A canyon is a deep, narrow valley with steep sides. “Canyon” comes from the Spanish word cañon, which means “tube” or “pipe.” The term “gorge” is often used to mean “canyon,” but a gorge is almost always steeper and narrower than a canyon.

The movement of rivers, the processes of weathering and erosion, and tectonic activity create canyons.

River Canyons
The most familiar type of canyon is probably the river canyon. The water pressure of a river can cut deep into a riverbed. Sediments from the riverbed are carried downstream, creating a deep, narrow channel.

Rivers that lie at the bottom of deep canyons are known as entrenched rivers. They are entrenched because, unlike rivers in wide, flat floodplains, they do not meander and change their course.

The Yarlung Zangbo Grand Canyon in Tibet, a region of southwestern China, was formed over millions of years by the Yarlung Zangbo River. This canyon is the deepest in the world—at some points extending more than 5,300 meters (17,490 feet) from top to bottom. Yarlung Zangbo Canyon is also one of the world’s longest canyons, at about 500 kilometers (310 miles).

Weathering and Erosion
Weathering and erosion also contribute to the formation of canyons. In winter, water seeps into cracks in the rock. This water freezes. As water freezes, it expands and turns into ice. Ice forces the cracks to become larger and larger, eroding bits of stone in the process. During brief, heavy rains, water rushes down the cracks, eroding even more rocks and stone. As more rocks crumble and fall, the canyon grows wider at the top than at the bottom.

When this process happens in soft rock, such as sandstone, it can lead to the development of slot canyons. Slot canyons are very narrow and deep. Sometimes, a slot canyon can be less than a meter (three feet) wide, but hundreds of meters deep. Slot canyons can be dangerous. Their sides are usually very smooth and difficult to climb.

Some canyons with hard, underlying rock may develop cliffs and ledges after their softer, surface rock erodes. These ledges look like giant steps.

Sometimes, entire civilizations can develop on and around these canyon ledges. Native American nations, such as the Hopi and Sinagua, made cliff dwellings. Cliff dwellings were apartment-style shelters that housed hundreds of people. The shaded, elevated ledges in Walnut Canyon and Canyon de Chelly, in Arizona, U.S.A., provided protection from hostile neighbors and the burning desert sun.

Hard-rock canyons that are open at one end are called box canyons. The Hopi and Navajo people often used box canyons as natural corrals for sheep and cattle. They simply built a gate on the open side of the box canyon, and closed it when the animals were inside.

Limestone is a type of hard rock often found in canyons. Sometimes, limestone erodes and forms caves beneath the earth. As the ceilings of these caves collapse, canyons form. The Yorkshire Dales, an area in northern England, is a collection of river valleys and canyons created by limestone cave collapses.

Tectonic Uplift
Canyons are also formed by tectonic activity. As tectonic plates beneath Earth’s crust shift and collide, their movement can change the area’s landscape. Sometimes, tectonic activity causes an area of Earth’s crust to rise higher than the surrounding land. This process is called tectonic uplift. Tectonic uplift can create plateaus and mountains. Rivers and glaciers that cut through these elevated areas of land create deep canyons.

The Grand Canyon, in Arizona, is a product of tectonic uplift. The Grand Canyon, up to 447 kilometers (277 miles) long, 29 kilometers (18 miles) wide, and 1.8 kilometers (6,000 feet) deep, is the largest canyon in the United States. The Grand Canyon has been carved, over millions of years, as the Colorado River cuts through the Colorado Plateau. The Colorado Plateau is a large area that was elevated through tectonic uplift millions of years ago. Geologists debate the age of the canyon itself—it may be between five million and 70 million years old.

Canyons Reveal Earth’s History
Canyons are like silent journals of an area’s history over thousands or even millions of years. By studying the exposed layers of rock in a canyon wall, experts can learn about how the climate changed, what kind of organisms were alive at certain times, and perhaps even how the canyon may change in the future.

For example, geologists studying layers of rock in the Columbia River Gorge, in the U.S. states of Washington and Oregon, discovered that the oldest rocks there are at least 17 million years old. They also found out the rocks are dark-black basalt, made from hardened lava. From this, geologists determined that the rocks formed when volcanoes erupted and their lava spilled out onto the land. Over millions of years, the Columbia River and Ice Age glaciers cut through the area and exposed its volcanic beginnings.

Canyons are also important to paleontology, or the study of fossils. Fossils are often best preserved in dry, hot areas. Since canyons usually form under the same conditions, they are good places to examine fossils.

The layers of sediment revealed by a canyon can make it easier to date fossils. For example, a new area of dinosaur tracks was discovered in the U.S. state of Utah at Glen Canyon National Recreation Area in 2009. These tracks reveal new information about a group of dinosaurs called ornithopods. Paleontologists analyzed the layers of rock surrounding the fossils to estimate how old they were. These new dinosaur tracks show that ornithopods were alive 20 million years earlier than scientists thought.

Geologists study canyons to determine how the landscape will change in the future. The erosion patterns and thickness of different layers can reveal the climate during different years. A series of very dry years will have very thin layers of rock, when little erosion took place. The overall pattern of erosion and layering reveals the rate of water flow, from both the river and rain, through a canyon.

Geologists estimate that the Grand Canyon, for example, is being eroded at a rate of 0.3 meters (one foot) every 200 years. The Colorado Plateau, the geologic area where the Grand Canyon is located, is a very stable area. Geologists expect the Grand Canyon to continue to deepen as long as the Colorado River flows.

Submarine Canyons
Some of the deepest canyons lie beneath the ocean. These submarine canyons cut into continental shelves and continental slopes—the edges of continents that are underwater.

Some submarine canyons were carved by rivers that flowed during periods when the sea level was lower, and the continental shelves were exposed. The Hudson Canyon extends 750 kilometers (450 miles) into the Atlantic Ocean, from the mouth of the Hudson River, in the U.S. states of New York and New Jersey. At least part of the Hudson Canyon was the riverbed during the last ice age, when sea levels were much lower.

Submarine canyons can also develop when powerful ocean currents sweep away sediments. Just as rivers erode land, these currents carve deep canyons in the ocean floor. Strong currents of the Atlantic Ocean prevent Whittard Canyon, about 400 kilometers (248 miles) south of the coast of Ireland, from filling with sediment. Scientists studying Whittard Canyon believe glacial water mixed with seawater to rush into the submarine canyon thousands of years ago.

The formation of some submarine canyons is still a mystery. Monterey Canyon is a deep submarine canyon off the coast of the U.S. state of California. It has been compared to the Grand Canyon because of its size. It is 152 kilometers (95 miles) long and 3.2 kilometers (two miles) deep at its deepest point. Geologists still aren’t certain how Monterey Canyon was formed. One theory is that the canyon was formed by an ancient outlet of the Sacramento or Colorado Rivers. Another theory is that it was formed by tectonic activity—an earthquake splitting apart the rock with enormous force. Scientists believe the canyon was formed 25 million to 30 million years ago.

The depth of submarine canyons makes them hard to explore. Scientists usually use remotely operated vehicles (ROVs) to conduct studies. Sometimes, they can use a submersible, a special kind of submarine. The Monterey Bay Aquarium Research Institute (MBARI) uses a vehicle called Ventana to explore Monterey Canyon. Through the Ventana and other research vehicles, MBARI scientists have discovered new species of organisms living in the canyon, from tiny sea anemones to giant squid.