of rock. On a large landscape scale, these features are known as rift valleys.
Grade - (1) The amount of metamorphism found in a rock. (2) The relative quantity of metallic minerals in an ore. (3) The steepness of a slope as measured as a percentage.
Graded Stream - A stream that has a long profile that is in equilibrium with the general slope of the landscape. A graded profile is concave and smooth. A stream can maintain its grade through a balance between erosion, transport, and deposition. Erosion removes material from high points in the profile, and deposition fills in low points.
Gradient - The steepness of a slope as measured in degrees, percentage, or as a distance ratio (rise/run).
Gradient Wind - A horizontal wind in the upper atmosphere that moves parallel to curved isobars. Results from a balance between the pressure gradient force, the Coriolis effect, and the centripetal force.
Grain - (1) The relative textural size of the mineral particles that make up a rock (fine - less than 1 mm, medium - 1 to 3 mm, coarse - 3 to 5 mm, and very coarse - greater than 5 mm), sediment, or a soil (see soil texture) (2) Fracture direction associated with a rock. (3) The topographical trend in a landscape in terms of direction. (4) A measurement unit of weight equal to 0.0648 grams.
Granite - Medium to coarse-grained igneous rock that is rich in quartz and potassium feldspar. Granite contains at least 20 percent quartz by volume. Other minerals common in this rock include muscovite, biotite, and amphibole. It is derived from felsic magma.
Granitic Magma - Felsic magma that generates mainly granitic rocks.
Granodiorite - Medium to coarse-grained igneous rock that is rich in quartz and at least 65% plagioclase feldspar. Quite similar in appearance to granite but darker in color. A granodiorite contains at least 20 percent quartz by volume. Other minerals common in this rock include biotite and hornblende. Derived from intermediate to felsic magma.
Granule - (1) A spherical-shaped soil aggregate. (2) A rock particle that is between 2.0 and 4.0 mm in diameter according to the Wentworth Scale.
Graphic Scale - A way of expressing the scale of a map with a graphic.
Graphite - A naturally occurring mineral form of crystalline carbon that is mainly found in metamorphic rocks where there is a metamorphic reduction of sedimentary carbon. Graphite can also be found in igneous rocks.
Grass (Graminoids) - A type of plant that has long, slender leaves that extend from a short stem or the soil surface. More than 9,000 species of grass exist, including cereals, grains, lawn grass, and bamboo. This group of plants includes many species that are very important to humans. Grasses are used for food for people and their livestock, insulation, clothing, fuel, paper, building materials, and the production of beverages.
Grassland - A type of common ecosystem on our planet whose dominant species are various species of grass. Grasslands are often found in regions where average precipitation is insufficient to support the growth of shrubs or forests.
Graupel - A type of precipitation that consists of a snow crystal and a raindrop frozen together. Also called snow pellets.
Gravel - A term used to describe unconsolidated sediments composed of rock fragments. Gravel rock fragments have a size greater than 2.0 millimeters (0.079 inches) but less than 63 millimeters (2.48 inches) (Wentworth Scale 4.0 to 64 millimeters). The term pebble can be synonymous. Compare with clay, silt, sand, cobble, and boulder.
Gravel Train - See valley train.
Gravimeter - A scientific instrument used to measure specific gravity.
Gravimetric Method - A method used to measure the quantity of water in a soil. Involves weighing the mass of a soil sample before and after oven drying at 105°C (221°F) for 24 hours.
Gravitational Water - Water that moves through soil due to gravitational forces. Soil water in excess of hygroscopic water and capillary water.
Gravity (Gravitation) - A natural process where any body of mass found in the Universe attracts other nearby bodies with a force proportional to the product of their masses and inversely proportional to the distance that separates them. First proposed by Sir Isaac Newton in 1686.
Gravity Wave - is a vertical wave generated in a gas or fluid or the interface between two of these media. Despite its name, this phenomenon is not directly related to gravity. Gravity waves are a common feature of the lower atmosphere and can be produced by mountains, thunderstorms, and fronts, which displace air vertically, producing successive bands of cloud and cloud-free air. Once the air is displaced, it produces a wave that moves away from the point of displacement in an up-and-down motion. The vertical distance between the crests and troughs of these waves becomes smaller and smaller with distance traveled. At some point, the waves will no longer exist.
Grazing Food Chain - Is a highly idealized model that describes the trophic flow of organic energy in a community or ecosystem. This model begins with energy from the Sun, captured by primary producers (mainly autotrophic plants) via photosynthesis, which forms the first trophic level. At the next level of the food chain, a portion of the organic energy in producers is transferred to primary consumers (heterotrophic herbivores) through the consumption of producers. The consumption of primary consumers by secondary consumers (primary carnivores) creates the next trophic level. The graphic model shown ends with a tertiary consumer level that gets its energy by consuming the secondary consumers. Grazing food chains typically only have four to five levels.
Great Circle - An imaginary circle drawn on the surface of a sphere (like the Earth) from a plane running through the center of this object. A great circle divides the sphere into two halves. On the Earth, the equator or any line representing longitude would be examples of a great circle. Also called an orthodrome or Riemannian circle.
Green Mud - A fine-grained deposit composed of a high percentage of glauconite found on the continental slope.
Greenhouse Effect - The greenhouse effect causes the atmosphere to trap more heat energy at the Earth's surface and within the atmosphere by absorbing and reemitting longwave radiation. Of the longwave energy emitted back to space, 90% is intercepted and absorbed by greenhouse gases. Without the greenhouse effect, the Earth's annual mean global temperature would be -18°C (-0.4°F), rather than the present 15°C (59°F). Over the last few centuries, human activities have directly or indirectly increased the concentration of the major atmospheric greenhouse gases. Scientists predict that this increase may enhance the greenhouse effect, making the planet warmer. Some experts estimate that the Earth's annual mean global temperature has already increased by 0.3 to 0.6°C (0.5 to 1.0°F) since the beginning of this century, because of this enhancement.
Greenhouse Gas - The various gases responsible for the greenhouse effect to operate in a planet's atmosphere. On Earth, these gases include water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFXClX), and tropospheric ozone (O3).
Greensand - A sand or sandstone rock that contains a high percentage of glauconite.
Greenwich Mean Time (GMT) - A former standard for determining world time as measured at Greenwich, England (location of the Prime Meridian). Replace in 1928 with Universal Time (UT).
Greenwich Meridian - A (prime) meridian reference standard that passes through Greenwich Royal Observatory, England, and has a longitude measurement of 0°.
Greywacke - A type of sedimentary sandstone that contains a considerable quantity of clay mixed with its sand particles (Image Source: Wikipedia Commons, image is in the public domain). Unlike typical sandstone, these rocks are poorly sorted and contain particles over a wide size range. Greywackes are believed to form because of submarine avalanches or strong turbidity currents at the margin of continental shelves. Greywackes can be grey, brown, yellow, or black in color.
Grid North - The direction of North as measured on the Universal Transverse Mercator grid system.
Grid South - The direction of South as measured on the Universal Transverse Mercator grid system.
Grike - A vertical crack up to 0.5 meters (1.5 feet) wide in limestone bedrock formed when a natural joint is made larger because of solution weathering.
Grivation - The angle measured between Grid North on a map and Magnetic North measured in the field.
Groove - A large striation or scratch on a rock surface because of glacial abrasion.
Gross Primary Productivity - The total amount of organic chemical energy or biomass fixed by the processes of photosynthesis. Compare with net primary productivity.
Gross Secondary Productivity - The total amount of organic chemical energy or biomass assimilated by consumer organisms. Compare with net secondary productivity.
Gross Sediment Transport - The total amount of sediment transported along a shoreline in a specific time.
Ground Fog - Another name for radiation fog.
Ground Frost - Frost that penetrates the soil surface in response to freezing temperatures.
Ground Ice - A general term used to describe all bodies of ice in the ground surface of the permafrost layer. Also called anchor ice. Some forms of ground ice include pore ice, needle ice, ice wedge, segregated ice, sand wedge, and ice lenses.
Ground Information - Ancillary data collected that is associated with a feature on remote sensing imagery that comes from other data sources, like maps and field measurements.
Ground Moraine - A thick layer of till deposited by a melting glacier.
Groundwater - The water that occupies the pore spaces found in some types of bedrock.
Groundwater Flow - An underground topographic flow of groundwater because of gravity.
Groundwater Recharge - The replenishment of groundwater with surface water.
Growing Degree Day - A calculation derived from measurements of mean daily temperature that determines the quantity of heat energy available to support the growth of plants. This measure is commonly used by farmers and gardeners to forecast plant development rates and to approximate dates of flowering and crop maturity. Growing degree days are calculated as a sum of the mean daily temperature minus a base temperature (usually 10°C or 50°F) for each day during the growing season.
Growing Season - Time of the year when local weather conditions (i.e., rainfall and temperature) allow for normal plant growth.
Groyne - A solid wall or jetty built by humans at roughly right angles to an ocean shoreline or from a stream bank to significantly reduce the erosive movement of sediment caused by longshore drift or stream flow.
Grus - A deposit of unsorted clay particles and angular-shaped quartz fragments that are the result of local weathering of granite.
Guano - A localized concentration of bird or cave-dwelling bat excrement. Guano is common on some islands that support large populations of nesting seabirds that feed mainly on ocean fish. This material is very nutrient-rich, especially in nitrogen, phosphate, and potassium, and is used as a fertilizer.
Gulf Stream - A major warm ocean current that originates in and around the Caribbean Sea, flows up the United States east coast, and then moves across the North Atlantic to Northern Europe and West Africa.
Gully Erosion - The formation of steep-sided stream channels because of the erosion of soils and sediments by short-lived runoff and stream flow.
Gumbo - A term used to describe a wet, sticky, and heavy clay soil.
Gust - A sudden, rapid increase in wind speed over a relatively short period of time.
Gust Front - A boundary layer in the lower atmosphere found ahead of a thunderstorm that separates cold storm downdrafts from warm, humid surface air. Winds in this severe weather event are strong and fast.
Gustiness Factor - A measure of the variability of wind speed over a given period of time. Calculated by finding the wind speed mathematical range associated with gusts and the lulls, which are then compared to the mean wind speed.
Guyot - A type of seamount that has a top eroded off by wave action. Guyots have flat tops that are usually more than 200 meters (600 feet) below sea level.
Gymnosperms - A type of tree that does not cover its seeds in a structure such as a fruit or a nut. Most gymnosperms place their seeds in cones. Representatives of this group include the conifers.
Gypcrete - A hard crust of gypsum located at or near the ground surface. Commonly found in deserts.
Gypsum - Sedimentary rock created by the chemical precipitation of calcium, sulfur, and oxygen (CaSO4•2H2O). Commonly found in desert soils as a surface crust or a compact, hard subsurface layer.
Gyre - The arrangement of surface ocean currents into a large continental-scale circular pattern of flow.
Gyttja - A black colored mud rich in organic matter that is associated with lakes and ponds. It contains the partially decomposed remains of aquatic plants that once thrived in a lake or pond system.
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