U,V,W,X,Y,Z Index Glossary Geology Science
U,V,W,X,Y,Z Index Glossary Geology Science
uranium-thorium-lead dating
A form of isotope dating that relies on the extremely long half-life of radioactive isotopes of uranium, which decay into isotopes of lead, to determine the age of rocks in which uranium and lead are present.
Ultrabasic Rock
An igneous rock with a very low silica content and rich in minerals such as hypersthene, augite and olivine. These rocks are also known as ultramafic rocks.
uniformitarianism
The hypothesis that current geologic processes, such as the slow erosion of a coast under the impact of waves, have been occurring in a similar manner throughout the Earth's history and that these processes can account for past geologic events. See also catastrophism.
upwarped mountain
A mountain consisting of a broad area of the Earth's crust that has moved gently upward without much apparent deformation, and usually containing sedimentary, igneous, and metamorphic rocks.
uranium-lead dating
A form of radiometric dating that relies on the extremely long half-life of radioactive isotopes of uranium, which decay into isotopes of lead, to determine the age of rocks in which uranium and lead are present.
Unconsolidated
A term used when referring to sediment that has not been lithified into a rock. Uncemented.
Underemployment
A condition among a labor force such that a portion of the labor force could be eliminated without reducing the total output. Some individuals are working less than they are able or want to, or they are engaged in tasks that are not entirely productive.
Underpopulation
Economically, a situation in which an increase in the size of the labor force will result in an increase in per worker productivity.
Uniform Region
A territory with one or more features present throughout and absent or unimportant elsewhere.
Unit Cell
The smallest sample of a substance that has a complete representation of its atomic structure. A crystal structure is formed by repetition of the unit cell in three dimensions.
Uplift
A structurally high area in Earth's crust. Formed by movements that bend the crust into a structure such as a dome or an arch.
Upwelling
Movement of cold water from the floor of a lake or ocean up into a shallow area.
Ultramafic rock
An igneous rock consisting dominantly of mafic minerals, containing less than 10 percent feldspar. Includes dunite, peridotite, amphibolite, and pyroxenite.
Unconformity
A surface that separates two strata. It represents an interval of time in which deposition stopped, erosion removed some sediments and rock, and then deposition resumed (see also Angular unconformity ).
Unconsolidated material
Nonlithified sediment that has no mineral cement or matrix binding its grains.
Upwelling current
The upward movement of cold bottom water in the sea, which occurs when wind or currents displace the lighter surface water.
A deep valley with a flat floor and very steep walls. Shaped in cross-section like the letter "U". Valleys with this geometry are frequently cut by a glacier.
unsaturated zone
the zone immediately below the land surface where the pores contain both water and air, but are not totally saturated with water. These zones differ from an aquifer , where the pores are saturated with water.
Vadose Water
Water that exists in the pore spaces of a rock or soil, between the ground surface and the water table.
Valence Electrons
Electrons in the outermost shell of an atom. The electrons that are typically involved in making chemical bonds.
vent
An opening in the Earth's surface through which lava, gases, and hot particles are expelled. Also called volcanic vent and volcano.
Valley Glacier
A glacier that occurs in a mountainous region and occupies a valley. Also known as an Alpine Glacier.
Varve
A thin layer of fine-grained sediment deposited in the still waters of a lake. Varves are frequently associated with glaciation and represent a yearly sedimentation cycle - a silty, light-colored layer deposited in summer and a darker, organic-rich clay layer deposited during winter.
Vertical Exaggeration
In making sketches of landscapes and cross-sections the vertical dimension is frequently exaggerated to show detail. Vertical exaggeration is a number that represents the magnitude of this exaggeration. It is a proportion between the vertical scale and the horizontal scale. For example, a cross section with a vertical exaggeration of 4 has a vertical scale that is four times the horizontal scale (in this example the vertical scale could be 125 while the horizontal scale is 1100).
Vesicle
Spherical or elongated cavities in an igneous rock that are created when a melt crystallizes with bubbles of gas trapped inside.
Viscosity
The resistance of a fluid to flow. Fluids with a high viscosity resist flow. Fluids with a low viscosity flow freely.
Volcanic Ash
Sand-sized particles of igneous rock that form when a spray of liquid magma is blown from a volcanic vent by escaping gas.
Volcanic Ash Fall
An accumulation of volcanic ash produced by an eruption. These can be very thick near the vent and decrease to a light dusting in a downwind direction.
Volcanic Bomb
A projectile of hot magma or rock that is blown from the vent during a volcanic eruption. These solidify in flight and frequently form an elongated rock of streamlined shape.
Volcanic Breccia
A rock made up of pyroclastic fragments that are at least 64 millimeters in diameter.
Volcanic Cone
A cone-shaped hill or mountain composed of pyroclastic debris and/or lava which builds up around a volcanic vent during eruptions.
Volcanic Neck
A vertical intrusion with the geometry of a volcanic pipe. An erosional remant of a volcanic pipe.
Volcanic Pipe
A vertical or near vertical tunnel which connects a magma reservoir to the surface. Magma and gas travel up this tube to produce the eruption. After the eruption the tube can be filled with a cooling magma which preserves its shape as an intrusive body.
V-shaped valley
A valley with a narrow bottom and a cross section shaped like the letter "V". Valleys of this shape are almost always cut by stream erosion.
van der Waals bond
A relatively weak kind of intermolecular bond that forms when one side of a molecule develops a slight negative charge because a number of electrons have temporarily moved to that side of the molecule, and this negative charge attracts the nuclei of the atoms of a neighboring molecule, while the side of the molecule with fewer electrons develops a slight positive charge that attracts the electrons of the atoms of neighboring molecules.
Vadose zone
The region in the ground between the surface and the water table in which pores are not filled with water. Also called the unsaturated zone.
Valence electron
An electron of the outermost shell of an atom; one of those most active in bonding.
Vector
A mathematical element that has a direction and magnitude, but no fixed position. Examples are force and gravity.
Volcanic block
A pyroclastic rock fragment ranging from about fist- to car-sized.
Volcanic ejecta blanket
A collective term for all the pyroclastic rocks deposited around a volcano, especially by a volcanic explosion.
Volcanic emanations
Gases, especially steam, emitted from a vent or released from lava.
Vein
A fracture that has been filled with mineral material.
- An opening in the Earth's surface through which lava, gases, and hot particles are expelled. Also called volcanic vent and volcano.
Ventifact
A rock that has been shaped or polished by the sandblasting effect of wind-blown sand.
-A stone that has been flattened and sharpened by wind abrasion. Ventifacts are commonly found strewn across a desert floor.
volcanic arc
A chain of volcanoes fueled by magma that rises from an underlying subducting plate.
volcanic crater
A steep, bowl-shaped depression surrounding a vent. A volcanic crater forms when the walls of a vent collapse inward following an eruption.
volcanic dome
A bulb-shaped solid that forms over a vent when lava so viscous that it cannot flow out of the volcanic crater cools and hardens. When a volcanic dome forms, it traps the volcano's gases beneath it. They either escape along a side vent of the volcano or build pressure that causes another eruption and shatters the volcanic dome.
volcanic rock
See extrusive rock.
volcanic tuff
A solid rock made up of tephra that have consolidated and become cemented together. Also called tuff.
volcanism
The set of geological processes that result in the expulsion of lava, pyroclastics, and gases at the Earth's surface.
volcano
The solid structure created when lava, gases, and hot particles escape to the Earth's surface through vents. Volcanoes are usually conical. A volcano is "active" when it is erupting or has erupted recently. Volcanoes that have not erupted recently but are considered likely to erupt in the future are said to be "dormant." A volcano that has not erupted for a long time and is not expected to erupt in the future is "extinct."
wastewater
water that has been used in homes, industries, and businesses that is not for reuse unless it is treated.
wastewater-treatment return flow
water returned to the environment by wastewater-treatment facilities.
water cycle
water cycle - the circuit of water movement from the oceans to the atmosphere and to the Earth and return to the atmosphere through various stages or processes such as precipitation, interception, runoff, infiltration, percolation, storage, evaporation, and transportation.
water quality
a term used to describe the chemical, physical, and biological characteristics of water, usually in respect to its suitability for a particular purpose.
water use
water that is used for a specific purpose, such as for domestic use, irrigation, or industrial processing. Water use pertains to human's interaction with and influence on the hydrologic cycle, and includes elements, such as water withdrawal from surface- and ground-water sources, water delivery to homes and businesses, consumptive use of water, water released from wastewater-treatment plants, water returned to the environment, and instream uses, such as using water to produce hydroelectric power.
Watershed
drainage basin -the land area that drains water to a particular stream, river, or lake. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large watersheds, like the
- The geographic area that contributes runoff to a stream. It can be outlined on a topographic map by tracing the points of highest elevation (usually ridge crests) between two adjacent stream valleys. The watershed of a large river usually contains the watersheds of many smaller streams. Also referred to as a "drainage basin".
welded tuff
A volcanic igneous rock that forms when still-warm tephra accumulates at the Earth's surface. Because the particles are still warm and soft, they can weld together under the weight of overlying deposits, forming a hard rock.
Watthour
(Wh)-an electrical energy unit of measure equal to one watt of power supplied to, or taken from, an electrical circuit steadily for one hour.
well water
an artificial excavation put down by any method for the purposes of withdrawing water from the underground aquifers. A bored, drilled, or driven shaft, or a dug hole whose depth is greater than the largest surface dimension and whose purpose is to reach underground water supplies or oil, or to store or bury fluids below ground.
wave-cut bench
A relatively level surface formed when waves erode the base of a cliff, causing the overlying rock to fall into the surf. A wave-cut bench stands above the water and extends seaward from what remains of the cliff
wave refraction
The process by which a wave approaching the shore changes direction due to slowing of those parts of the wave which enter shallow water first, causing a sharp decrease in the angle at which the wave approaches until the wave is almost parallel to the coast.
weathering
The process by which exposure to atmospheric agents, such as air or moisture, causes rocks and minerals to break down. This process takes place at or near the Earth's surface. Weathering entails little or no movement of the material that it loosens from the rocks and minerals. See also erosion
Water mass
A mass of water that fills part of an ocean or lake and is distinguished by its uniform physical and chemical properties, such as temperature and salinity.
Wave steepness
The maximum height or amplitude of a wave divided by its wavelength.
wetland
A lake, marsh, or swamp that supports wildlife and replenishes the groundwater system
wind abrasion
The process by which wind erodes bedrock through contact between the bedrock and rock particles carried by the wind
windward
Of, located on, or being the side of a dune, hill, or ridge facing into the wind. See also leeward
Wadi
A stream valley in an arid region that is dry except during the rainy season.
Warping
A slight bend, uplift or subsidence of Earth's crust on a regional scale.
Water Cycle
The movement of water between the atmosphere, ground and surface water bodies through the processes of evaporation, precipitation, infiltration, percolation, transpiration and runoff. Also known as the "hydrologic cycle".
Water Quality
An assessment of the physical, chemical and biological characteristics of water, especially how they relate to the suitability of that water for a particular use.
Water Table
the top of the water surface in the saturated part of an aquifer
- A level beneath the Earth's surface, below which all pore spaces are filled with water and above which the pore spaces are filled with air. The top of the zone of saturation in a subsurface rock, soil or sediment unit.
Water Table
The level below the land surface at which the subsurface material is fully saturated with water. The depth of the water table reflects the minimum level to which wells must be drilled for water extraction.
Wave-Cut Terrace
A long, level surface formed by wave erosion during a time when sea level was higher.
Wavelength
An interval of repetition in a wave-like disturbance. The distance between two successive crests or two successive troughs.
Withdrawal
A removal of water from a surface or ground water source for use.
X-ray Diffraction
A technique used to identify minerals by bombarding them with X-rays. Planes of repetition within the atomic structure of the mineral diffract the X-rays. The pattern of diffraction is unique for each mineral structure and can be used for identification.
xenolith
A preexisting rock embedded in a newer igneous rock. Xenoliths are formed when a rising magma incorporates the preexisting rock. If the preexisting rock does not melt, it will not be assimilated into the magma and will therefore remain distinct from the new igneous rock that surrounds it.
xeriscaping
a method of landscaping that uses plants that are well adapted to the local area and are drought-resistant. Xeriscaping is becoming more popular as a way of saving water at home.
Yazoo Stream
A tributary that parallels the main channel for a considerable distance. Joining of these streams is normally blocked by a natural levee along the larger stream.
Yellow Ground
Oxidized kimberlite. A yellow soil that is characteristic of the area above a kimberlite diamond pipe.
Yield
The quantity of water, coal, gold or other resource that can be produced from a deposit.
yeild point
The maximum stress that a given rock can withstand without becoming permanently deformed.
Youth
The earliest stage in the development of a landscape. During this stage streams are actively downcutting and flowing straight for long distances with frequent waterfalls and rapids. The valleys are typically steep sided and v-shaped.
zone of ablation
The part of a glacier in which there is greater over all loss than gain in volume. A zone of ablation can be identified in the summer by an expanse of bare ice. See also zone of accumulation
zone of accumulation
The part of a glacier in which there is greater overall gain than loss in volume. A zone of accumulation can be identified by a blanket of snow that survives summer melting. See also zone of ablation.
Zeolite
A group of hydrous aluminosilicates that are similar to the feldspars. They easily lose and regain their water of hydration and they fuse and swell when heated. Zeolites are frequently used in water softening, ion exchange and absorbent applications
Zoned
A crystal that grew while temperatures were changing or while the composition of the parent solution was changing.
Zone of Aeration
A zone between the land surface and the water table where pore spaces are filled mainly with air. Water that exists in the pore space in this zone is referred to as "soil moisture".
Zone of Saturation
The zone beneath the water table where all pore spaces are completely filled with water. Water that exists within this zone is known as "ground water".
Zoning
The public regulation of land and building use to control the character of a place.