A Index Glossary geology Science
A Index Glossary geology Science
Aa
A term of Hawaiian origin. Used in reference to a basaltic lava that occurs in flows with a fissured, rough and jagged surface.
A-horizon
The uppermost layer of a soil, containing organic material and leached minerals.
Algal mat
A layered communal growth of algae observed in fossils an in present day tidal zones associated with carbonate sedimentation.
Alkali metal
A strongly basic metal like potassium or sodium.
Angstrom
A length of 10 to the minus tenth meter or one hundred millionth of a centimeter.
Artesian well
A well that penetrates an aquiclude to reach an aquifer containing water under pressure. Thus water in the well rises above the surrounding water table.
Atmosphere
(unit)- A unit of pressure equal to 101,325 newtons per square meter, or about 14.7 pounds per square inch.
Acidic Rock
An igneous rock that has a relatively high silica content. Examples are granite and rhyolite. Also see entries for basic, intermediate and ultrabasic rocks
Acre-Foot
(acre-ft)-The volume of water needed to flood one acre of land to a depth of one foot. Equivalent to 43,560 cubic feet, 1,233 cubic meters or 325,851 gallons. One of the most common units of measure used for reservoir capacity. Also used in mineral resource calculations (an acre-foot of coal is a block of coal one acre in area and one foot thick - it weighs approximately 1,800 tons(.
Accessibility
A locational characteristic that permits a place to be reached by the efforts of those at other places.
Accessibility Resource
A naturally occurring landscape feature that facilitates interaction between places.
Air Mass
A very large body of atmosphere defined by essentially similar horizontal air temperatures. Moisture conditions are also usually similar throughout the mass.
ablation
The loss of snow and ice from a glacier, caused primarily by melting.
abrasion
A form of mechanical weathering that occurs when loose fragments or particles of rocks and minerals that are being transported, as by water or air, collide with each other or scrape the surfaces of stationary rocks.
absolute dating
The fixing of a geological structure or event in time, as by counting tree rings.
accretionary
wedge A mass of sediment and oceanic lithosphere that is transferred from a subducting plate to the less dense, overriding plate with which it converges.
ar
A sharp ridge of erosion-resistant rock formed between adjacent cirque glaciers.
aridity
index The ratio of a region's potential annual evaporation, as determined by its receipt of solar radiation, to its average annual precipitation.
atomic number
The number of protons in the nucleus of a given atom. Elements are distinguished from each other by their at-omic numbers.
accretionary wedge
A mass of sediment and oceanic lithosphere that is transferred from a subducting plate to the less dense, overriding plate with which it converges.
accumulation
The increase in a glacier's volume, caused primarily by snowfall.
acid rain
Rain that contains such acidic compounds as sulfuric acid and nitric acid, which are produced by the combination of atmospheric water with oxides released when hydrocarbons are burned. Acid rain is widely considered responsible for damaging forests, crops, and human-made structures, and for killing aquatic life.
aeration zone
See zone of aeration.
aftershock
A ground tremor caused by the repositioning of rocks after an earthquake. Aftershocks may continue to occur for as long as two years after the initial earthquake. The intensity of an earth quake's aftershocks decreases over time.
aggradation
The process by which a stream's gradient steepens due to increased deposition of sediment.
alloy
A metal that is manufactured by combining two or more molten metals. An alloy is always harder than its component metals. Bronze is an alloy of copper and tin.
Alluvial Fan
A fan-shaped wedge of sediment that typically accumulates on land where a stream emerges from a steep canyon onto a flat area. In map view it has the shape of an open fan. Typically forms in arid or semiarid climates.
- A triangular deposit of sediment left by a stream that has lost velocity upon entering a broad, relatively flat valley.
alpine glacier
A mountain glacier that is confined by highlands.
andesite
The dark, aphanitic, extrusive rock that has a silica content of about 60% and is the second most abundant volcanic rock. Andesites are found in large quantities in the
andesite line
The geographic boundary between the basalts and gabbros of the
acid
a substance that has a pH of less than 7, which is neutral. Specifically, an acid has more free hydrogen ions (H+) than hydroxyl ions (
alkaline
sometimes water or soils contain an amount of alkali (strongly basic) substances sufficient to raise the pH value above 7.0 and be harmful to the growth of crops.
alkalinity
the capacity of water for neutralizing an acid solution.
appropriation doctrine
the system for allocating water to private individuals used in most Western states. The doctrine of Prior Appropriation was in common use throughout the arid west as early settlers and miners began to develop the land. The prior appropriation doctrine is based on the concept of "First in Time, First in Right." The first person to take a quantity of water and put it to Beneficial Use has a higher priority of right than a subsequent user. Under drought conditions, higher priority users are satisfied before junior users receive water. Appropriative rights can be lost through nonuse; they can also be sold or transferred apart from the land. Contrasts with Riparian Water Rights.
aquaculture
farming of plants and animals that live in water, such as fish, shellfish, and algae.
aqueduct
a pipe, conduit, or channel designed to transport water from a remote source, usually by gravity.
artificial recharge
an process where water is put back into ground-water storage from surface-water supplies such as irrigation, or induced infiltration from streams or wells.
anticline
A convex fold in rock, the central part of which contains the oldest section of rock. See also syncline.
aphanitic
Of or being an igneous rock containing grains that are so small as to be barely visible to the naked eye.
Aquiclude
A subsurface rock, soil or sediment unit that does not yield useful quanties of water.
-An impermeable body of rock that may absorb water slowly but does not transmit it.
aquitard
A layer of rock having low permeability that stores groundwater but delays its flow.
arete
A sharp ridge of erosion-resistant rock formed between adjacent cirque glaciers. It often results from the erosive activity of alpine glaciers flowing in adjacent valleys.
aridity index
The ratio of a region's potential annual evaporation, as determined by its receipt of solar radiation, to its average annual precipitation.
Arroyo
A flat-bottom gully with steep sides that is a channel for an intermittent stream.
- A small, deep, usually dry channel eroded by a short-lived or intermittent desert stream.
artesian
Of, being, or concerning an aquifer in which water rises to the surface due to pressure from overlying water.
atom
The smallest particle that retains all the chemical properties of a given element.
atomic mass
1. The sum of protons and neutrons in an atom's nucleus. 2. The combined mass of all the particles in a given atom.
aureole
A section of rock that surrounds an intrusion and shows the effects of contact metamorphism.
Alluvia
Clay, silt, gravel, or similar detrital material deposited by running water.
Alluvial Soils
Soils deposited through the action of moving water. These soils lack horizons and are usually highly fertile.
altitude
Height of an object in the atmosphere above sea level.
Alkali
Used in reference to materials that are rich in sodium and/or potassium.
Alluvium
An unconsolidated accumulation of stream-deposited sediments, including sands, silts, clays or gravels.
- A deposit of sediment left by a stream on the stream's channel or flood plain. deposits of clay, silt, sand, gravel, or other particulate material that has been deposited by a stream or other body of running water in a streambed, on a flood plain, on a delta, or at the base of a mountain.
Angle of Repose
The maximum angle that a soil, sediment or other loose material can be placed or accumulate and be stable. The angle of repose varies for different types of materials and different moisture conditions.
Angular Unconformity
An erosional surface that separates rock units of differing dips. The rocks below the surface were deposited, deformed and eroded. The younger rocks above then accumulated upon the erosional surface.
Aquifer
A subsurface rock or sediment unit that is porous and permeable. To be an aquifer it must have these traits to a high enough degree that it stores and transmits useful quantities of water.
- (artesian) An aquifer that is bounded above and below by impermeable rock or sediment layers. The water in the aquifer is also under enough pressure that, when the aquifer is tapped by a well, the water rises up the well bore to a level that is above the top of the aquifer. The water may or may not flow onto the land surface.
- (confined) An aquifer that is bounded above and below by impermeable rock or sediment layers. There may or may not be enough pressure in the aquifer to make it an "artesian aquifer".
- (unconfined) An aquifer that is not overlain by an impermeable rock unit. The water in this aquifer is under atmospheric pressure and is recharged by precipitation that falls on the land surface directly above the aquifer.
Asthenosphere
A portion of the upper mantle that is directly below the lithosphere. A zone of low strength in the upper mantle defines the top of the asthenosphere. This weak zone allows the plates of the lithosphere to slide across the top of the asthenosphere.
Astrobleme
An ancient circular scar on Earth's surface produced by the impact of a meteorite or comet.Use our Google maps page to get close up images of these meteor impact structures.
Atoll
A ring-shaped group of coral islands that are surrounded by deep ocean water and that enclose a shallow lagoon.
Arkose
A sandstone that contains at least 25% feldspar. Easily recognized because the feldspar grains are typically pink and angular in shape.
Antebellum
Before the war; in the
Anthracite
A hard coal containing little volatile matter.
-The highest rank of coal. By definition, a coal with a fixed carbon content of over 91% on a dry ash-free basis. Anthracite coals have a bright luster, break with a conchoidal fracture, a semi-metallic luster and are difficult to ignite. Frequently referred to by the layman as "hard coal".
-Anthracite burns with an extremely hot, blue flame and very little smoke, but it is difficult to ignite and both difficult and dangerous to mine.
Arroy
A deep gully cut by a stream that flows only part of the year; a dry gulch. A term normally used only in desert areas.
atlas
A bound collection of maps.