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GABBRO: A black, coarse-grained, intrusive igneous rock, composed of calcic feldspars and pyroxene; the intrusive equivalent of basalt.

GEOCHEMICAL CYCLE: The set of processes that carry a particular chemical from reservoir to reservoir in Earth's systems.

GEOLOGIC TIME: The time from the formation of the Earth to the present, divided into periods of time during which known geological events have taken place.

GEOLOGIC TIME SCALE: The division of geologic history into eras, periods, and epochs accomplished through stratigraphy and paleontology.

GEOLOGY: The science of Earth-how it originated, how it evolved, how it works, and how human intervention can affect it.

GEOTHERM: A curve on a temperature-pressure or temperature-depth graph that describes how temperature in the Earth changes with depth. Different tectonic provinces are characterized by more or less rapid increases of temperature with depth.

GEOTHERMAL ENERGY: Energy generated by using the heat energy of the crust, especially in volcanic regions.

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 m to 10,000 km. (See also Continental glacier; Valley glacier.)

GLACIER SURGE: A period of unusually rapid movement of a glacier, sometimes lasting more than a year.

GLASS: A rock formed when magma or molten rock is cooled too rapidly to allow crystal growth.

GLASSY: Adjective indicating that a material does not have an orderly, repeating, three-dimensional array of atoms.

GLOBAL CHANGE: A change in climate that has worldwide effects on the environment, life, and other components of Earth.

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

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 longitudinal profile is unbroken by resistant ledges, lakes, or waterfalls and that exactly maintains the slope, velocity, and discharge required to carry its sediment load in equilibrium without erosion or sedimentation.

GRANITE: A felsic, coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodium-rich plagioclase feldspar, and micas. Also sometimes a metamorphic product. (Compare Rhyolite.)

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

GRANODIORITE: A plutonic rock similar to granite in composition, except that plagioclase feldspar is present in greater abundance than orthoclase feldspar; the intrusive equivalent of dacite.

GRANULITE: A nonfoliated regional metamorphic rock with coarse interlocking grains, generally formed under conditions of relatively high pressure and temperature.

GRAVEL: The coarsest clastic sediment, consisting mostly of particles larger than 2 mm and including cobbles and boulders.

GREENHOUSE EFFECT: A global warming effect in which carbon dioxide and water vapor absorb infrared radiation from Earth's surface and radiate it back to the surface.

GREENSCHIST: A schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism of mafic volcanic rocks.

GREENSTONE: A field term applied to any low-grade metamorphosed mafic igneous rock (for example, basalt, gabbro, or diabase). Chlorite accounts for their greenish cast.

GROUNDWATER: The mass of water in the ground (below the unsaturated zone) occupying the total pore space in the rock and moving slowly where permeability allows.

GROUNDWATER TABLE: The upper surface of the saturated zone of groundwater. Also called the "water table."

GUYOT: A flat-topped submarine mountain or seamount.