Grand Canyon. Photo: Josh Sorenson pexels.com
Looking across our oceans, the world beneath is hidden from view. This hidden world shapes our planet in profound ways. I love learning about the processes, patterns and features that make up the roughly-70 per cent of Earth covered by oceans, and it’s striking how much we still don’t know. What we do know, though, is often astounding in scale.
Janine Krippner
The Grand Canyon in Arizona, United States, and canyons like it are surprisingly common along the edges of continental shelves. Scientists estimate there may be around 10,000 submarine canyons worldwide.
A continental shelf is the part of the seafloor that extends out from land, sloping gently downward until it reaches the continental slope, where the seabed steepens dramatically. Many submarine canyons cut down through these shelf edges. They form through erosion, the gradual removal of material grain by grain, or through more sudden and dramatic mass-wasting events such as landslides. Over long timescales, these processes carve deep channels into the seafloor, slicing downward through layers of rock.
Because canyons expose those layers, they offer a window through time. The rocks represent millions, or even billions, of years of Earth’s history. Some layers are sedimentary, built from deposits of older material ranging from fine clay and mud to coarse gravel and boulders. Over time, as these deposits are buried and compacted, where they can harden into rock such as sandstone and mudstone.
Volcanic rocks may also appear in canyon walls, and some expose stacked lava flows, ash-rich deposits from explosive eruptions, or even the remains of underground magma bodies that slowly cooled into granite. A canyon may also cut into metamorphic rocks, material that was buried deep enough to be transformed by pressure and heat into new rock types such as schist or gneiss. These rocks are far older than the canyon itself.
The Grand Canyon illustrates this. Its oldest exposed rocks are about 1.8 billion years old, its youngest roughly 270 million. The canyon began to form around five million years ago. This rich history is revealed across its depth that reaches 1.6 kilometres.
Submarine canyons can be even larger. Zhemchug Canyon near Alaska, for example, has a relief (or depth) of roughly 2.6 kilometres. Closer to home, we have spectacular examples too, including Cook Strait Canyon between Te Ika-a-Māui/the North Island and Te Waipounamu/the South Island, and the Kaikōura Canyon offshore of the Kaikōura Peninsula.
These giants of the deep matter. Submarine canyons can act as major pathways for transporting sediment from coastal regions down to the deep ocean floor. They can support ecosystems, shaping biodiversity by influencing currents, nutrients, and habitat structure. They can be sites of major capable of triggering tsunamis that may impact our coastlines with little or no warning.
From life-supporting systems to potential disasters, submarine canyons remind us that the seafloor is dynamic, complex, and deeply connected to the world we live in. The more we understand how it is shaped, the better we can understand our planet and our place within it.
Birds eye photography of brown-rock plateau. Photo: Lukas Kloeppel: pexels.com
