“Continental slope” or “slope,” as used in UNCLOS Article 76, means that part of the continental margin lying between the continental shelf and the continental rise. The continental slope may not be uniform or abrupt and may locally take the form of terraces. The continental slope’s gradients are usually greater than 1.5 degrees.
In LOAC-governed situations under the “other rules of international law” clauses in UNCLOS, a different definition may apply. The same may be the situation if the UN Charter supersedes UNCLOS or if jus cogens norms apply.
Consolidated Glossary ¶ 22 defines the “continental slope as “[t]hat part of the continental margin that lies between the shelf and the rise[,s]imply called the slope in [UNCLOS] Art. 76.3. The slope may not be uniform or abrupt, and may locally take the form of terraces. The gradients are usually greater than 1.5E.”
UNCLOS Article 76(3), defines the continental margin as “the submerged prolongation of the land mass of the coastal State, and consist[ing] of the seabed and subsoil of the [continental] shelf, the slope and the rise. It does not include the deep ocean floor with its oceanic ridges or the subsoil thereof.” Article 76(4)(a)(ii) requires a coastal State to establish the outer edge of the continental margin wherever the margin extends beyond 200 nautical miles from the baselines from which the territorial sea’s breadth is measured by “a line delineated in accordance with [Article 76(7)] by reference to fixed points not more than 60 nautical miles from the foot of the continental slope.” (Article 76[a][i] gives another measuring option, not relevant to this analysis.) Article 76(4)(b) says that “in the absence of evidence to the contrary,” “the foot of the continental slope shall be determined as the point of maximum change in the gradient.” Article 76(1) defines the continental shelf.
This might be compared with a geomorphological definition of the continental shelf:
… [T]he continental shelf is only one part of the submerged prolongation of land territory offshore. It is the inner-most of three geomorphological areas — the continental slope and the continental rise are the other two — defined by changes in the angle at which the seabed drops off toward the deep ocean floor. The shelf, slope and rise, taken together, are known as the continental margin. Worldwide, there is a wide variation in the breadths of these areas.
The ILA Committee on the Outer Continental Shelf comments:
Article 76(4)(b) … provides that in the absence of evidence to the contrary, the foot of the continental slope shall be determined as the point of maximum change in the gradient at its base. The reference to these two approaches to determine the foot of the slope indicates that the foot of the slope can be determined on the basis of geomorphological and/or geological characteristics.
Article 76(4)(b) does not establish a precedence between the two approaches … A coastal State may opt to present evidence to the contrary to locate the foot of the slope, or, if such evidence is not available, present evidence on the maximum change of gradient at the foot of the slope.