Gabbro: A black, coarse-grained, intrusive igneous rock, composed of calcic feldspars and pyroxene. The intrusive equivalent of basalt.

GAS: is A fluid (such as air) that has neither independent shape nor volume, but tends to expand indefinitely.

Geochronology: The science of absolute dating and relative dating of geologic formations and events, primarily through the measurement of daughter elements produced by radioactive decay in minerals.

Geologic cycle: 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.

Geomorphology: The science of surface landforms and their interpretation on the basis of geology and climate.

Geosyncline: A major downwarp in the Earth's crust, usually more than 1000 kilometers in length, in which sediments accumulate to thicknesses of many kilometers. The sediments may eventually be deformed and metamorphosed during a mountain-building episode.

Geotherm: A curving surface within Earth along which the temperature is constant.

Glacial rebound: Epeirogenic uplift of the crust that takes place after the retreat of a continental glacier, in response to earlier subsidence under the weight of the ice.

Glacier: A mass of ice and surficial snow that persists throughout the year and flows downhill under its own weight. The size range is from 100 meters to 10,000 kilometers.

Glass: A rock formed when magma is too rapidly cooled (quenched) to allow crystal growth.

Gneiss: A coarse-grained regional metamorphic rock that shows compositional banding and parallel alignment of minerals.

Graben: A downthrown block between two normal faults of parallel strike but converging dips; hence a tensional feature. See also horst.

Graded bedding: 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.

Graded stream: 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.

Granite: A coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodic plagioclase feldspar, and micas. Also sometimes a metamorphic product.

Granitization: The formation of metamorphic granite from other rocks by recrystallization with or without complete melting.

Granular snow: Snow that has been metamorphosed into small granules of ice.

Granulite: A metamorphic rock with coarse interlocking grains and little or no foliation.

Gravel: The coarsest of alluvial sediments, containing mostly particles larger than 2 mm in size and including cobbles and boulders.

Gravitation: is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass.

Gravity anomaly: 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.

Gravity survey: The measurement of gravity at regularly spaced grid points with repetitions to control instrument drift.

Greenhouse effect: 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.

Greenschist: A metamorphic schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism.

Ground moraine: A glacial deposit of till with no marked relief, interpreted as having been transported at the base of the ice.

Groundwater: The mass of water in the ground below the phreatic zone, occupying the total pore space in the rock and moving slowly downhill where permeability allows.

Gully: A small steep-sided valley or erosional channel from 1 meter to about 10 meters across.

Guyot: A flat-topped submerged mountain or seamount found in the ocean.