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African Plate
The African Plate is a major tectonic plate covering the African continent and adjoining Atlantic and Indian Ocean floor, slowly splitting into the Nubian and Somalian plates.
Aftershock
An aftershock is a smaller earthquake that follows a larger mainshock in the same area. Aftershock sequences follow statistical patterns, allowing seismologists to forecast them.
Agate
Agate is a banded variety of chalcedony, a fine-grained quartz, that forms in colorful layers filling cavities in volcanic rocks, with Brazil the major commercial source.
Ageostrophic wind
The ageostrophic wind is the component of the actual wind left after subtracting the geostrophic wind, the balanced flow, and it produces divergence and vertical motion.
Aggradation
Aggradation is the raising of a land surface, most often a river bed or floodplain, through net sediment deposition, occurring when sediment supply exceeds a river's transport capacity.
Aggregation (cloud physics)
Aggregation in cloud physics is the process by which ice crystals collide and stick together inside clouds, forming large irregular snowflakes; it is a key process in precipitation growth.
Agnew Mine
Agnew Mine is an underground gold mine in Western Australia owned by Gold Fields, 23 km west of Leinster, producing 245.3 koz of gold in 2025.
Agua Nueva Formation
Agua Nueva Formation: Upper Cretaceous (Cenomanian-Turonian) marine limestone-marl-bentonite unit of northeastern Mexico, defined by Muir 1934, type locality Cañon de la Borrega, Sierra de Tamaulipas; also an Eagle Ford-like shale play.
Aguja Formation
Aguja Formation: Late Cretaceous (Campanian-Maastrichtian) sandstone, shale, clay, and lignite unit in Trans-Pecos Texas and Coahuila/Chihuahua, Mexico; overlies Pen Formation, underlies Javelina Formation; named by Adkins 1933; fossiliferous.
Agulhas leakage
Agulhas leakage is the transport of warm, salty Indian Ocean water into the South Atlantic south of Africa, via Agulhas rings and other eddies, feeding the global overturning circulation.
Ahafo North mine
Ahafo North is an open-pit gold mine in Ghana's Ahafo Region, operated by Newmont, with first gold poured in 2025 and commercial production declared that year.
Ahafo South mine
Ahafo South is a large gold mine in Ghana's Ahafo region, near Kenyase and operated by Newmont Ghana Gold Limited, with commercial production since 2006.
Air mass
An air mass is a large body of air whose temperature and humidity are horizontally uniform, so it behaves as a single unit; most weather occurs at fronts.
Air quality model
An air quality model is a numerical model that simulates the emission, transport, chemical transformation, and deposition of air pollutants, predicting concentrations over regions thousands of kilometers wide.
Air sparging
Air sparging, also called in situ air stripping, is a groundwater remediation method first used in Germany in 1985 that injects air into contaminated aquifers to strip volatile organic contaminants.
Airborne electromagnetic survey
An airborne electromagnetic survey is a geophysical method in which aircraft carry or tow electromagnetic transmitters and receivers to map ground conductivity, first tested in Canada in 1948.
Airborne fraction
The airborne fraction is the fraction of human CO2 emissions remaining in the atmosphere rather than absorbed by land and ocean sinks, averaging about 44 percent since 1959.
Airborne laser scanning
Airborne laser scanning (ALS) is a remote sensing method in which an aircraft-mounted lidar measures ground and vegetation distances to produce 3D point clouds and terrain models.
Airborne snow observatory
The Airborne Snow Observatory (ASO) is a NASA airborne system pairing scanning lidar with an imaging spectrometer to map mountain snow depth and snow water equivalent, first flown in 2013.
Aircore drilling
Aircore drilling is a shallow mineral exploration drilling method that uses compressed air to lift cuttings up the drill string for analysis, developed in Western Australia in the 1970s.
Akademik Mstislav Keldysh (Академик Мстислав Келдыш)
Akademik Mstislav Keldysh (Академик Мстислав Келдыш) is a Russian oceanographic research vessel, built in Finland and entered in 1981, best known as support ship for the Mir submersibles.
Akyem Gold Mine
The Akyem Gold Mine is a large open-pit gold mine in Ghana's Eastern Region, one of the country's largest, sold by Newmont to Zijin Mining in 2025.
Al Roker
Al Roker (Albert Lincoln Roker Jr.) is an American weather presenter, television host, and author who has been the weather anchor of NBC's TODAY since 1996.
Alamos Gold Inc.
Alamos Gold Inc. is a Canadian intermediate gold producer headquartered in Toronto, listed as AGI, with mines in Ontario and Mexico producing 545,400 ounces of gold in 2025.
Albite
Albite is a rock-forming mineral, the sodium end member of the plagioclase feldspar series with formula NaAlSi₃O₈, named in 1815 by Gahn and Berzelius from the Latin albus, white.
Aleutian Low
The Aleutian Low is a semi-permanent low-pressure center over the North Pacific near the Aleutian Islands, strongest in winter, displaced northward and almost nonexistent in summer.
Aleutian subduction zone
The Aleutian subduction zone is the 3,000-km boundary where the Pacific plate subducts beneath North America along the Aleutian trench, source of the great 1964 Alaska earthquake.
Alex Beresford
Alex Beresford, also known as Alexander Beresford, is a British weather presenter for ITV Weather and Good Morning Britain who challenged Piers Morgan live on air in 2021.
Alfisols
Alfisols are a soil order in USDA Soil Taxonomy: moderately leached soils with a clay-enriched subsurface and base saturation of 35% or greater, mostly in temperate regions.
Alfred Wegener Institute for Polar and Marine Research
The Alfred Wegener Institute for Polar and Marine Research (Alfred-Wegener-Institut, AWI) is a German institute in Bremerhaven that coordinates German polar research, founded in 1980.