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Alcântara Formation
The Alcântara Formation (Formação Alcântara) is a Cenomanian rock unit of the Itapecuru Group in the São Luís Basin, Maranhão, Brazil, exposed in coastal cliffs near the town of Alcântara.
Alcobaça Formation
Alcobaça Formation: Kimmeridgian (Late Jurassic) mixed carbonate-siliciclastic unit of the central Lusitanian Basin, Portugal, between the Abadia and Lourinhã formations; formally defined 2021, seven members, fossil-rich including Guimarota mammal beds.
Aleke Tag
Aleke Tag, also known as Akato or Yusupu Aleketag, is a 6,065-meter ultra-prominent peak in the eastern Kunlun, China, first climbed by Janne Corax in September 2010.
Aletsch Glacier
Aletsch Glacier (Aletschgletscher), Switzerland's largest and longest Alpine glacier at 22.6 km and 83 km², a UNESCO World Heritage site retreating since 1860 and projected to lose most of its volume by 2100.
Aleutian Islands
Aleutian Islands: volcanic island arc separating the North Pacific from the Bering Sea, homeland of the Unangax̂ (Aleut) people, with 52 volcanoes, great megathrust earthquakes, Russian fur-colonization history, and the World War II Attu and Kiska campaign.
Aleutian Ridge
Aleutian Ridge: mostly submerged cordillera of the Aleutian arc, Alaska, carrying the Aleutian Islands on a pedestal above the Aleutian Trench; Pacific plate subducts beneath North America at 55-90 mm/yr.
Alexander Archipelago
The Alexander Archipelago is a group of over 1,100 islands along southeastern Alaska's coast, holding 7 of the 10 largest U.S. islands, shaped by fjords, glaciation, the Tongass National Forest, and endemic wildlife.
Alexander Island
Alexander Island (Isla Alejandro I) is Antarctica's largest island, about 400 km long, off the Antarctic Peninsula's west coast, separated by George VI Sound, named for Tsar Alexander I, sighted 1821, with Mount Hall at 3078 m.
Aliso
Aliso (volcano number 352031) is a largely unexplored Ecuadorian volcanic complex east of Antisana, a 4,267 m stratovolcano with rhyolitic and dacitic lava domes, last confirmed eruption 2450 BCE.
Allegheny Front
The Allegheny Front is a sharp, east-facing escarpment in the Appalachian Mountains, extending about 174 miles from northern Pennsylvania into eastern West Virginia and separating the Appalachian Plateaus from the Valley and Ridge.
Allegheny Mountains
Allegheny Mountains: Appalachian subrange running from Pennsylvania through western Maryland and eastern West Virginia into Virginia; bounded by the Allegheny Front, topping 4,840 feet at Back Allegheny Mountain, with heavy snowfall and the Allegheny National Forest.
Allegheny Plateau
The Allegheny Plateau is a deeply dissected upland in the Appalachian region, covering much of New York, Pennsylvania, Ohio, and West Virginia, rising to more than 4,000 feet.
Allen Formation
Allen Formation: Upper Cretaceous (83.5–66.0 Ma, late Campanian–early Maastrichtian) sedimentary unit in the Neuquén Basin, Río Negro, Argentina, recording the first Atlantic transgression and yielding diverse vertebrate fossils including Kuruleufenia xenopoides and Aeolosaurus.
Alor Island
Alor Island (Pulau Alor), largest island of Indonesia's Alor Archipelago in East Nusa Tenggara, volcanic inner Banda Arc island of about 2,119 km² with 40,000-year-old Makpan cave occupation, high seismicity, and Kalabahi as main town.
Aloyon Island
Aloyon Island, also associated with the village name Aluyon (Aloyan, Aloyun), is a small low-lying island in Burdeos municipality, Quezon, Calabarzon, Philippines, at 14.88°N 121.98°E, elevation 3 m, near Icol, Cabalao, and San Rafael islands.
Alps
The Alps, Europe's great fold mountain system from the Col d'Altare to the Semmering, formed by Cretaceous and Tertiary collision of the Adriatic and European margins; now facing severe glacier retreat, tourism pressure, and Alpine Convention governance.
Altai Mountains (Алтайские горы)
Altai Mountains (Алтайские горы): mountain system across Russia, Mongolia, China, and Kazakhstan, north of the Tianshan; source of the Ob and Irtysh, core of the Altai-Sayan Ecoregion, with the Golden Mountains of Altai World Heritage site (1998).
Altiplano
Altiplano: high internally drained plateau of the central Andes (Bolivia, Peru), northern part of the Altiplano-Puna plateau, 1,800 km long, second highest plateau on Earth, formed by crustal shortening at a subduction margin, not collision.
Alvord Desert
Alvord Desert: a large dry playa (6 by 11 miles) in southeastern Oregon east of Steens Mountain, a former Pleistocene lake bed with hot springs, borax-mining history, and BLM wilderness study area status.
Ama Dablam
Ama Dablam (6,812 m) is a technical granite-and-ice peak in Nepal's Khumbu, Sagarmatha National Park, first ascended March 13, 1961 via the southwest ridge by Gill, Romanes, Bishop, and Ward; the Yellow Tower is its crux.
Amalfi Coast
The Amalfi Coast (Costiera Amalfitana) is a 40 km cliff-lined shore on the Sorrentine Peninsula, Salerno, Italy; UNESCO World Heritage site since 1997, heart of the medieval Maritime Republic of Amalfi and its terraced lemon groves.
Amaltheenton Formation
Amaltheenton Formation, Upper Pliensbachian (Lias delta) mudstone unit of the German Jurassic, below the Posidonia Shale; dark-grey pyritic claystones with siderite concretions, shallow-marine, organic-lean.
Amami Ōshima
Amami Ōshima (奄美大島) is the largest island (712.4 km²) of the Amami archipelago, Kagoshima Prefecture, Japan, between Kyushu and Okinawa; subtropical, home of the Amami Rabbit, and part of the 2021 UNESCO World Heritage serial site.
Amatolo - Winterberg Highlands
Amatolo-Winterberg Highlands, FEOW freshwater ecoregion 577, montane freshwaters in South Africa's Eastern Cape; headwaters of Buffalo, Great Fish, Keiskamma, Swart Kei; endemic endangered Sandelia bainsii and critically endangered Barbus trevelyani.
Amba Farit
Amba Farit (also Monte Amba Farit, Mount Amba Ferit) is a mountain in the Amhara Region, Ethiopia, near Koso Beret and T'ire Neza. GeoNames lists 3,824 m at 10.93333 N, 38.96667 E; other sources give 3,975 to 4,270 m.
Ambrym (island)
Ambrym: volcanic island in Vanuatu's Malampa Province, a basaltic shield volcano with a 12-km caldera and the active Benbow and Marum craters, former lava lakes, ~5 Gg/day SO2, eruptions since 1774.
Amchitka Island
Amchitka Island, a 40-mile Aleutian island in the Rat Island group, hosted three US underground nuclear tests (Long Shot 1965, Milrow 1969, Cannikin 1971, the largest US underground test); assessments found no test-attributable radionuclide leakage.
Ameghino Formation
Ameghino Formation, also called Nordenskjöld Formation, a Kimmeridgian–Berriasian (Upper Jurassic–lowermost Cretaceous) radiolarian-rich mudstone and tuff sequence, oldest marine unit of the Larsen Basin, Antarctic Peninsula; type locality Longing Gap; yields ammonites, fishes, ichthyosaurs, and a plesiosaur.
Amelia Island
Amelia Island, the Isle of Eight Flags, is a barrier island in Nassau County, Florida, the northernmost on Florida's Atlantic coast, claimed by France, Spain, Britain, and the United States.
American Cordillera
American Cordillera: the continuous western mountain system of the Americas, from Alaska through the Rockies, Sierra Madre, and Central American highlands to the 4,500-mile Andes, built by Pacific subduction.