Characteristics of Continental Margins

Abstract

Two major types of morphological features dominate the surface of the Earth—the continents and the oceans, the latter being by far the most significant in that they cover about 71% of the surface. These features are separated by the coastline, which is a transitory boundary. A much more fundamental subdivision of the Earth's surface is in terms of geological provinces composed of either continental or oceanic lithosphere (crust and uppermost mantle), each with their own characteristic lithological, geochemical, and physical properties. The continental margin is the zone separating the thin oceanic crust of the deep ocean basins from the thick continental crust. Continental margins underlie about 28% of the oceans, with the transition from continental to oceanic crust commonly occurring beneath the outer part of continental margins. Thus, a substantial proportion of continental crust (about 20%) lies beneath the oceans. The boundary between these crustal provinces marks the real physical outer edge of the prolongation of the continent beneath the ocean. It is this boundary that is commonly viewed as the absolute natural limit of a claim that a coastal state can make for sovereign rights to explore and exploit the natural resources of the deep seabed and subsoil. The geomorphological and geological characteristics of a continental margin are a function of its tectonic, magmatic, and sedimentary history. Continental margins can differ greatly in their dimensions and style depending on their age and their tectono-magmatic and depositional setting. Important province boundaries, such as the continent-ocean boundary (COB), are always difficult to define with any accuracy on a map because of their complexity and transitional nature. Given this, and the development of legal thought over the last 50 years on defining the various marine jurisdictional zones, it is little wonder that the definition of the outer limit of the seabed and subsoil resource regime on continental margins, the "legal" continental shelf, is a complex blend of legal, geomorphological, geological, and geodetic concepts that can be confusing even to experts in the individual fields.