The steepest part of the continental margin, where it descends down to the ocean floor, is known as the:

Continental slope

The continental slope is said to be the steepest part of the continental margin, where there is a sudden decrease in slope or it suddenly descends down to the ocean floor and the sea floor begins to level out towards the abyssal plain. Moreover, this portion of the sea floor also marks the end of the continental margin called the continental rise.

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