Glossary of Terms: G

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 the 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 are induced by astronomical, biological, lithological, or other perturbations, life responds to these changes by growth and metabolism.

 

Galaxy

An assemblage of millions to hundreds of billions of stars.

 

Gall-Peters Projection

Map projection system that reduces the area distortion found in Mercator projections.

 

Gamete

A haploid reproductive cell.

 

Gamma Radiation

A type of ionizing, electromagnetic radiation that readily penetrates the body tissues of organisms. Has a wavelength less than 0.03 nanometers.

 

Gap

A spatial opening in a plant community. Can be caused by natural death or by some other abiotic or biotic disturbance.

 

Gas

A state of matter where molecules are free to move in any direction they like. The state of matter where the substance completely fills any container that it occupies.

 

Gelifluction

Form of mass movement in periglacial environment where a permafrost layer exists. It is characterized by the movement of soil material over the permafrost layer and the formation of lobe-shaped 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 layer of permafrost within one meter of the soil surface.

 

Gene(s)

Organic material that allows organisms to pass on the inheritance of adaptations or traits. In most organisms these adaptations are coded through the organic molecule DNA. New adaptations appear by way of mutations.

 

Gene Frequency

Frequency of alleles at an individual or population level.

 

Gene Pool

Sum total of all the genes found in the individuals of the population of a particular species.

 

General Circulation Model (GCM)

Computer-based climate model that produces future forecast of weather and climate conditions for regions of the Earth or the complete planet. Uses complex mathematical equations and physical relationships to determine a variety of climate variables in a three-dimensional grid.

 

Generalist Species

Species that can survive and tolerate a broad range of environmental conditions.

 

Genetic Adaptation

Changes in the genetic makeup of organisms of a species due to mutations that allow the species to reproduce and gain a competitive advantage under changed environmental conditions.

 

Genetic Diversity

Genetic variability found in a population of a species or all of the populations of a species. Also see biodiversity, ecosystem diversity, and species diversity.

 

Genus

A group in the classification of organisms. Classification level above the species group. It consists of similar species. Similar genera (plural form of genus) are grouped into a family.

 

Geocoding

The conversion of features found on an analog map into a computer-digital form. In this process, the spatial location of the various features is referenced geographically to a coordinate system used in the computer's software system.

 

Geodesy

The science that measures the surface features of the Earth.

 

Geographical Coordinate System

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 information in a computer database with spatial coordinates on a digital map.

 

Geographic Cycle

Theory developed by William Morris Davis that models the formation of river-eroded landscapes. This theory suggests that landscapes go through three stages of development (youth, maturity, and old age) and argues that the rejuvenation of landscapes arises from tectonic uplift of the land.

 

Geographic Isolation

See spatial isolation.

 

Geographic Range

Spatial distribution of a species. The geographic ranges of species often fluctuate over time.

 

Geography

The study natural and human constructed phenomena relative to a spatial dimension.

 

Geoid

True shape of the Earth, which deviates 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.

 

Geology

The field of knowledge that studies the origin, structure, chemical composition, and history of the Earth and other planets.

 

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.

 

Geomorphology

The field of knowledge that investigates the origin of landforms on the Earth and other planets.

 

Geostationary Orbit

Satellite that has an orbit that keeps it over the same point on the Earth at all times. This is accomplished by having the satellite travel in space at the same angular velocity as the Earth.

 

Geostrophic Wind

Horizontal wind in the upper atmosphere that moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force.

 

Geothermal Energy

Heat energy derived from the Earth's interior.

 

Germination

The beginning of vegetative growth of a plant from a seed.

 

Glacial (glaciation)

(1) Period of time during an ice age when glaciers advance because of colder temperatures.

(2) Involving glaciers and moving ice. Usually pertaining to processes associated with glaciers.

 

Glacial Drift

A generic term applied to all glacial and glaciofluvial deposits.

 

Glacial Ice

A very dense form 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 Milk

Term used to describe glacial meltwater which has a light colored or cloudy appearance because of clay-sized sediment held in suspension.

 

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

The backwards movement of the snout of a 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 was formed from glacial erosion. At the base of many of these valleys are cirques.

 

Glacial Uplift

Upward movement of the Earth's crust following isostatic depression from the weight of the continental glaciers.

 

Glacial Valley

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 glacial trough.

 

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.

 

Glaciofluvial

Geomorphic feature whose origin is related to the processes associated with glacial meltwater.

 

Glaze

Coating of ice that forms when rain falls on a surface with a temperature below freezing.

 

Gleization

A soil formation process that occurs in poorly drained environments. Results in the development of extensive soil organic layer over a layer of chemically reduced clay that takes on a blue color.

 

Gleysol Soil

Soil order (type) of the Canadian System of Soil Classification. This soil type is found in habitats that are frequently flooded or permanently waterlogged. Its soil horizons show the chemical signs of oxidation and reduction.

 

Global Positioning System (GPS)

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 a number of specialized satellites to a hand held receiving unit. The receiving unit uses triangulation to calculate altitude and spatial position on the Earth's surface.

 

Global Warming

Warming of the Earth's average global temperature because of an increase in the concentration of greenhouse gases. A greater concentration in greenhouse gases in the atmosphere is believed to result in an enhancement of the greenhouse effect.

 

Glucose

Simple six-carbon sugar. The chemical formula for glucose is C₆H₁₂O₆.

 

Gneiss

A metamorphosed coarse grained igneous rock. In this rock you get the recrystallization of quartz, feldspars, micas and amphiboles into bands.

 

GOES (Geostationary Operational Environmental Satellite)

Series of geostationary meteorological satellites launched by the United States starting in 1968. The main purpose behind these satellites was to use a variety of remote sensing devices for weather forecasting and environmental monitoring.

 

Graben Fault

This fault is produced when tensional stresses result in the subsidence of a block of rock. On a large scale these features are known as Rift Valleys.

 

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. Stream's maintain their grade through a balance between erosion, transportation, and deposition. Erosion removes material from bumps in the profile and deposition fills in dips.

 

Gradient

The steepness of a slope as measured in degrees, percentage, or as a distance ratio (rise/run).

 

Gradient Wind

Horizontal wind in the upper atmosphere that moves parallel to curved isobars. Results from a balance between pressure gradient force, Coriolis force, and centripetal force.

 

Granite

Medium to coarse grained igneous rock that is rich in quartz and potassium feldspar. Derived from felsic magma.

 

Granitic Magma

Felsic magma that generates mainly granitic rocks.

 

Graphic Scale

Way of expressing the scale of a map with a graphic.

 

Grass

Type of plant that has long slender leaves that extend from a short stem or the soil surface.

 

Grassland

Ecosystem whose dominant species are various types of grass. Found in regions where average precipitation is not great enough to support the growth of shrublands or forest.

 

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. These rock fragments have a size that is greater than 2 millimeters.

 

Gravitational Water

Water that moves through soil due to gravitational forces. Soil water in excess of hygroscopic water and capillary water.

 

Gravity

Is the process where any body of mass found in the universe attracts other bodies with a force proportional to the product of their masses and inversely proportional to the distance that separates them. First proposed by Sir Issac Newton in 1686.

 

Grazing Food Chain

Model describing the trophic flow of organic energy in a community or ecosystem.

 

Great Circle

An imaginary circle drawn on the Earth's surface that has its center synchronize to the center of the planet. The equator is a great circle.

 

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 re-emitting longwave energy. Of the longwave energy emitted back to space, 90% is intercepted and absorbed by greenhouse gases. Without the greenhouse effect the Earth's average global temperature would be -18° Celsius, rather than the present 15° Celsius. In the last few centuries, the activities of humans have directly or indirectly caused the concentration of the major greenhouse gases to increase. Scientists predict that this increase may enhance the greenhouse effect making the planet warmer. Some experts estimate that the Earth's average global temperature has already increased by 0.3 to 0.6° Celsius, since the beginning of this century, because of this enhancement.

 

Greenhouse Gases

Gases responsible for the greenhouse effect. These gases include: water vapor (H₂O), carbon dioxide (CO₂); methane (CH₄); nitrous oxide (N₂O); chlorofluorocarbons (CFxClx); and tropospheric ozone (O₃).

 

Greenwich Mean Time (GMT)

Former standard world time as measured at Greenwich, England (location of the Prime Meridian). Replace in 1928 with Universal Time (UT).

 

Grid North

The direction north as measured on the Universal Transverse Mercator grid system.

 

Grid South

The direction south as measured on the Universal Transverse Mercator grid system.

 

Gross Primary Productivity

Total amount of chemical energy fixed by the processes of photosynthesis.

 

Gross Secondary Productivity

Total amount of chemical energy assimilated by consumer organisms.

 

Gross Sediment Transport

The total amount of sediment transported along a shoreline in a specific time period.

 

Ground Fog

See radiation fog.

 

Ground Frost

Frost that penetrates the soil surface in response to freezing temperatures.

 

Ground Ice

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 Moraine

A thick layer of till deposited by a melting glacier.

 

Groundwater

Water that occupies the pore spaces found in some types of bedrock.

 

Groundwater Flow

Underground topographic flow of groundwater because of gravity.

 

Groundwater Recharge

The replenishment of groundwater with surface water.

 

Gulf Stream

Warm ocean current that originates in and around the Caribbean and flows across the North Atlantic to northwest Europe.

 

Gust Front

A boundary found ahead of a thunderstorm that separates cold storm downdrafts from warm humid surface air. Winds in this phenomenon are strong and fast.

 

Gymnosperm

Plant that bears naked seeds. Representatives of this group include the conifers.

 

Gypsum

Sedimentary rock created by the chemical precipitation of calcium, sulfur, and oxygen.

 

Gyre

Arrangement of surface ocean currents into a large macro-scale circular pattern of flow.

 

 

Citation: Pidwirny, M. (2006). "Glossary of Terms: G". Fundamentals of Physical Geography, 2nd Edition. Date Viewed. http://www.physicalgeography.net/physgeoglos/g.html