Chapter 12 Seafloor Highs in Article 76 of the Law of the Sea Convention

A Scientific Introduction and Some Comments on CLCS Practice

In: New Knowledge and Changing Circumstances in the Law of the Sea
Walter R. Roest
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The seafloor of the world’s ocean is not flat but characterized by large topographic variations. The average depth of the ocean is around 3500 m, but the deepest parts of the ocean exceed 10 km in depth. Significant elevations exist in all ocean basins, and these features stand often several kilometres high above the surrounding abyssal plains. They include mid-ocean ridges, plateaux, guyots, volcanos, etc. Seafloor highs are one of the most contentious issues when dealing with the definition of the continental shelf under article 76 of the United Nations Convention on the Law of the Sea. According to the Convention, coastal States can extend sovereign rights over the continental shelf beyond 200 nautical miles based on natural prolongation of their land territory under the sea. This requires submitting scientific and technical information on the outer limits of the continental shelf to the Commission on the Limits of the Continental Shelf.

In recent years, modern shipborne measurements as well as satellite derived maps of the seafloor have contributed to a much better understanding of the seafloor and the processes that shape it. As a result, many coastal States include hitherto poorly known seafloor highs within their continental margins, if they are morphologically connected to them. It is fair to say that new knowledge significantly increases the areal extent of the continental shelf beyond 200 nautical miles as compared to initial estimates.

After a scientific description of some of the most prominent types of seafloor highs and their tectonic origin and evolution, I will discuss some criteria that can be applied to decide whether a given seafloor high may contribute to the continental shelf beyond 200 nautical miles of a coastal State. In particular, the case of a special class of seafloor highs, ridges, will be examined. The complexities of that issue will be further discussed in light of recent cases examined by the Commission on the Limits of the Continental Shelf.

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