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Pacific Coastal and Marine Science Center

Law of the Sea - Outer Limits of the US Continental Margins

Sunrise on the Arctic Ocean, August 6, 2010, by Helen Gibbons, USGS.The Convention on the Law of the Sea (LOS) sets forth a process by which a coastal nation (hereafter called State) may establish the outer limits of its extended continental shelf (ECS), i.e., that portion of the seafloor and sub-seafloor of the continental margin that extends beyond 200 nautical miles (nm) where States can exert their sovereign rights.

Defining the ECS is likely to be the most important land expansion of the United States in the 21st century, with the potential to add between one and two million square kilometers of seafloor, similar to the size of the state of California. It rivals the addition of the EEZ in terms of its importance to governance and sovereign land rights. Resources in the ECS are estimated to be worth about $1.2 trillion (in 2000 dollars!).

While the U.S. is not party to LOS, accession to LOS is a stated priority for the last three Presidential administrations. It is in the U.S. national interest to collect accurate and precise data on the outer limits of its continental shelf.

Although much of the LOS project is to define the outer limits of the ECS, the project offers unprecedented scientific opportunities to understand processes of continental margins and deep ocean floor, e.g., tectonics of the Arctic Ocean, deep-sea fan processes of the Gulf of AK, and gas hydrate occurrence in the Bering Sea. The LOS project provides USGS the first opportunity to conduct blue-water marine geology and geophysics seismic research in almost two decades. Many of the ECS areas are in deep-water where few data have been collected (e.g., Gulf of AK).

With recent technological advances, the quality of data collected for ECS provides new resolution and new insights into the geology of the sea floor - hence the scientific opportunities. The information needed to define the extended continental shelf includes physiography, sediment thickness, and geology. USGS, with its national knowledge and expertise of the U.S. continental margins, has responsibility for developing a database of sediment thickness and continental margin geology, assessing existing data, and developing scientific opportunities.

USGS map of survey tracklinesArctic Ocean; click for larger view.The LOS work was conducted as part of the U.S. Interagency Task Force on the ECS, using ECS guidelines CLCS, 1999). The project reviewed, compiled, collected (as authorized) and synthesized geologic and geophysical data from offshore seabed areas beyond 200 nm. USGS led three of the seven regions that the Task Force established for further study, and USGS contributed to all of the seven regional teams in geologic expertise, data management, and GIS analysis. USGS plans, budgets, coordinates, and ultimately conducts complex, lengthy field experiments in the regions beyond 200 nautical miles, and ensures that the criteria are met for defining these ECS limits. Part of the USGS responsibilities for these field programs is to also publish the significant scientific findings from the surveys.

This project provides a mechanism for USGS to research and advise the Department of State about the geologic criteria used in submissions to UNCLOS made by other States and the recommendations regarding these submissions. This advisory role involves identifying and becoming familiar with the public literature on the particular continental margin and then apply LOS Article 76 criteria to assess how reasonable the limits are. The geology-legal nexus is critical for determining how morphologic ridges are treated, particularly along the Pacific west coast (Mendocino Ridge) and Hawaiian Necker Island (Necker Ridge).

Finally, this project includes a task to investigate the mineral resource potential of the EEZ and potential ECS. This Task deals with mineralization and the identification of strategic and critical minerals in the potential ECS that will enable the federal government to make informed decisions about boundary negotiations, resource development, and resource conservation.

Field activities

USGS Seismic Survey in the Atlantic Ocean, 2014 (Field Activity 2014-011-FA)

USGS Marine Geophysical Survey in Central-Western Bering Sea, 2011 (Field Activity 2011-034-FA)

USGS Marine Geophysical Survey in Central Gulf of Alaska, 2011 (Field Activity N-01-11-GA)

USGS Seismic Survey in the Arctic Ocean, 2010 (Field Activity 2010-018-FA)

2010 Extended Continental Shelf (ECS)

2009 ECS

2008 ECS


Cover of the publication.USGS Publication, Open-File Report 2010-1117: “Environmental Assessment for a Marine Geophysical Survey of Parts of the Arctic Ocean, August-September 2010”

According to the United Nations Convention on the Law of the Sea (UNCLOS), individual nations' sovereign rights extend to 200 nautical miles (n.mi.) (370 km) offshore in an area called the continental shelf. These rights include jurisdiction over all resources in the water column and on and beneath the seabed. Article 76 of UNCLOS also establishes the criteria to determine areas beyond the 200 n.mi. (370 km) limit that could be defined as "extended continental shelf," where a nation could extend its sovereign rights over the seafloor and sub-seafloor. This jurisdiction provided in Article 76 includes resources on and below the seafloor but not in the water column. The United States has been acquiring data to determine the outer limits of its extended continental shelf (ECS) in the Arctic and has a vested interest in declaring and receiving international recognition of the reach of its extended continental shelf.


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