SUBMARINE CANYON

The edge of the continental margin is incised by a number of valleys or submarine canyons that extend from the shelf to the deep sea floor. Thus, they are somewhat similar to canyons carved into the edge of a plateau or mountain front on land. These erosional features were apparently formed by scouring out a portion of the shelf. But what was the scouring agent? What force could erode such sizable canyons in the ocean? One clue to their origin is the presence of a fan or apron of sediment at the mouth of the submarine canyon. Other canyons also have sediment fans. Indeed, we can imagine that the joining of these adjacent fans has built up the sediment prism that we call the continental rise. So, whatever force carved the canyon also may have transported the sediment down the canyon. Two mechanisms have been suggested, and they may have operated together. One is that canyons were carved during a time when sea level was lower than present and rivers were able to flow out to the edge of the shelf. The presence of many (but not all) canyons offshore from major rivers supports this suggestion. Another theory is that canyons were carved by turbidity currents. These are dense, sediment-laden, currents that are triggered by earthquakes and flow down the continental margin. The speed and erosive power of these currents is considerable. Both processes may have been involved in forming the canyons.