This page lists 51 geological features that start with A, from “Abyssal plain” to “Anticlinal ridge.” The list covers landforms, rock types, mountain ranges, volcanic features, glacier zones, and famous sites from around the world. Geologists, students, teachers, hikers, and curious readers use these terms to read landscapes, understand how Earth changes, and plan trips to remarkable places.
Geological features are the natural landforms, rocks, and structures that make up Earth’s surface and the layers beneath it. They form over millions of years through forces like volcanic activity, erosion, glaciers, and moving plates. The Appalachian Mountains show how dramatic this change can be, since they were once as tall as the Himalayas.
Below you’ll find the table with the geological feature, region, characteristics, and description for each entry.
Geological Feature: This is the name of each item, so you can quickly find a specific term or browse the full alphabetical list.
Region: This tells you where in the world each feature is found, which helps you locate examples or plan a visit.
Characteristics: This gives a short summary of the key traits of each feature, so you can compare them at a glance.
Description: This explains what each feature is, how it forms, and why it matters, giving you the background in plain language.
Geological features
| Geological Feature | Region | Characteristics |
|---|---|---|
| Abyssal plain | Ocean basins worldwide | Flat, deep seafloor, covered in fine sediment |
| Aa lava | Hawaii, Iceland, volcanic regions worldwide | Rough, jagged, clinkery surface |
| Alluvial fan | Arid mountain fronts such as Death Valley, USA | Fan-shaped, built from sediment deposits |
| Alluvial plain | River valleys worldwide | Flat, fertile, built from river sediment |
| Alluvium | River valleys and floodplains worldwide | Loose sand, silt, clay, and gravel |
| Amphitheater | Mountains and canyons worldwide | Bowl-shaped, steep-walled |
| Andesite line | Western Pacific | Boundary between volcanic rock types |
| Anticline | Folded mountain belts such as the Appalachians | Arch-shaped fold, oldest rocks at the core |
| Aquifer | Worldwide, e.g. Ogallala Aquifer, USA | Underground, water-bearing rock or sediment |
| Arch | Utah, USA, and arid regions worldwide | Natural opening, carved by erosion |
| Arete | Alps, Rockies, Himalayas | Narrow, sharp ridge |
| Arroyo | American Southwest | Dry, steep-sided gully, floods in storms |
| Ash fall deposit | Volcanic regions worldwide | Fine volcanic particles, blankets large areas |
| Ash flow tuff | Volcanic regions such as the western USA | Welded volcanic ash and pumice |
| Asthenosphere | Beneath Earth’s tectonic plates | Soft, slowly flowing upper mantle layer |
| Atoll | Pacific and Indian Oceans, e.g. Bikini Atoll | Ring-shaped coral reef around a lagoon |
| Avalanche chute | Mountain ranges worldwide | Steep, repeatedly scoured slope path |
| Alluvial terrace | River valleys worldwide | Step-like bench above current river level |
| Alpine glacier | Alps, Andes, Himalayas, Rockies | Valley-confined ice mass |
| Alps | Europe (France, Switzerland, Italy, Austria) | Young, folded, high mountain range |
| Andes | South America | Longest continental mountain range |
| Appalachian Mountains | Eastern North America | Old, worn-down, folded ridges |
| Atacama Desert | Chile | Extremely dry, high-altitude desert |
| Aleutian Trench | North Pacific, near Alaska | Deep ocean trench, subduction zone |
| Aleutian Islands | Alaska, USA | Volcanic island arc |
| Afar Triangle | Ethiopia, Eritrea, Djibouti | Rift junction, below sea level, hot and volcanic |
| African Rift Valley | East Africa | Long, branching continental rift |
| Adirondack Mountains | New York, USA | Domed, ancient, glacially sculpted |
| Alps of New Zealand (Southern Alps) | South Island, New Zealand | Fast-rising, glaciated range |
| Alpine Fault | South Island, New Zealand | Major strike-slip fault, about 600 km long |
| Altiplano | Peru, Bolivia, Chile | High, flat plateau about 12,000 feet up |
| Aso Caldera | Kyushu, Japan | One of the world’s largest calderas, active |
| Ayers Rock (Uluru) | Northern Territory, Australia | Giant sandstone monolith, red in color |
| Arthur’s Seat | Edinburgh, Scotland | Extinct volcano, steep hill |
| Agate | Worldwide, e.g. Brazil, Uruguay, Lake Superior | Banded, microcrystalline quartz |
| Amethyst geode | Brazil, Uruguay | Hollow rock lined with purple quartz |
| Anhydrite deposit | Evaporite basins worldwide | Sulfate mineral beds from evaporated seawater |
| Agglomerate | Volcanic regions worldwide | Coarse volcanic rock with rounded fragments |
| Active volcano | Pacific Ring of Fire and worldwide | Erupted recently, may erupt again |
| Alluvial gold placer | Alaska, California, Australia | Gold concentrated in stream gravel |
| Alcove | Southwestern USA canyons | Shallow, arched hollow in a cliff |
| Apron (talus apron) | Mountain bases worldwide | Slope of fallen rock debris |
| Archipelago | Pacific, Mediterranean, Caribbean | Island group, often volcanic |
| Ablation zone | Glaciers worldwide | Area where glacier ice melts away |
| Accumulation zone | Glaciers worldwide | Area where snow builds up into ice |
| Anastomosing river | Alberta, Canada, and flat floodplains | Network of interlinked channels |
| Abrasion platform | Rocky coasts worldwide | Flat rock surface cut by waves |
| Alkali flat | Great Basin, USA, and arid basins worldwide | Dry lakebed coated in salts |
| Arkose | Worldwide | Feldspar-rich, pinkish sandstone |
| Aphanitic rock | Volcanic regions worldwide | Fine-grained igneous texture |
| Anticlinal ridge | Appalachians, Zagros Mountains | Long ridge shaped by an upward fold |
Descriptions
Abyssal plain
A vast, nearly flat area of the deep ocean floor, usually 10,000 to 20,000 feet down. It covers about half of Earth’s ocean floor.
Aa lava
A type of lava flow with a sharp, rubbly surface that is hard to walk on. The word comes from Hawaiian.
Alluvial fan
A fan or cone of sand, gravel, and silt dropped where a fast stream leaves a mountain valley and spreads onto flatter land.
Alluvial plain
A flat stretch of land formed from sediment laid down by rivers over thousands of years. These plains often make excellent farmland.
Alluvium
Loose sediment deposited by flowing water. It is the material that builds up floodplains, deltas, and fans.
Amphitheater
A large, rounded, bowl-like landform with steep walls, often carved by glaciers or erosion. Bryce Canyon’s main bowl is a famous example.
Andesite line
A boundary in the Pacific separating the andesite-rich volcanic arcs on the continental side from the basalt-dominated ocean basin.
Anticline
A fold in rock layers that bends upward like an arch. Anticlines are important because they can trap oil and gas.
Aquifer
A layer of rock, sand, or gravel that holds and carries groundwater. Aquifers supply drinking water and irrigation for billions of people.
Arch
A natural rock opening formed when wind, water, and weathering wear away softer rock and leave a span behind. Arches National Park holds over 2,000.
Arete
A thin, knife-edged ridge left between two glacier-carved valleys or cirques. Hikers often cross them on exposed mountain routes.
Arroyo
A dry creek bed with steep walls that fills with fast water after heavy rain. They are common in desert regions.
Ash fall deposit
A layer of volcanic ash settled from an eruption cloud. It can blanket land for hundreds of miles and make soils very fertile over time.
Ash flow tuff
Rock made from a hot, fast-moving cloud of ash and gas that settles and fuses together. It is also called ignimbrite.
Asthenosphere
A weak, partly molten layer of the upper mantle beneath the rigid plates. Its slow flow lets tectonic plates move across the surface.
Atoll
A ring of coral reef and small islands enclosing a lagoon. Atolls typically form on the rims of sunken volcanoes.
Avalanche chute
A steep channel on a mountain slope where snow slides down again and again. Trees are usually missing or stunted along its path.
Alluvial terrace
A flat, step-like landform left behind when a river cuts down into its older floodplain.
Alpine glacier
A river of ice that forms in high mountains and flows slowly down a valley. Alpine glaciers carve U-shaped valleys and sharp peaks.
Alps
Europe’s best-known mountain range, stretching about 750 miles. It was formed by the collision of the African and Eurasian plates.
Andes
The world’s longest continental mountain range, running about 4,300 miles along South America’s west coast. It formed through ocean plate subduction and holds many volcanoes.
Appalachian Mountains
An ancient mountain range stretching from Canada to Alabama. Once as tall as the Himalayas, it has been softened by hundreds of millions of years of erosion.
Atacama Desert
One of the driest places on Earth, with some weather stations recording almost no rainfall for years. Its salt flats and Mars-like landscapes draw scientists.
Aleutian Trench
A curved ocean trench about 4,000 miles long along the Aleutian Islands. It marks where the Pacific Plate dives beneath the North American Plate.
Aleutian Islands
A chain of over 300 volcanic islands stretching west from Alaska. They sit above the subduction zone and have many active volcanoes.
Afar Triangle
A low, hot region where three tectonic rifts meet. It is one of the few places where scientists can watch a continent splitting apart.
African Rift Valley
A huge system of cracks and valleys running through East Africa where the continent is slowly pulling apart. It contains lakes, volcanoes, and early human fossil sites.
Adirondack Mountains
A rounded mountain region in northern New York made of very old rock. Glaciers carved its many lakes and valleys.
Alps of New Zealand (Southern Alps)
A mountain range running along New Zealand’s South Island. It is rising rapidly along the Alpine Fault and holds many glaciers.
Alpine Fault
A long fault where the Pacific and Australian plates slide past each other. It tends to produce a large earthquake every few centuries.
Altiplano
A broad, high plateau in the central Andes. It is the largest high plateau outside Tibet and holds Lake Titicaca.
Aso Caldera
A huge volcanic crater about 15 miles across, with an active volcano at its center. Towns and farms sit inside its rim.
Ayers Rock (Uluru)
A massive sandstone rock rising about 1,140 feet above the Australian desert. It is sacred to the Anangu people and changes color at sunrise and sunset.
Arthur’s Seat
A rocky hill rising about 823 feet above Edinburgh. It is the remains of a volcano that was active around 340 million years ago.
Agate
A colorful, banded variety of chalcedony that forms in cavities of volcanic rock. It is popular among rock collectors.
Amethyst geode
A hollow stone with a cavity of purple quartz crystals inside. These form from silica-rich fluids in volcanic gas bubbles.
Anhydrite deposit
A rock layer made of calcium sulfate that was left behind when ancient seas dried up. It often sits beside salt and gypsum beds.
Agglomerate
A rock made of large, rounded chunks of volcanic material fused in a finer matrix. It usually forms near volcanic vents.
Active volcano
A volcano that has erupted in recent history and could erupt again. Around 500 are considered active worldwide.
Alluvial gold placer
A deposit where flowing water has gathered heavy gold particles in riverbed gravels. Placers fueled many gold rushes.
Alcove
A shallow, curved recess in a rock wall created by weathering and seeps. Ancient peoples often built homes inside them.
Apron (talus apron)
A sloping skirt of loose rock and sediment collected at the base of a cliff or mountain.
Archipelago
A group or chain of islands clustered together in a sea or ocean. Many form from volcanic activity or rising seafloor.
Ablation zone
The lower part of a glacier where more ice melts or evaporates than snow arrives. The glacier’s end moves back or forward based on this balance.
Accumulation zone
The upper part of a glacier where snowfall exceeds melt. This snow compresses into the ice that feeds the glacier.
Anastomosing river
A river that splits into several connected channels separated by stable, vegetated islands. It differs from braided rivers because its banks are stable.
Abrasion platform
A flat or gently sloping rock shelf at the base of a sea cliff, worn smooth by waves carrying sand and pebbles.
Alkali flat
A flat dry lakebed covered with a crust of alkaline salts. It forms where lake water evaporates in dry climates.
Arkose
A sandstone containing a lot of feldspar, often pink or reddish. It forms near granite sources where erosion is rapid.
Aphanitic rock
Igneous rock with crystals too small to see without magnification. It forms from lava that cools quickly at or near the surface.
Anticlinal ridge
A ridge that follows the crest of an anticline. Many of the long ridges in Pennsylvania are examples.