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Antarctic circles of latitude
The Antarctic Circle, at about 66°33′48.8″ south, marks where the Sun stays above or below the horizon for a full day, first crossed by James Cook in 1773.
Antarctic Circumpolar Wave
The Antarctic Circumpolar Wave (ACW) is a proposed pattern of coupled sea surface temperature, pressure, and sea-ice anomalies propagating eastward around the Southern Ocean, identified in 1996.
Antarctic Intermediate Water
Antarctic Intermediate Water (AAIW) is a cold, low-salinity water mass of the Southern Ocean, marked by a salinity minimum at 700–1000 m that spreads northward into all three ocean basins.
Antarctic land bridge
The Antarctic land bridge, formalized as the Weddellian Isthmus, was a land connection between southern South America and the Antarctic Peninsula until roughly 55 million years ago.
Antarctic Plate
The Antarctic Plate is a major tectonic plate comprising Antarctica and the surrounding ocean floor, bounded almost entirely by mid-ocean spreading ridges. East and West Antarctica moved separately until about 11 million years ago.
Anthropocene
The Anthropocene, a term coined by Paul Crutzen in 2000, is a proposed geological epoch of dominant human impact on Earth, rejected for formal status by the IUGS in 2024.
Anticline
An anticline is an arch-shaped fold in layered rock whose core holds the stratigraphically oldest strata, formed when crustal compression buckles the layers; anticlines are major petroleum traps.
Antigorite
Antigorite is a soft green magnesium silicate mineral of the serpentine group, described in 1840 and named after the Valle Antigorio in Italy. It forms above about 250 °C and carries water deep into subduction zones.
Antillanca Volcanic Complex
The Antillanca Volcanic Complex is a volcanic field in the southern Andes of Chile, southeast of Lago Puyehue, whose highest peak is the Casablanca stratovolcano.
Anvil cloud
An anvil cloud, formally called incus, is the flat, spreading icy top of a mature cumulonimbus storm cloud, formed where rising air meets a stable layer.
Apatite
Apatite is a group of hexagonal phosphate minerals, the most abundant phosphate mineral in Earth's crust and the main industrial source of phosphorus, named from the Greek for 'to deceive'.
Aphotic zone
The aphotic zone is the part of the ocean below roughly 1,000 meters where sunlight does not penetrate, comprising the bathypelagic, abyssopelagic, and hadopelagic zones and lit only by bioluminescence.
Apparent resistivity
Apparent resistivity is the resistivity a subsurface would have if it were homogeneous and isotropic, computed from DC survey measurements and inverted to recover a true resistivity model.
April 2015 Nepal earthquake
The April 2015 Nepal earthquake, known as the Gorkha earthquake, was a magnitude 7.8 quake that struck central Nepal on April 25, 2015, killing nearly 9,000 people.
Aptian
The Aptian is a stage of the Early Cretaceous, spanning roughly 121.4 to 113.1 million years ago. Defined by Alcide d'Orbigny in 1840, it is named after Apt, France.
Aquatic Sciences
Aquatic Sciences is a peer-reviewed Swiss journal covering limnology, hydrology, and aquatic research, published by Springer; it continues the Swiss Journal of Hydrology, founded in 1920.
Aqueduct 4.0
Aqueduct 4.0 is the 2023 version of the World Resources Institute's Aqueduct water risk framework, mapping 13 open-data indicators of water quantity, quality, and reputational risk worldwide.
Aquifer
An aquifer, also known as a groundwater basin, is an underground layer of soil or rock that stores groundwater and transmits it to wells, springs, and streams.
Aquitanian
The Aquitanian is the lowest stage of the Miocene, spanning 23.03 to 20.45 million years ago, named after Aquitaine in France, and defined by a boundary section in northwestern Italy.
Aquitard
An aquitard is a saturated geologic formation of low permeability, usually clay, silt, or shale, that slows groundwater, confines aquifers, and allows slow vertical leakage between them.
Arabian Plate
The Arabian Plate is a tectonic plate carrying the Arabian Peninsula and parts of the Middle East, formed when it rifted from Africa about 25 million years ago.
Arabian-Nubian Shield
The Arabian-Nubian Shield is an exposure of juvenile Neoproterozoic crust in Egypt, Sudan, Arabia, and surrounding countries, split in two by the Red Sea rift.
Aragonite
Aragonite is an orthorhombic calcium carbonate mineral, CaCO₃, less stable than calcite at surface conditions and the classic first example of mineral polymorphism, named in 1788 in Spain.
Aragonite saturation state
The aragonite saturation state (Ωarag) measures how close seawater is to chemical equilibrium with aragonite, a form of calcium carbonate that marine organisms use to build shells and skeletons.
Araguainha crater
The Araguainha crater, also called the Araguainha Dome (Domo de Araguainha), is a 40 km-wide impact crater in central Brazil and the largest confirmed impact structure in South America.
Archean
The Archean, also spelled Archaean, is the second eon of Earth's geological history, lasting from 4.031 to 2.5 billion years ago and forming most of the Precambrian.
Arctic Circle
The Arctic Circle, also called the Arctic Polar Circle, is the latitude at about 66° 34′ north, above which the sun never sets on the summer solstice.
Arctic front
An arctic front is the boundary separating deep, cold arctic air from shallower, less cold polar air, distinct from the polar front and linked to intense snow squalls.
Arctic ice pack
The Arctic ice pack is the drifting sea ice covering the Arctic Ocean and adjoining seas, floating on the ocean and reaching its minimum extent each September.
Arctic Oscillation
The Arctic Oscillation, also called the Northern Annular Mode, is a climate variability pattern where pressure over the Arctic fluctuates against mid-latitude pressure, reshaping winter weather across North America, Europe, and Asia.