This page lists 29 geological features that start with O, from “Oasis” to “Ocean Floor Spreading”. The list mixes landforms, rock types, volcanoes, wetlands, caves and the processes that shape the planet. You can use it to study earth science, plan a trip to a famous landmark, or find a topic for a school project.
Geological features are the natural forms and processes that make up the surface and interior of a planet. They range from small grains of rock to entire mountain belts. Even Mars has one, “Olympus Mons”, the largest known volcano in the solar system.
Below you’ll find the table with the feature, region, characteristics and description for each entry.
Feature: The name of each geological feature, listed alphabetically so you can scan the page quickly and find the one you want.
Region: Where in the world you can find the feature, from a single national park to places across the globe.
Characteristics: A short set of key traits, such as size, shape or rock type, that helps you tell features apart at a glance.
Description: A plain-language explanation of what the feature is, how it forms and why it matters, with well-known examples where they exist.
Geological features
| Feature | Region | Characteristics |
|---|---|---|
| Oasis | Deserts worldwide, especially the Sahara | Fed by groundwater or springs, green patch in arid land |
| Outcrop | Worldwide | Exposed bedrock, visible rock at the surface |
| Oxbow Lake | River floodplains worldwide | U-shaped, cut-off meander, still water |
| Outwash Plain | Glaciated regions such as Iceland, Alaska and northern Europe | Flat, sandy and gravelly, formed by glacial meltwater |
| Orogeny | Mountain belts worldwide | Mountain building, folding and faulting |
| Ocean Trench | Pacific, Atlantic and Indian Ocean margins | Deep, narrow, found at subduction zones |
| Oceanic Crust | Ocean floors worldwide | Thin, dense, basalt-rich |
| Ocean Basin | All major oceans | Large bowl-shaped depression, covered by seawater |
| Oceanic Ridge | Mid-Atlantic, East Pacific and Indian Ocean | Underwater mountain chain, volcanic spreading center |
| Ophiolite | Oman, Cyprus, California, Newfoundland | Slice of oceanic crust, uplifted onto land |
| Obsidian Flow | Volcanic regions such as Oregon and Iceland | Glassy, black, silica-rich lava |
| Old Faithful | Yellowstone National Park, USA | Cone geyser, erupts roughly every 90 minutes |
| Ol Doinyo Lengai | Tanzania | Active volcano, erupts carbonatite lava |
| Okavango Delta | Botswana | Inland delta, seasonal flooding, wetlands |
| Ojos del Salado | Chile and Argentina | Highest active volcano, about 22,615 feet |
| Olympus Mons | Mars | Shield volcano, about 13.6 miles high |
| Okefenokee Swamp | Georgia and Florida, USA | Blackwater wetland, peat-filled, spans about 438,000 acres |
| Ouachita Mountains | Arkansas and Oklahoma, USA | Folded ridges, east-west trend |
| Ozark Plateau | Missouri, Arkansas and Oklahoma, USA | Limestone, dolomite, caves and springs |
| Olgas | Northern Territory, Australia | Domed conglomerate rocks, 36 formations |
| Organ Pipes | Otago, New Zealand and Namibia | Columnar basalt, vertical columns |
| Owens Valley | California, USA | Rift valley, between the Sierra Nevada and White Mountains |
| Ordos Plateau | Inner Mongolia, China | Desert plateau, loess-covered, enclosed by the Yellow River bend |
| Olivine Beach | Hawaii, USA (Papakolea) and Guam | Green sand, volcanic mineral grains |
| Oolite | Bahamas, Persian Gulf and Utah | Small round grains, limestone |
| Overhang | Cliffs and cave mouths worldwide | Rock projecting past the base, shelter-like |
| Ore Deposit | Mining regions worldwide | Concentrated minerals, economically valuable metals |
| Oregon Caves | Oregon, USA | Marble cave, dissolved by acidic water |
| Ocean Floor Spreading | Mid-ocean ridges worldwide | Plate divergence, new crust formation |
Descriptions
Oasis
A fertile spot in a desert where water reaches the surface from underground. Oases support plants, animals and settlements, and have long served as stops on trade routes.
Outcrop
A place where bedrock or older deposits show through the soil and vegetation. Geologists rely on outcrops to study rock layers directly.
Oxbow Lake
A curved lake formed when a river meander is cut off from the main channel. It often sits quietly on the floodplain and slowly fills with sediment.
Outwash Plain
A broad, flat area of sand and gravel laid down by meltwater streams flowing from a glacier. Icelandic sandurs are well-known examples.
Orogeny
The process by which mountain ranges form when tectonic plates collide and crumple the crust. The Alps and Himalayas are products of orogenies.
Ocean Trench
A long, steep-sided depression in the seafloor where one plate dives beneath another. Trenches hold the deepest points on Earth, such as the Mariana Trench.
Oceanic Crust
The outer rock layer beneath the oceans. It is thinner and denser than continental crust and is constantly created at mid-ocean ridges and recycled at subduction zones.
Ocean Basin
A huge depression in Earth’s crust that holds an ocean. Basins include the continental shelves, slopes, abyssal plains and ridges.
Oceanic Ridge
A long underwater mountain range where new crust forms as plates pull apart. The Mid-Atlantic Ridge is the most famous example.
Ophiolite
A piece of ancient seafloor and upper mantle pushed up onto continents. Ophiolites let geologists study the deep ocean floor without diving.
Obsidian Flow
A thick lava flow that cooled so quickly it formed volcanic glass. Newberry Volcano in Oregon has a large, easily visited obsidian flow.
Old Faithful
One of the world’s best-known geysers, shooting hot water up to about 185 feet. Its eruptions are frequent and predictable enough that visitors can plan around them.
Ol Doinyo Lengai
A volcano in the East African Rift known as the Mountain of God. Its lava is unusual, rich in carbonates and dark when fresh but quickly turning white.
Okavango Delta
A vast inland delta where the Okavango River spreads across the Kalahari instead of reaching the sea. Seasonal floods turn it into a rich wildlife habitat.
Ojos del Salado
The tallest volcano on Earth, sitting high in the Andes along the Chile-Argentina border. It is very dry, with little glacial ice despite its height.
Olympus Mons
The largest known volcano in the solar system, roughly two and a half times the height of Mount Everest. It is a broad shield volcano on Mars.
Okefenokee Swamp
One of North America’s largest intact freshwater swamps. Its tea-colored water and floating peat islands make it a distinctive wetland landscape.
Ouachita Mountains
A range of ridges and valleys in the central United States. Unlike most American ranges, its ridges run east to west, a result of ancient collisions.
Ozark Plateau
A heavily eroded upland of the central US known for caves, springs and clear rivers. Much of it sits on soluble rock, which produces karst landscapes.
Olgas
Also called Kata Tjuta, this group of large rounded rock domes lies near Uluru. The domes are made of coarse conglomerate and glow red at sunset.
Organ Pipes
Tall, vertical columns of volcanic rock that look like organ pipes. They form when thick lava cools and cracks into regular columns.
Owens Valley
A deep valley in eastern California, flanked by high mountain ranges. Its steep walls come from faulting that has pulled the land apart over millions of years.
Ordos Plateau
A high, dry plateau inside the great northern loop of the Yellow River. Parts are covered by desert and parts by wind-blown loess soil.
Olivine Beach
A beach with green sand made of olivine crystals eroded from volcanic rock. Papakolea Beach on the Big Island of Hawaii is a famous example.
Oolite
A sedimentary rock made of tiny, round grains that form as calcium carbonate coats particles in warm, shallow water. The grains look like fish roe.
Overhang
A rock ledge that sticks out beyond the base of a cliff, often because softer layers below have eroded faster. Early people used many overhangs as shelters.
Ore Deposit
A natural concentration of metal-bearing minerals rich enough to mine. Ore deposits form through volcanic, hydrothermal and sedimentary processes.
Oregon Caves
A cave system in the Siskiyou Mountains carved into marble rather than the more usual limestone. It contains many flowstone and stalactite formations.
Ocean Floor Spreading
The process by which new oceanic crust forms at ridges and moves outward over time. It helped confirm the theory of plate tectonics.