G and H Index Glossary geology Science
G and H Index Glossary geology Science
Gabbro
A black, coarse-grained intrusive igneous rocks that is the compositional equivalent of basalt. Composed of calcium-rich feldspars, pyroxene and possibly olivine, but containing little if any quartz.
Gaging Station
A facility on a stream, lake, canal, reservoir or other water body where instruments are installed to automatically monitor the water. Measurments such as stage, discharge, water temperature and pH are automatically taken and transmitted to hydrologists via satellite, radio or telephone. Measurements from these stations are useful for a wide variety of flood prediction, water management, recreation and navigation purposes.
Geochronology
A study of the time relationships of rock units. Includes methods of both relative and absolute dating.
Geosyncline
A major trough or downwarp of the Earth's crust, in which great thicknesses of sedimentary and/or volcanic rocks have accumulated.
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.
glacial
Produced by, transported by, or concerning a glacier.
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 under lying 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
The mass of rocks and finely ground material carried by a glacier, then deposited when the ice melted. Creates an unstratified material of varying composition.
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.
gravity
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.
Geothermal Gradient
The progressive increase of temperature with depth into the Earth.
Geyser
A hot spring that intermittently erupts a spray of steam and hot water. Caused by the heating of ground water within a confined opening in hot rock.
Glacial Rebound
A very gradual uplift of Earth's crust that occurs after the weight of a thick continental ice sheet (which produced subsidence) has melted away.
Glacial Striations
Grooves and scratches on a bedrock surface that were produced the movement of a glacier. The orientation of the striations gives evidence to the direction of glacial movement.
Glacial Valley
A valley with a U-shaped cross section that was cut by an alpine glacier.
Glacier
A thick mass of ice that forms on land from an accumulation and recrystallization of snow significant enough to persist through the summer and grow year by year. There are two basic types of glaciers 1) valley (or alpine) glaciers that creep downslope under the influence of gravity, and 2) continental glaciers that flow outward from a thick central area under their own weight.
Glass
An amorphous (without crystal structure) igneous rock that forms from very rapid cooling of magma. The rapid cooling does not provide enough time for crystal growth.
Graded Bedding
A rock layer that has a progressive change in particle size from top to bottom. Most common is a sequence with coarse grains at the bottom and fining upwards, which is typically caused by a declining current velocity within the depositional environment.
Granite
A coarse-grained, intrusive igneous rock composed primarily of light colored minerals such as quartz, orthoclase, sodium plagioclase and muscovite mica. Granite is thought to be one of the main components of continental crust.
Gravel
Clastic sedimentary particles of any composition that are over 2 mm in diameter.
Geomorphology
The study of the arrangement and form of the Earth's crust and of the relationship between these physical features and the geologic structures beneath.
gage height
the height of the water surface above the gage datum (zero point). Gage height is often used interchangeably with the more general term, stage, although gage height is more appropriate when used with a gage reading.
greywater
wastewater from clothes washing machines, showers, bathtubs, hand washing, lavatories and sinks.
ground-water recharge
inflow of water to a ground-water reservoir from the surface. Infiltration of precipitation and its movement to the water table is one form of natural recharge. Also, the volume of water added by this process.
Ghetto
Originally, the section of a European city to which Jews were restricted. Today, commonly defined as a section of a city occupied by members of a minority group who live there because of social restrictions on their residential choice.
Glaciation
Having been covered with a glacier or subject to glacial epochs.
globe
A true-to-scale map of the Earth that duplicates its round shape and correctly represents areas, relative size and shape of physical features, distances, and directions.
The shortest distance between two places on the Earth's surface. The route follows a line described by the intersection of the surface with an imaginary plane passing through the Earth's center.
grid
A pattern of lines on a chart or map, such as those representing latitude and longitude, which helps determine absolute location.
Geomorphic cycle
An idealized model of erosion wherein a plain is uplifted epeirogenically, then dissected by rapid streams (youth), then rounded by d0wnslope movements into a landscape of steep hills (maturity), and finally reduced to a new peneplain at sea level (old age).
Geotherm
A curving surface within Earth along which the temperature is constant.
Glassiness
The content of extent of glass in an igneous rock.
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.
Gravity survey
The measurement of gravity at regularly spaced grid points with repetitions to control instrument drift.
Greenschist
A metamorphic schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism.
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.
Gyre
The circular rotation of the waters of each major sea, driven by prevailing winds and the Coriolis effect.
Growing Season
The period from the average date of the last frost (in the
Greenhouse Effect
A warming of the atmosphere caused by carbon dioxide and water vapor in the lower portions of the atmosphere capturing heat that is radiated from and reflected by Earth's surface.
Greenstone
A low-grade metamorphic rock that frequently contains green minerals such as chlorite, epidote and talc.
Ground Moraine
A blanket of till that is deposited during the retreat of a glacier.
Ground Water
Water that exists below the water table in the zone of saturation. Ground water moves slowly in the same direction that the water table slopes.
Ground Water Recharge Area
A location where surface water or precipitation can infiltrate into the ground and replenish the water supply of an aquifer.
Heat conduction
The transfer of the rapid vibrational energy of atoms and molecules, which constitutes heat energy, through the mechanism of atomic or molecular impact.
Heat engine
A device that transfers heat from a place of high temperature to a place of lower temperature and does mechanical work in the process.
Hill
A natural land elevation, usually less than 1000 feet above its surroundings, with a rounded outline. The distinction between hill and mountain depends on the locality.
Hooke's Law
The principle that the stress within a solid is proportional to the strain. It holds only for strains of a few percent or less.
Hypsometric diagram
A graph that shows in any way the relative amounts of the Earth's surface at different elevations with regard to sea level.
A tributary to a U-shaped glacial valley which, instead of entering the valley at the same level as the main stream, enters at a higher elevation, frequently as a waterfall. These different stream levels are a result of the rapid downcutting of the glacier being much faster than the slower downcutting of the tributary stream.
Hard Water
Water that has a significant amount of dissolved calcium and magnesium ions. This water performs poorly with most soaps and detergents and leaves a scaly deposit in containers where it is heated or evaporates. It can frequently be improved through the use of home-based water treatment systems.
Heat Flow
The movement of heat energy from the core of the Earth towards the surface.
Hogback
A narrow ridge with steeply inclined sides of nearly equal slopes. Formed by differential erosion of steeply dipping rock units.
Hornfels
A nonfoliated metamorphic rock that is typically formed by contact metamorphism around igneous intrusions.
Horst
An elongated block of high topographic relief that is bounded on two sides by steep normal faults. Produced in an area of crustal extension.
Hot Spring
A natural spring that delivers water to the surface that is of higher temperature than the human body.
Humus
The dark portion of a soil that consists of organic material that is well enough decayed that the original source material can not be identified.
Hydrocarbon
Any organic chemical compound (gaseous, liquid or solid) that is composed of carbon and hydrogen. The term is frequently used in reference to fossil fuels, specifically crude oil and natural gas.
Hydrograph
A graph that shows the change of a water-related variable over time. Example A stream discharge hydrograph shows the change in discharge of a stream over time.
Heavy Industry
Manufacturing activities engaged in the conversion of large volumes of raw materials and partially processed materials into products of higher value; hallmarks of this form of industry are considerable capital investment in large machinery, heavy energy consumption, and final products of relatively low value per unit weight (see Light Industry).
hemisphere
Half of the Earth, usually conceived as resulting from the division of the globe into two equal parts'north and south or east and west.
Hinterland
The area tributary to a place and linked to that place through lines of exchange, or interaction.
Horizon
A distinct layer of soil encountered in vertical section.
half-life
The time necessary for half of the atoms of a parent isotope to decay into the daughter isotope.
hanging wall
The section of rock that lies above the fault plane in a dip-slip fault See also footwall.
headland
A cliff that projects out from a coast into deep water.
historical geology
The study of the history, origin, and evolution of the Earth and all of its life forms and geologic structures.
Holocene Epoch
The second epoch of the Quaternary Period, beginning approximately 10,000 years ago and continuing to the present time. See also Pleistocene Epoch.
hook
A spit that curves sharply at its coastal end.
horn
A high mountain peak that forms when the walls of three or more cirques intersect.
hardness
The degree of resistance of a given mineral to scratching, indicating the strength of the bonds that hold the mineral's atoms together. The hardness of a mineral is measured by rubbing it with substances of known hardness.
-a water-quality indication of the concentration of alkaline salts in water, mainly calcium and magnesium. If the water you use is "hard" then more soap, detergent or shampoo is necessary to raise a lather.
Headwater
(1) the source and upper reaches of a stream; also the upper reaches of a reservoir.
(2) the water upstream from a structure or point on a stream.
(3) the small streams that come together to form a river. Also may be thought of as any and all parts of a river basin except the mainstream river and main tributaries.
hydroelectric power water use
the use of water in the generation of electricity at plants where the turbine generators are driven by falling water.
hydrologic cycle
The perpetual movement of water among the mantle, oceans, land, and atmosphere of the Earth
- the cyclic transfer of water vapor from the Earth's surface via evapotranspiration into the atmosphere, from the atmosphere via precipitation back to earth, and through runoff into streams, rivers, and lakes, and ultimately into the oceans.
hot spot
An area in the upper mantle, ranging from 100 to 200 km in width, from which magma rises in a plume to form volcanoes. A hot spot may endure for ten million years or more.
hydraulic conductivity
The extent to which a given substance allows water to flow through it, determined by such factors as sorting and grain size and shape
hydraulic gradient
The difference in potential between two points, divided by the lateral distance between the points
hydraulic lifting
The erosion of a stream bed by water pressure.
hydrogen bond
An intermolecular bond formed with hydrogen
hydrolysis
A form of chemical weathering in which ions from water replace equivalently charged ions from a mineral, especially a silicate.
hydrothermal deposit
A mineral deposit formed by the precipitation of metallic ions from water ranging in temperature from 500 to 7000 C
hydrothermal metamorphism
The chemical alteration of preexisting rocks that is caused by the action of hot water.
hypothesis
A tentative explanation of a given set of data that is expected to remain valid after future observation and experimentation
Hydrography
The study of the surface waters of the Earth.
Hydroponics
The growing of plants, especially vegetables, in water containing essential mineral nutrients rather than in soil.
Hydrology
The science of Earth's water, its movement, abundance, chemistry and distribution on, above and below Earth's surface.
Hydrothermal
Pertaining to hot water, the actions of hot water or the products produced by the actions of hot water.
Hydrothermal Vein
A deposit of minerals precipitated in a fracture by the actions of hot water or gases associated with a magmatic source.
Hypocenter
A point beneath earth's surface where the vibrations of an earthquake are thought to have originated. Also known as the focus.