Canyons are created primarily by rivers cutting down through rock over millions of years, though tectonic forces, glaciers, and even underwater currents can carve them too. The basic formula is simple: flowing water carries sediment that grinds against bedrock like sandpaper, slowly deepening a channel while the land around it rises or holds steady. But the details of how that process plays out, and why canyons look so different from one another, involve a fascinating interplay of rock type, climate, and geological timing.
How Rivers Cut Through Solid Rock
The main force behind most canyons is abrasion. A river by itself has limited power against solid rock, but a river loaded with sand, gravel, and boulders becomes a cutting tool. As water rushes downstream, it drags this sediment along the riverbed and against the walls, grinding the rock away grain by grain. Over thousands to millions of years, this process can carve channels hundreds of meters deep.
Hydraulic action plays a supporting role. When fast-moving water slams into cracks in the rock, it compresses air inside those cracks, gradually widening them and prying loose chunks of stone. In softer or heavily fractured rock, this alone can remove significant material. Chemical dissolution also contributes, particularly in limestone landscapes where slightly acidic water slowly eats away the rock itself.
None of this happens at a constant pace. Canyon cutting is often dominated by rare, powerful flood events rather than the everyday flow of a river. Flash floods carry enormous volumes of sediment at high speed, doing more erosive work in hours than calm water accomplishes in years.
The Tug-of-War Between Uplift and Erosion
A river can only carve a deep canyon if the land beneath it rises or if its base level (the lowest point it can erode to, usually sea level) drops. This is where tectonic uplift becomes essential. As geological forces push a plateau or mountain range upward, rivers cutting across that surface gain steeper gradients and more erosive energy. The canyon deepens not just because the river digs down, but because the land is simultaneously moving up around it.
Research in the Peruvian Andes illustrates this relationship clearly. When tectonic shortening rates were high, uplift along fault lines outpaced the rivers’ ability to cut downward. The result was closed-off basins with no outlet, not canyons. It was only after shortening rates dropped significantly, around 10 million years ago, that rivers finally gained enough erosive power relative to uplift to carve through the divides and begin incising deep canyons along the plateau edge. The lesson: canyon formation requires that erosion and uplift be in the right balance. Too much uplift too fast, and the river can’t keep up. A slowdown in uplift, or an increase in water flow, tips the scales toward incision.
Why Rock Type Shapes the Canyon Walls
The kind of rock a river encounters determines whether a canyon has sheer vertical cliffs, gentle slopes, or a stair-step profile. Hard, resistant layers like limestone or well-cemented sandstone hold their shape and form cliff faces. Softer layers like shale erode quickly and create slopes or ledges between the cliffs.
The Grand Canyon is the classic example. Its layered walls alternate between resistant limestone and sandstone forming vertical cliffs and weaker shale forming angled slopes between them. As a river’s erosive signal moves upstream, it cuts quickly through weak rock units and gets “hung up” on harder layers, creating sharp changes in the channel’s slope called knickzones. This uneven resistance is what gives many canyons their distinctive terraced appearance rather than smooth, uniform walls.
Massive, uniform rock produces a very different result. When a river cuts through a single thick formation of sandstone with no weak layers to exploit, the walls stay steep and close together, often producing the dramatic slot canyons found across the American Southwest.
Slot Canyons: Carved by Flash Floods
Slot canyons are an extreme version of river-carved canyons. They’re extremely narrow, typically less than 5 meters wide, yet can reach depths of up to 100 meters. Their smooth, nearly vertical walls and sinuous curves make them some of the most visually striking geological features on Earth.
These canyons form in massive sandstone formations where abrasion dominates the erosion process. Wire Pass and Buckskin Gulch in Utah, for instance, cut through the thick Navajo Sandstone. Incision in these canyons happens almost exclusively during flash floods. For much of the year, the channels may contain only standing or slow-moving water, but during intense storms, floodwaters surge through with width-to-depth ratios sometimes as low as 1:1, meaning the water is as deep as the channel is wide. These concentrated, sediment-laden torrents scour the walls into the smooth, undulating shapes that make slot canyons so distinctive.
The Grand Canyon’s Timeline
The Grand Canyon is the most studied canyon on Earth, yet scientists have debated its age for decades. Hypotheses have ranged from younger than 6 million years to older than 70 million years. The current weight of evidence supports the younger end of that range. Geomorphic data from the western Grand Canyon indicates a relatively stable landscape with no deep canyon from at least 70 million years ago through about 17 million years ago. The canyon as we know it most likely formed after the Colorado River integrated into its current course around 6 million years ago, with the bulk of incision happening since then.
Six million years may sound like a long time, but for carving a gorge over 1,600 meters deep through hard rock, it represents a surprisingly fast rate of erosion. The combination of a large, powerful river, significant tectonic uplift of the Colorado Plateau, and the abrupt drop in base level as the river found a path to the Gulf of California all accelerated the process.
Glacial and Subglacial Canyons
Not all canyons owe their existence to rivers flowing in the open air. Glaciers carve canyons too, though they tend to produce broader, U-shaped valleys rather than the narrow V-shapes typical of river canyons. The sheer weight and slow grinding of ice, embedded with rock debris, can excavate enormous volumes of material.
Even more remarkable are canyons hidden beneath ice sheets. Beneath Greenland’s ice lies a mega-canyon stretching 750 kilometers long, likely carved before the ice sheet even existed. Subglacial meltwater, flowing under immense pressure at the base of the ice, continues to shape these buried channels. The topography of such canyons influences how water drains from the ice sheet’s interior to its edges, affecting ice flow patterns across the entire island. These features are invisible from the surface and were only discovered through ice-penetrating radar surveys.
Canyons on the Ocean Floor
Some of the largest canyons on Earth are underwater, carved into the continental shelf and slope. Submarine canyons form through a process driven by turbidity currents: fast-moving, sediment-heavy flows that rush downhill along the ocean floor. These currents act much like underwater avalanches, scouring channels as they transport enormous quantities of terrestrial sediment and organic carbon into the deep sea. The deposits they leave behind form some of the largest sedimentary accumulations on the planet. Some submarine canyons began as river valleys during ice ages when sea levels were much lower, then continued to deepen through turbidity current erosion as the seas rose.
The Deepest Canyon on Earth
The Grand Canyon gets the most attention, but it’s far from the deepest. That distinction belongs to the Yarlung Tsangpo Grand Canyon in Tibet, where the river passes between two Himalayan peaks. The canyon’s average depth is about 2,268 meters (7,440 feet), and at its deepest point it plunges 6,009 meters (19,714 feet) from rim to river. That’s nearly four times the depth of the Grand Canyon. The extreme depth results from the same basic process, river erosion combined with rapid tectonic uplift, but amplified by the extraordinary rate at which the Himalayas continue to rise.
Canyons Beyond Earth
The solar system’s largest canyon isn’t on Earth at all. Valles Marineris on Mars stretches roughly 4,000 kilometers long and reaches depths of up to 7 kilometers. Unlike most canyons on Earth, it wasn’t carved by flowing water. Instead, it formed primarily through tectonic rifting: the planet’s crust pulled apart and collapsed as magma shifted beneath the surface. Some scientists have proposed that subsurface water erosion and catastrophic outbursts from pressurized underground aquifers may have helped widen and deepen portions of the system after the initial rifting. Volcanic activity and lake formation within the rift zone further modified its shape over time, much as rifting and volcanism occur together on Earth in places like East Africa.