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G


gabbro
Any of a group of dark, dense, phaneritic, intrusive rocks that are the plutonic equivalent to basalt.
geochronology
The study of the relationship between the history of the Earth and time.
geologic time scale
The division of all of Earth history into blocks of time distinguished by geologic and evolutionary events, ordered sequentially and arranged into eons made up of eras, which are in turn made up of periods, which are in turn made up of epochs.
geology
The scientific study of the Earth, its origins and evolution, the materials that make it up, and the processes that act on it.
geophysics
The branch of geology that studies the physics of the Earth, using the physical principles underlying such phenomena as seismic waves, heat flow, gravity, and magnetism to investigate planetary properties.
geyser
A natural spring marked by the intermittent escape of hot water and steam.
glacial abrasion
The process by which a glacier erodes the underlying bedrock through contact between the bedrock and rock fragments embedded in the base of the glacier. See also glacial quarrying.
glacial drift
A load of rock material transported and deposited by a glacier. Glacial drift is usually deposited when the glacier begins to melt.
glacial erratic
A rock or rock fragment transported by a glacier and deposited on bedrock of different composition. Glacial erratics range from a few millimeters to several yards in diameter.
glacial quarrying
The process by which a glacier erodes the underlying bedrock by loosening and ultimately detaching blocks of rock from the bedrock and attaching them instead to the glacier, which then bears the rock fragments away. See also glacial abrasion.
glacial till
Drift that is deposited directly from glacial ice and therefore not sorted. Also called till. See also glacial drift.
glacier
A moving body of ice that forms on land from the accumulation and compaction of snow, and that flows downslope or outward due to gravity and the pressure of its own weight.
gneiss
A coarse-grained, foliated metamorphic rock marked by bands of light-colored minerals such as quartz and feldspar that alternate with bands of dark-colored minerals. This alternation develops through metamorphic differentiation.
graben
A block of rock that lies between two faults and has moved downward to form a depression between the two adjacent fault blocks. See also horst.
graded bed
A bed formed by the deposition of sediment in relatively still water, marked by the presence of particles that vary in size, density, and shape. The particles settle in a gradual slope with the coarsest particles at the bottom and the finest at the top.
graded stream
A stream maintaining an equilibrium between the processes of erosion and deposition, and therefore between aggradation and degradation.
gradient
The vertical drop in a stream's elevation over a given horizontal distance, expressed as an angle.
granite
A pink-colored, felsic, plutonic rock that contains potassium and usually sodium feldspars, and has a quartz content of about 10%. Granite is commonly found on continents but virtually absent from the ocean basins.
gravity
1. The force of attraction exerted by one body in the universe on another. Gravity is directly proportional to the product of the masses of the two attracted bodies. 2. The force of attraction exerted by the Earth on bodies on or near its surface, tending to pull them toward the Earth's center.
gravity anomaly
The difference between an actual measurement of gravity at a given location and the measurement predicted by theoretical calculation.
groin
A structure that juts out into a body of water perpendicular to the shoreline and is built to restore an eroding beach by intercepting longshore drift and trapping sand.
guyot
A seamount, the top of which has been flattened by weathering, wave action, or stream erosion.

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