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Amphibole
Amphibole, or the amphibole group, is a supergroup of rock-forming inosilicate minerals defined by a double chain of silica tetrahedra, abundant in intermediate igneous and metamorphic rocks.
Amphibolite
Amphibolite is a coarse-grained metamorphic rock composed mainly of hornblende and plagioclase, formed from basalt or gabbro, and quarried for highway aggregate and as dimension stone sold as black granite.
Amplitude versus offset
Amplitude versus offset (AVO) is a seismic analysis technique that measures how reflected wave amplitude changes with source-receiver distance, used as a direct hydrocarbon indicator.
Amplitude versus offset analysis
Amplitude versus offset (AVO) analysis measures how seismic reflection amplitude changes with source-receiver offset, and uses that variation to constrain subsurface rock properties and fluid content, notably free gas.
AMSU-A
AMSU-A, the Advanced Microwave Sounding Unit-A, is a 15-channel microwave radiometer flown on polar-orbiting weather satellites since 1998 to measure atmospheric temperature from the surface to about 45 km.
Amundsen Sea Low
The Amundsen Sea Low (ASL) is a quasi-stationary climatological low-pressure center off West Antarctica, the deepest of three lows around Antarctica and a key control on the region's climate.
An Inconvenient Truth
An Inconvenient Truth is a 2006 American documentary directed by Davis Guggenheim, following former Vice President Al Gore on the lecture circuit explaining global warming.
An'ei eruption of Mount Sakurajima
The An'ei eruption of Mount Sakurajima (安永噴火) was a large flank eruption that began in 1779 in Kagoshima Prefecture, Japan, killing about 150 people and creating the An'ei Islands.
Anabatic wind
An anabatic wind is a wind that blows up a hillside or mountain slope during the day, driven by solar heating of the surface and strongest in the afternoon.
Analcime
Analcime, historically called analcite, is a hydrated sodium aluminosilicate zeolite mineral with the formula NaAlSi₂O₆·H₂O, best known for its trapezohedral crystals that mimic cubic symmetry and its close relationship to leucite.
Anammox
Anammox, short for anaerobic ammonium oxidation, is a microbial process in which bacteria convert ammonium and nitrite into nitrogen gas, discovered in 1995 and now used to treat wastewater.
Anatoxin-a
Anatoxin-a (ATX-a) is a potent neurotoxic bicyclic amine alkaloid produced by freshwater cyanobacteria, which acts on the nicotinic acetylcholine receptor and can cause neurological disorders and death.
Andahua-Orcopampa
Andahua-Orcopampa, the Valley of the Volcanoes, is a monogenetic volcanic field in southern Peru's Central Volcanic Zone: a 60 x 60 km scoria cone and lava field, 4,713 m high, last erupting 1490 CE.
Andalusite
Andalusite is an aluminum nesosilicate mineral, Al₂SiO₅, one of three polymorphs alongside kyanite and sillimanite. It forms in metamorphosed clay-rich rocks and is used chiefly as a raw material for refractory ceramics.
Andesite
Andesite is a fine-grained volcanic rock of intermediate composition between basalt and rhyolite, named after the Andes and common around the Pacific Ring of Fire.
Andisols
Andisols, known as Andosols in the World Reference Base, are volcanic-ash soils of USDA Soil Taxonomy, covering under 1 percent of land but rich in organic carbon.
Andradite
Andradite is a calcium-iron garnet mineral, Ca₃Fe₂(SiO₄)₃, named in 1868 for José Bonifácio de Andrada e Silva and best known for its green gem variety, demantoid.
Andrus
Andrus (Mount Andrus), volcano 390023, is a 2,978 m trachytic shield volcano with a 4.5 km caldera in the Ames Range, Marie Byrd Land, Antarctica; no Holocene eruptions are known.
Anemometer
An anemometer, also called a wind gauge or wind meter, is an instrument that measures wind speed. The commonest design is the cup anemometer, brought into general use in 1846.
Anglesite
Anglesite is a dense, transparent lead sulfate mineral (PbSO₄) formed in the oxidized zones of lead ore deposits by weathering of galena, and named for Anglesey, Wales.
Angular unconformity
An angular unconformity is an erosional surface where younger beds lie at an angle to older, tilted or folded strata, recording deposition, deformation, erosion, and renewed deposition.
Anhydrite
Anhydrite is a calcium sulfate mineral, CaSO4, close to gypsum but lacking water in its structure, named in 1804 and used in cement and plaster.
Anisotropy of magnetic susceptibility
Anisotropy of magnetic susceptibility (AMS) is the dependence of a rock's magnetic susceptibility on field direction, defined by three principal susceptibilities forming an ellipsoid that records mineral fabric.
Ankerite
Ankerite is a calcium iron carbonate mineral of the dolomite group, Ca(Fe²⁺,Mg)(CO₃)₂, named in 1825 for Austrian mineralogist Matthias Joseph Anker and found chiefly in iron formations and hydrothermal veins.
Annual exceedance probability
Annual exceedance probability is the probability, expressed as a percentage, that a flood or rainfall event of a given magnitude will be equaled or exceeded in any given year.
Anorthite
Anorthite is the calcium-rich end member of the plagioclase feldspar series, with formula CaAl₂Si₂O₈, an essential mineral of basic igneous rocks like gabbro and basalt.
Anorthosite
Anorthosite is a coarse-grained plutonic igneous rock made almost entirely of plagioclase feldspar. Rare on Earth, it forms the Moon's bright highlands and hosts ilmenite deposits.
Anoxic waters
Anoxic waters, also called anoxic sea water, are bodies of water lacking dissolved oxygen, occurring naturally in basins like the Black Sea and increasingly in coastal waters receiving nutrient pollution.
Antapaccay
Antapaccay is an open-pit copper mine in Yauri, Espinar province, in the Cusco region of southern Peru, wholly owned by Glencore and opened in 2012.
Antarctic Bottom Water
Antarctic Bottom Water (AABW) is the coldest, densest water mass in the ocean, forming around Antarctica's margins and spreading north to fill much of the deep ocean.