Water And Oceanography

Submarine Canyons or Continental Shelves and Submarine Canyons

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The transitional zone between a shallow continental shelf and a deep ocean basin has always been an interesting place for scientific research, not only because of the different oceanographic processes that take place within the region, but also because of the significant biodiversity that can be experienced within this zone. However, not all aspects of edges of continental shelves have been documented adequately in literature and according to experts, the submarine canyon is one such aspect, which lacks in-depth studies with regard to its oceanographic processes. This article will introduce the different terms that are required in describing a submarine canyon and will look into its placement, its characteristics, as well as its formation to a certain extent.

Active and passive continental margins and its link to submarine canyons

Usually, a continental margin is known as a ‘passive continental margin’ because of its landward and shallow continental shelf, the deeper continental slope, the continental rise and a flat abyssal plain. However, there are instances where the continental margin becomes active and in such instances, these features would change into a much steeper continental slope, which ends in an active ocean trench and an irregular ocean bottom. The importance of ‘active continental margins’ is that these areas are more prone to volcanic eruption and earthquakes than the ‘passive continental margins’ that are deemed stable. At the same time, submarine canyons are much more common and much deeper when it comes to steeper continental slopes of active continental margins than in the gradual slopes existing within passive continental margins.

Continental shelf

Before discussing submarine canyons, it is necessary to understand what a continental shelf is. A continental shelf is a shallow and almost flat platform, which extends seawards from the edge of the continent. While its beginning is covered mostly by sand, the deeper end of the continental shelf is covered in fine-grained mud. The continental shelf can extend up to thousands of kilometers, while its depth can vary from few meters to even 200 meters.

Continental slope

Continental slope is the seabed that extends from the seaward edge of the continental shelf into the deep ocean. In the continental slope, the seabed can change from a depth of 600 feet to over 6000 feet within a horizontal length of around 10 miles. Most expeditions covering submarine canyons include trips to the canyons located within this region.

Definition and characteristics of a submarine canyon

In simple terms, a submarine canyon can be described as a steep-sided valley cut into the sea floor of the continental slope. In some instances, the canyon can extend into the continental shelf as well. The steep walls of the canyon can show multiple erosions and these are likely to be the result of bioerosion and or slumping. These erosions can extend into all substrates from unlithified sediment to crystalline rock.

Some submarine canyons are found to be extensions of large rivers while many canyons continue to extend through the continental rise as submarine channels. These can extend into hundreds of kilometers in length.

Formation of submarine canyons

Although there are multiple theories with regard to how the submarine canyons came into being, the most acceptable theory to date would be the effects of turbidity currents and underwater landslides. It is believed that dense and sediment laden currents flow down-slope following being dislodged from the upper slopes. Such dislodging may take place as a result of earthquakes and in most instances, the dislodged sediments are loosely deposited or else deposited very rapidly. Mass wasting and slumping are two other terms used to describe similar displacements of material on the canyon slopes, although they would be somewhat slower in displacement than a landslide.  

Some early theorists believed that the submarine canyons were carved during the glacial period where rivers were flowing to the edges of a continental shelf. However, the theory has long lost its validity with regard to the formation of certain other canyons, which are not associated with rivers.

Although scientists use these theories to describe the phenomenon of submarine canyons, it is widely accepted that with further understanding, some of these theories will become outdated in the near future.

More about this author: Dr Pandula Siribaddana

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