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Geological Glossary - A

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A horizon The top layer of soil. It is usually dark and contains organic material formed do to the decay of vegetable matter.
A-horizon The top layer of soil. Plant and other organic debris builds up in this layer. This is the part of the soil generally referred to as 'top soil'.
A-type earthquake Events with clear P-waves and S-waves occurring under volcanoes at depths of 1-10 km.
AFMAG Audio Frequency Magnetic technique in which natural electromagnetic fields in the audio-frequency range are used to study lateral changes in earth resistivity.
AMT Audio MagnetoTelluric method in which magnetotelluric measurements are made in the audio frequency range (10-10,000 Hz) for medium to shallow depth investigation.
Agung Active volcano, elevation 3141 m (10380 ft) in Bali, Indonesia (8.342ø S 115.508ø E). Agung is known to have erupted three times, the last in 1964.
Anthropocene New proposed epoch covering human geological impact on the Earth
Archean The time interval between 3800-2500 million years ago. The Archean is one of the Precambrian time intervals.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
a'a' 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.
ablation (glacial) All processes which include melting evaporation (sublimation) wind erosion and calving (breaking off of ice masses) that remove snow or ice from a glacier or snowfield. The term also refers to the amount of snow or ice removed by these processes.
abrasion The wearing away of a rock by friction, rubbing, scraping, or grinding of a river's, glacier's, or wind's load.
absolute date An estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.
absolute time Geological time measured in a specific number of years.
abundance O 46.6 Mg 2.09
Si 27.62 Ti .044
Al 8.13 H 0.14
Fe 5.00 Mn 0.1
Ca 3.63 Mn 0.1
Na 2.83 S 0.05
K 2.59 C 0.03
abyssal floor The relatively flat surface of the deep ocean floor spreading from an oceanic ridge. It consists of the abyssal plain and the abyssal hill.
abyssal hill The hill rising as much as 1000m above the surrounding floor of the ocean. They exist seaward of the abyssal plain where they become covered by sediment.
abyssal plain An area of the ocean floor with a slope of less than 1 in 1,000 or flat, nearly level areas that occupy the deepest portions of many ocean basin.
abyssal plain The broad expanse of sea floor lying between about 3 and 6 kilometers (-2 to 4 miles) below sea level.
accelerometer A seismograph for measuring ground acceleration as a function of time.
accreted terrane Small crustal fragments, island arc, or seamount which are transported by the moving oceanic plate and are added to a continental mass at the subduction zone.
accretion A process that adds part of one tectonic plate to a larger plate along a convergent (collisional) plate boundary.
accretionary wedge Accumulation on the ocean floor of sedimentary material scraped off the subducted plate by the upper plate.
accumulation (glacial) All processes including snowfall condensation avalanching snow transport by wind and freezing of liquid water that add snow or ice to a glacier floating ice or snow cover. The term also includes the amount of snow or other solid precipitation added to a glacier or snowfield by these processes.
accumulative minerals Minerals that make up the extract assemblage during fractional crystallization of a magma.
actinolite A bright to gray-green member of the amphibole mineral family. In addition to silica, it contains calcium, magnesium, and iron. Actinolite is a non-hazardous relative of asbestos and is a common mineral in metamorphic rocks.
active fault A fault along which slip has occurred in historical (or Holocene) time or earthquake foci are located.
active lobe of a delta. The site on a delta where functioning distributary channels cause the delta to grow seaward.
active margin The edge of a plate bordered by a trench.
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. "
actualism The interpretation of ancient rocks by applying the results of analyses of modern-day geologic processes in accordance with the principle of uniformitarianism.
adaptation A feature of an organism that serves one or more functions useful to the organism.
adaptive breakthrough An evolutionary innovation that affords a group of organisms a special ecologic opportunity and often leads to the adaptive radiation of that group.
adaptive radiation Evolutionary diversification of a taxon's morphology, ecology, physiology, behavior and other characteristics over a geologically short time interval leading to the appearance of a number of new taxa.
aeon see
aftershock An earthquake which follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake. Generally, major earthquake are followed by a larger number of aftershock decreasing in frequency with time.
age geologic. The division of geologic time smaller than an epoch.
aggregation The process of combination of very small volcanic ash or dust particles in volcanic plumes in the atmosphere to form larger particles which settle out rapidly to the earth's surface due to their large diameter.
albedo The percentage of solar radiation reflected from the earth's surface. This percentage is higher for ice than for land or water and usually higher for land than for water.
alcove A large niche or recession formed in a steep cliff.
algal mat A layered communal growth of algae observed in fossil an in present day tidal zones associated with carbonate sedimentation.
algal ridge The durable structure formed by coralline algae that buttresses the front of a modern coral reef.
alkali metal A strongly basic metal like potassium or sodium.
alkane One of the group of hydrocarbon of the paraffin series, for example methane, ethane and propane.
allelochemicals Chemicals produced by one species that enter the environment and influence other species.
allochthonous clay Clay originating in an environment different from the final sedimentary environment (rocky substrate soil or upstream sediment subjected to erosion).
alluvail fan A low, cone shaped deposit of terrestrial sediment formed where a stream undergoes an abrupt reduction of slope.
alluvium An unconsolidated terrestrial sediment composed of sorted or unsorted sand, gravel, and clay deposited during comparatively recent geologic time by a stream or river. Alluvial deposits include mid channel bar, point bar, floodplain deposits, alluvial fans, and fluvial terrace.
alluvlal fan A low cone-shaped structure that form where an abrupt reduction in slope -- for example, the transition from a highland area to a broad valley -- causes a stream to slow down.
alpine glaciers A glacier occupying a valley, usually high in mountainous terrain.
amniote egg The type of egg laid by reptiles and birds having a nutritious yolk and a hard outer shell to protect the embryo from the dry environment. The amniote egg is named for the amnion a sac that contains the embryo.
amphibole " A family of silicate mineral forming prism or needlelike crystal. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. Actinolite has no aluminum; it and is needle-shaped and light green. Blue amphibole contains sodium and, of course, is bluish in color. "
amphibolite "A family of silicate mineral forming prism or needlelike crystal. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. Actinolite has no aluminum; it and is needle-shaped and light green. Blue amphibole contains sodium and, of course, is bluish in color. "
amplitude The maximum height of a wave crest or depth of a trough.
andesite A find-grained extrusive igneous rock intermediate in composition between rhyolite which is felsic and basalt which is mafic. Intermediate volcanic rock containing 54 to 62 percent silica and moderate amounts of iron and magnesium. Andesite mineral 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 cone associated with convergent plate margins.
angle of repose The steepest slope angle in which a particular sediment will lie without cascading down.
angstrom A length of 10 to the minus tenth meter or one hundred millionth of a centimeter.
angular unconformity An unconformity separating horizontal strata above from older strata that had been tilted and eroded.
anhydrite The mineral that consists of calcium sulfate (CaS04) or the rock composed of this mineral.
annual snowline A term used by glaciologists (scientists who study glacier) for the boundary where the amount of snow loss from melting equals the amount of snow accumulation from snowfall (also called firn limit).
anthracite The most highly metamorphosed form of coal, containing 92 to 98 percent of fixed carbon. It is black, hard, and glassy.
anticline A upward-curving (convex) fold in rock that resembles an arch. The central part contains the oldest section of rock
anticline A fold that is concave in a downward direction -- that is the vertex is the lowest point.
aphanitic An igneous rock texture in which individual mineral grain are too small to be distinguished with the naked eye.
aplite A light-colored igneous rock with the same mineral composition as granite: quartz, plagioclase feldspar, and potassium feldspar, but with a fine-grained, almost sugary texture
aquiclude An impermeable geologic formation or stratum which will not hold or transmit fluid.
aquifer Saturated permeable geologic unit that can store and transmit significant quantities of groundwater.
aquitard A geologic formation or stratum that significantly retards fluid movement.
arch A mass of rock in the shape of an arch. Arches are commonly formed by erosion of stratified rocks where harder rock rests upon weaker rock.
argillaceous A term used to describe clay-rich rocks.
argillic horizon A clay-rich layer of soil. Clay often form in overlying soil layers from the decomposition of feldspar and other mineral. The extremely fine clay particles are gradually carried down by water to accumulate into the argillic horizon.
argillite Name used for unusually hard, fine-grained sedimentary rocks, such as shale, mudstone, silttone, and claytone. Commonly black.
arkose A variety of sandstone containing abundant feldspar and quartz, frequently in angular, poorly sorted grain.
array An ordered arrangement of seismometers or geophones the data from which feeds into a central receiver.
arrival The appearance of seismic energy on a seismic record.
arrival time The time at which a particular wave phase arrives at a detector.
arroyo A steep-sided and flat-bottomed gully in an arid region that is occupied by a stream only intermittently, after rains.
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.
artificial recharge The unnatural addition of surface waters to groundwater. Recharge could result from reservoir storage basin, leaky canals, direct injection of water into an aquifer, or by spreading water over a large land surface.
arête A sharp, narrow ridge separating two glacial valley.
aseismic "Not associated with an earthquake as in aseismic slip. Also used to indicate an area with no record of earthquakes; an aseismic zone. "
ash Fine particles of rock material ejected during an explosive volcanic eruption (commonly intermediate to felsic events). Ash may be either solid or molten when first erupted and generally measures less than 0.10 inch in size (larger particles have other names).
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 pyroclastic is normally of very high temperature and moves rapidly down the slope or even along a level surface.
ashfall (subaerial) Volcanic ash that has fallen through the air. The resulting deposit is usually well sorted and exhibits a finely layered structure.
asthenosphere The layer below the lithosphere which is marked by low seismic wave velocities and high seismic wave attenuation and probably partially molten. The layer or shell of Earth below the lithosphere which is weak and in which large-scale isostatic adjustment take place, magma may be generated, and seismic waves are strongly attenuated. This zone of soft, easily deformed rock exists at depths of 100 kilometers to as deep as 700 kilometers.
astrobleme A circular erosional feature that has been ascribed to the impact of a meteorite or comet.
atmosphere The gaseous envelope surrounding the Earth. The mixture of gases that surrounds the Earth, being held thereto by gravity. It consists by volume of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and minute quantities of helium, krypton, neon, and xenon. The atmosphere is so compressed by its own weight that half is within 5.5 km of the Earth's surface.
atmosphere (unit) A unit of pressure equal to 101,325 newtons per square meter, or about 14.7 pounds per square inch.
atoll A continuous or broken circle of coral reef and low coral island surrounding a central lagoon.
augen Augen are relatively large, eye-shaped mineral grain in certain types of metamorphic rocks, especially schist and gneiss. (Augen = eyes in German)
authigenesis Process by which new mineral form in place within a sediment during deposition or in the early stage of diagenesis.
autochthonous clay Clay originating in the soft sediment or sedimentary rock where it is identified and where it formed either by neoformation (i.e., from dissolved elements) or by transformation (i.e., from mineral growth on preexisting sedimentary substrate).
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.
axial plane An imaginary plane that cuts through a fold dividing it as symmetrically as possible.
axis A plane intersecting the point where a folded bed shows maximum curvature.
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