Thursday, November 23rd, 2017
* A horizon-The uppermost layer of soil composed of a mixture of organic matter and leached and weathered minerals. (syn: topsoil)
* Aa- Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments. Aa flows commonly develop from pahoehoe flows as they cool and lose gas.
* Aa-A lava flow that has a jagged, rubbly, broken surface.
* Ablation area-The lower portion of a glacier where more snow melts in summer than accumulates in winter so there is a net loss of glacial ice. (syn: zone of wastage)
* Abrasion- wearing away of a rock by grinding action.
* Abrasion-The mechanical wearing and grinding of rock surfaces by friction and impact.
* Absolute age-Time measured in years.
* Absolute Date- An estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.
* Absolute dating - A technique that geologists use to assign specific dates to rock formations and geologic events.
* Absolute Humidity- The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapor in the air, usually expressed as grams of water vapor per cubic meter of air.
* Absolute humidity-See "humidity."
* Absolute magnitude-The brightness that a star would appear to have if it were 32 light-years (10 parsecs) away.
* Absolute Zero - Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes. The value is 0o Kelvin, -273.15o Celsius and -459.67o Fahrenheit.
* Absorption nebula-A cold nebula that absorbs light.
* Absorption spectrum-See "spectrum."
* Abyssal fan-A large, fan-shaped accumulation of sediment deposited at the bases of many submarine canyons adjacent to the deep-sea floor. (syn: submarine fan)
* Abyssal plain-A flat, level, largely featureless part of the ocean floor between the mid-oceanic ridge and the continental rise.
* Accreted terrain-A land mass that originated as an island arc or a microcontinent that was later added onto a continent.
* Accretion -Accumulation of dust and gas into larger bodies.
* Accumulation area-The upper part of a glacier where accumulation of snow during the winter exceeds melting during the summer, causing a net gain of glacial ice.
* Acid precipitation-A condition in which natural precipitation becomes acidic after reacting with air pollutants. (Often called "acid rain.")
* Active continental margin-A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: Andean margin)
* Active volcano- A volcano that is erupting; or one that, while not erupting at the present, has erupted within (geologically) recent time and is considered likely to do so in the (geologically) near future.
* Adiabatic rate- The rate of temperature change in the atmosphere due to the raising or lowering of an air mass. The "dry adiabatic rate" is 5.5 deg. F. Per 1000 feet, while the "wet" rate is 3.5 deg. F. Per 1000 feet.
* Adiabatic temperature changes-Temperature changes that occur without gain or loss of heat.
* Advection fog-Fog that forms when warm moist air from the sea blows onto cooler land, where the air cools and water vapor condenses at ground level.
* Advection-In meteorology, the horizontal component of a convection current in air, i.e. The surface movement that is commonly called wind.
* Aerosol-Any small particle that is larger than a molecule and suspended in air.
* Air mass- a large section of the lower atmosphere that has the same weather throughout.
* Air mass-A large body of air that has approximately the same temperature and humidity throughout.
* Air pressure (barometric pressure)- weight of the atmosphere per unit area
* Air- The mixture of gases that make up the earth's atmosphere. The principal gases that compose dry air are- Nitrogen (N2) 78.09%; Oxygen (O2) 20.946%; Argon (A) at 0.93%; and Carbon Dioxide (CO2) 0.033%. One other important constituents of air is water vapor (H2O) which varies from 0% to about 4%.
* Albedo -Reflectivity of an object; ratio of reflected light to incident light.
* Albedo-The reflectivity of a surface. A mirror or bright snowy surface reflects most of the incoming light and has a high albedo, whereas a rough flat road surface has a low albedo.
* Alberta Clipper- A fast moving winter storm originating in the Alberta, Canada region. Often these snow storms have high winds reaching 100 km/h which reduce visibility through blowing and drifting snow. Blizzard conditions are common in a well-developed Alberta Clipper.
* Alluvial fan-A fan-like accumulation of sediment created where a steep stream slows down rapidly as it reaches a relatively flat valley floor.
* Alluvium- A general term for clay, silt, sand, gravel or similar unconsolidated material deposited by a stream or other body of running water.
* Alpha Centauri -The closest bright star to our solar system.
* Alpine glacier-A glacier that forms in mountainous terrain.
* Alternative energy resources-All energy resources other than fossil fuels and nuclear fission; including solar energy; hydroelectric power; geothermal energy; wind energy; biomass energy; tidal, wave, and heat energy from the seas; and nuclear fusion.
* Altimeter - An instrument used to determine the altitude of an object with respect to a fixed level. The type normally used by meteorologists measures the altitude with respect to sea level pressure.
* Altimeter-A barometer with a scale calibrated in units of elevation rather than pressure.
* Altitude- height (number of degrees) of an object above the horizon.In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.
* Alto- A prefix to cloud-type names for clouds generally found between 3000 and 7000 meters. Alto comes from the Latin word meaning "middle." E.g., Altostratus.
* Altostratus cloud-A high-level stratus cloud.
* Amphibole-A group of double chain silicate minerals. Hornblende is a common amphibole.
* Anaerobic-Without oxygen; anaerobic bacteria are bacteria that live without oxygen.
* Andean margin-A continental margin characterized by subduction of an oceanic lithospheric plate beneath a continental plate. (syn: active continental margin)
* Andesite- Intermediate volcanic rocks containing 54 to 62 percent silica and moderate amounts of iron and magnesium. Andesite minerals commonly include plagioclase and hornblende, with lesser amounts of mica, pyroxene, and various accessory minerals. Andesites are aphanitic in texture and are usually medium dark in color. They occur with composite volcanic cones associated with convergent plate margins.
* Andesite-A fine-grained gray or green volcanic rock intermediate in composition between basalt and granite, consisting of about equal amounts of plagioclase feldspar and mafic minerals.
* Anemometer- an instrument that measures wind speed
* Aneriod Barometer- A barometer whose pressure sensor consists of an aneroid capsule, a thin, hollow disk partially evacuated and sealed. The difference between the exterior air pressure and the interior pressure causes the disk to expand or contract slightly. This movement is amplified by a spring or gear mechanism to indicate changes in air pressure on a scale or recording chart. These were the most common home barometers for many years and are still found on many walls. Aneroid means "without liquid."
* Angle of repose-The maximum slope or angle at which loose material remains stable.
* Angular unconformity-An unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted or folded older rocks.
* Anion-An ion that has a negative charge.
* Antarctic Ozone Hole- An extended area of extreme depletion of the ozone layer which forms over the southern polar region each austral spring (September through November).
* Antecedent stream-A stream that was established before local uplift started and cut its channel at the same rate the land was rising.
* Anticline-A fold in rock that resembles an arch; the fold is convex upward and the oldest rocks are in the middle.
* Anticyclone- An area of high pressure, also called a High, around which the winds circulate in a clockwise direction in the Northern Hemisphere (and counterclockwise in the Southern Hemisphere). It is usually responsible for fair, dry weather. When the area of highest pressure is elongated, it is called a high pressure ridge, or simply a ridge.
* Anticyclone-A system of rotating winds that develop where descending air spreads out over the Earth's surface. In the northern hemisphere, the Coriolis effect deflects the diverging winds to the right, forming a pinwheel pattern with the air spiraling clockwise. In the southern hemisphere, the Coriolis effect deflects winds leftward, and creates a counterclockwise spiral.
* Anticyclonic- The wind circulation pattern in Highs, or anticyclones, that has a sense of rotation opposite to that of cyclones and the Earth's rotation. Also, any rotational pattern that is opposite to that of cyclones and the Earth's rotation.
* Aphelion -The point in its orbit where a planet is farthest from the Sun.
* Apogee -The point in orbit farthest from the Earth.
* Apparent magnitude-The brightness of a star as seen from Earth.
* Aquifer- A body of rock that contains significant quantities of water that can be tapped by wells or springs.
* Aquifer-A porous and permeable body of rock that can yield economically significant quantities of ground water.
* Archean Eon-A division of geologic time 3.8 to 2.5 billion years ago. The oldest known rocks formed at the beginning of or just prior to the start of the Archean Eon.
* Arête-A sharp narrow ridge between adjacent valleys formed by glacial erosion.
* Artesian aquifer-An inclined aquifer that is bounded top and bottom by layers of impermeable rock so the water is under pressure.
* Artesian Well- A well in an aquifer where the groundwater is confined under pressure and the water level will rise above the top of the confined aquifer.
* Artesian well-A well drilled into an artesian aquifer in which the water rises without pumping and in some cases spurts to the surface.
* Artificial channel-Any channel dredged to modify the natural channel or to alter the course of a stream.
* Artificial levee-A wall built along the banks of a stream to prevent rising flood water from spilling out of the stream channel onto the flood plain.
* Artificial Recharge- The unnatural addition of surface waters to groundwater. Recharge could result from reservoirs, storage basins, leaky canals, direct injection of water into an aquifer, or by spreading water over a large land surface.
* Aseismic ridge-A submarine mountain chain with little or no earthquake activity.
* Ash (volcanic)-Fine pyroclastic material less than 2 mm in diameter.
* Ash- Fine particles of pulverized rock blown from an explosion vent. Measuring less than 1/10 inch in diameter, ash may be either solid or molten when first erupted. By far the most common variety is vitric ash (glassy particles formed by gas bubbles bursting through liquid magma).
* Ash Flow- A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure. The mass of pyroclastics is normally of very high temperature and moves rapidly down the slopes or even along a level surface.
* Ash flow-A mixture of volcanic ash, larger pyroclastic particles, and gas that flows rapidly along the Earth's surface as a result of an explosive volcanic eruption. (syn: nuee ardente)
* Ash-flow tuff-A pyroclastic rock formed when an ash flow solidifies.
* Aspect-The orientation of a slope with respect to the Sun; the geographic orientation or exposure of a slope.
* Asteroid- A rocky space object that can be from a few hundred feet to several hundred km wide.
* Asteroid number -Asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N.
* Asteroid-One of the many small celestial bodies in orbit around the Sun. Most asteroids orbit between Mars and Jupiter.
* Asthenosphere- The shell within the earth, some tens of kilometers below the surface and of undefined thickness, which is a shell of weakness where plastic movements take place to permit pressure adjustments.
* Asthenosphere-The portion of the upper mantle beneath the lithosphere. It consists of weak, plastic rock where magma may form and extends from a depth of about 100 kilometers to about 350 kilometers below the surface of the Earth.
* Astronomical unit (AU) -The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116 miles).
* Astronomy-Astronomy is the study of everything that lies beyond the Earth's atmosphere.
* Atmosphere -One atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth.
* Atmosphere- The envelope of gases that surround a planet's surface held by the planet's gravity. Over the Earth, the atmosphere is divided into several layers based on their properties. The most common layer designations are- the troposphere, the stratosphere, the mesosphere and the exosphere. Atmospheric chemists divide the atmosphere into the heterosphere, where gases are well mixed, and the homosphere, where gases are found in differentiated layers.
* Atmosphere-A mixture of gases, mostly nitrogen and oxygen, with smaller amounts of argon, carbon dioxide, and other gases. The atmosphere is held to the Earth by gravity and thins rapidly with altitude.
* Atmospheric inversion-See "inversion (atmospheric)."
* Atoll-A circular reef that surrounds a lagoon and is bounded on the outside by deep water of the open sea.-atom
* Aurora borealis -The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.
* Autumnal Equinox- see Equinox
* Autumnal- Pertaining to the Autumn season.
* Avalanche- A large mass of material falling or sliding rapidly due to the force of gravity. In many cases, water acts as a catalyst and/or lubricant. Avalanches often are classified by what is moving, such as a snow, ice, soil, or rock avalanche. A mixture of these materials is commonly called a debris flow.
* Axis of rotation- An imaginary straight line through a planet's poles on which the planet rotates. Earth's is tilted 23.5 degrees.

* B horizon-The soil layer just below the A horizon where ions leached from the A horizon accumulate.
* Back arc basin-A sedimentary basin on the opposite side of the magmatic arc from the trench, either in an island arc or in an Andean continental margin.
* Backshore-The upper zone of a beach that is usually dry but is washed by waves during storms.
* Bajada-A broad depositional surface extending outward from a mountain front formed by the merging of alluvial fans.
* Banded iron formation-Iron-rich layered sedimentary rocks precipitated from the seas mostly between 2.6 and 1.9 billion years ago, as a result of rising atmospheric oxygen concentrations.
* Bank-The rising slope bordering the side of a stream channel.
* Bar -A unit of pressure, equal to the sea-level pressure of Earth's atmosphere; 1 bar = 0.987 atmosphere = 101,300 pascals = 14.5 lbs/square inch = 100,000 Newtons per square meter.
* Bar-An elongate mound of sediment, usually composed of sand or gravel, in a stream channel or along a coastline.
* Barchan dune-A crescent shaped dune, highest in the center, with the tips facing downwind.
* Barometer- An instrument for measuring atmospheric pressure. There are three common types of barometers used widely today- mercury barometer; aneroid barometer; electronic barometer. See glossary entries for more on these types.
* Barometer-A device used to measure barometric pressure.
* Barometric pressure - The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).
* Barometric pressure-The pressure exerted by the Earth's atmosphere.
* Barrier island-A long, narrow, low-lying island that extends parallel to the shoreline.
* Barrier reef-A reef separated from the coast by a deep, wide lagoon.
* Basal slip-Movement of the entire mass of a glacier along the bedrock.
* Basalt- A dark, fine-grained volcanic rock.
* Basalt-A dark-colored, very fine grained, mafic, volcanic rock composed of about half calcium-rich plagioclase feldspar and half pyroxene.
* Base level-The deepest level to which a stream can erode its bed. The ultimate base level is usually sea level, but this is seldom attained.
* Basement rock-The older granitic and related metamorphic rocks of the Earth's crust that make up the foundations of continents.
* Basin-A low area of the Earth's crust of tectonic origin, commonly filled with sediment.
* Batholith-A large plutonic mass of intrusive rock with more than 100 square kilometers of surface exposed.
* Bauxite-A gray, yellow, or reddish brown rock composed of a mixture of aluminum oxides and hydroxides. It is the principle ore of aluminum.
* Baymouth bar-A bar that extends partially or completely across the entrance to a bay.
* Beach terrace-A level portion of old beach elevated above the modern beach by uplift of the shoreline or fall of sea level.
* Beach-Any strip of shoreline washed by waves or tides.
* Bed load-That portion of a stream's load that is transported on or immediately above the stream bed.
* Bedding-Layering that develops as sediments are deposited.
* Bedrock-The solid rock that underlies soil or regolith.
* Bed-The floor of a stream channel. Also the thinnest layer in sedimentary rocks, commonly ranging in thickness from a centimeter to a meter or two.
* Bench mark- A marker in the ground indicating the exact elevation above sea level.
* Benioff zone-An inclined zone of earthquake activity that traces the upper portion of a subducting plane in a subduction zone.
* Big Bang Hypothesis: The theory that the universe was originally a single dense sphere of hydrogen that exploded into a gigantic expanding cloud that eventually condensed into separate galaxies.
* Big bang-An event thought to mark the beginning of our Universe. The big bang theory postulates that 10 to 20 billion years ago, all matter exploded from an infinitely compressed state.
* Bioclastic sedimentary rock-Sedimentary rocks such as most limestone, that are composed of broken shell fragments and similar remains of living organisms. The fragments are clastic, but they are of biological origin.
* Biome-A community of plants living in a large geographic area characterized by a particular climate.
* Bioremediation-The use of microorganisms to decompose an environmental contaminant.
* Biosphere-The zone inhabited by life.
* Biotite-Black, rock forming mineral of the mica group.
* Bitumen-A general name for solid and semi-solid hydrocarbons that are fusible and soluble in carbon bisulfide. Petroleum, asphalt, natural mineral wax, and asphaltites are all bitumens.
* Black hole -An object whose gravity is so strong that the escape velocity exceeds the speed of light.
* Black hole-A small region of space that contains matter packed so densely that light cannot escape from its intense gravitational field.
* Black ice- Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also refers to thin, transparent ice on road surfaces.
* Black smoker-A jet of black water spouting from a fracture in the sea floor, commonly near the mid-oceanic ridge. The black color is caused by precipitation of very fine grained metal sulfide minerals as hydrothermal solutions cool by contact with sea water.
* Blackbody temperature -The temperature of an object if it is reradiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature.
* Blizzard- Although blizzard is often used to describe any major snow storm with strong winds, the technical definition for a blizzard requires- at least 3 hours in duration; low temperatures (usually less than minus 7C or 20F), strong winds (greater than 55 km/h or 35 mph), blowing snow which reduces visibility to less that 1 kilometer (0.6 miles). Surprisingly, snowfall need not be falling as long as the amount of snow in the air (falling or blowing) reduces visibility to less than 400m (0.25 miles).
* Blowout-A small depression created by wind erosion.
* Blue stars- These are the hottest stars, with a surface temperature of more than 37,000°F.
* Body wave-A seismic wave that travel through the interior of the Earth.
* Boiling point - The temperature at which a liquid changes to a vaporous state. The temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid. The boiling point of pure water at standard pressure is 100°C or 212°F.
* Bolide -An exploding meteorite.
* Bomb- Fragments of molten or semi-molten rock, several inches to several feet in diameter, which are blown out during an explosive volcanic eruption. Because of their semi-plastic condition, bombs are often modified in shape during their flight or upon impact.
* Braided stream-A stream that divides into a network of branching and reuniting shallow channels separated by mid-channel bars.
* Branching chain reaction-A nuclear fission reaction in which the initial reaction releases two or three neutrons, each of which triggers the fission of additional nuclei.
* Breccia -A course-grained rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix.
* Breccia-A coarse-grained sedimentary rock composed of angular, broken fragments cemented together.
* Brittle fracture-Rupture that occurs when a rock breaks sharply.
* Burial metamorphism-Metamorphism that results from deep burial of rocks in a sedimentary basin. Rocks metamorphosed in this way are usually unfoliated.
* Butte -A conspicuous, isolated, flattop hill with steep slopes.
* Butte-A flat-topped mountain, with several steep cliff faces. A butte is smaller and more tower-like than a mesa.