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U.S. Department of State

Diplomacy in Action

Defining the Limits of the U.S. Continental Shelf

The continental shelf is an important maritime zone, one that holds many resources and vital habitats for marine life. The majority of the world’s continental shelf is unknown and unmapped. Even so, the responsible use and preservation of this unique area depends on the collection of data to better understand where our rights on the continental shelf lie.


Given these important aspects, it is critical for the United States to accurately define the full extent of its continental shelf. Determining the extent of the continental shelf is a bit different than other maritime zones, such as the territorial sea or the exclusive economic zone, because it is not simply a matter of distance from shore. Under customary international law, as reflected in the Convention on the Law of the Sea, every coastal Country automatically has a continental shelf out to 200 nautical miles from its shore (or out to a maritime boundary with another coastal Country). In some cases, a coastal Country can have a continental shelf beyond 200 nautical miles if it meets certain criteria. Typically, the portion of continental shelf beyond 200 nautical miles is called the "extended continental shelf” or simply the ECS.



Defining the U.S. Extended Continental Shelf
The process to determine the outer limits of a country’s ECS requires the collection and analysis of data that describe the depth, shape, and geophysical characteristics of the seabed and sub-sea floor. Since 2001, U.S. agencies have been engaged in gathering and analyzing data to determine the outer limits of the U.S. ECS.


The rules for defining the ECS are based in international law, specifically Article 76 of the Convention on the Law of the Sea. About 80 coastal nations likely have a continental shelf that extends beyond 200 nautical miles. A coastal Country can use one of two formulas in any combination to determine the edge of its ECS. The Convention also provides two constraint lines that those two formulas cannot go past. Here, too, a country can use any combination of those constraint lines to maximize its shelf. Keep in mind this legal definition of the continental shelf is not the same as what a geologist would call continental shelf.


Why define the U.S. extended continental shelf?
The United States, like other countries, has an inherent interest in knowing, and declaring to others, the exact extent of our ECS and thus the sovereign rights we are entitled to exercise thereon. Specifically, the United States has sovereign rights over the resources on and under the seabed including "sedentary" creatures such as clams, crabs, and corals. While there may also be some hydrocarbon resources (oil, gas, gas hydrates) beyond 200 nautical miles, we'd expect to see more mineral resources, such as manganese nodules, ferromanganese crusts, and polymetallic sulfides. Defining the ECS in concrete geographical terms provides the specificity and certainty necessary for the development and conservation of these potentially resource-rich areas. While a coastal Country’s continental shelf may be coincident with its exclusive economic zone (EEZ) out to 200 nautical miles, the ECS is not an extension of the EEZ. Some of the sovereign rights that a coastal Country may exercise in the EEZ, especially rights to the resources of the water column (e.g., pelagic fisheries), do not apply to the ECS.


Preliminary studies have indicated that the U.S. ECS likely totals at least one million square kilometers -- an area about twice the size of California. While work in the Arctic Ocean has received the most attention, the United States also has ECS in other areas, such as the East Coast, the Gulf of Mexico, and the Bering Sea. Additional analyses and data collection suggest that we may have an even larger ECS in these and possibly other areas. As additional data are collected and existing data analyzed, we will begin to come to a more definitive conclusion as to the extent of the U.S. ECS. Because most of the ocean -- especially the deep ocean -- is unexplored, we are unsure exactly what the sea floor looks like. Given the size of the U.S. continental shelf, the resources we might find there may be worth many billions if not trillions of dollars.


Date: 05/26/2010 Description: Formula lines used for defining the extended continental shelf under Article 76 of the Law of the Sea Convention. - State Dept Image

Formula lines used for defining the extended continental shelf under Article 76 of the Law of the Sea Convention


Date: 05/26/2010 Description: Constraint lines used for defining the extended continental shelf under Article 76 of the Law of the Sea Convention. - State Dept Image

Constraint lines used for defining the extended continental shelf under Article 76 of the Law of the Sea Convention

Data Collection and Analysis

There are two primary datasets that a country needs to collect to determine the two formula lines and the 2500m isobath constraint line. The first is bathymetric data that provides a three-dimensional map of the ocean floor. The second required dataset is seismic reflection data, which provides a cross-section view of what's beneath the ocean floor. From that cross-section view, scientists can derive information on the thickness, geometry, and other characteristics of the geologic layers that are stacked on top of one another.



Since 2003, the United States has collected more than one million square kilometers of bathymetric data from fourteen cruises: Arctic Ocean (2003, 2004, 2007, 2008, 2009), Gulf of Alaska (2005), Gulf of Mexico (2007), Atlantic Ocean (2004, 2005, 2008), Northern Mariana Islands and Guam (2006, 2007), the Bering Sea (2003), Hawaii (2009), and U.S. West Coast/ Pacific Ocean (2009). Cruises for 2010 include the Northern Mariana Islands, Kingman Reef and Palmyra Atoll, and the third joint expedition with the United States and Canada in the Arctic Ocean. Much of this data collection was coordinated through the Joint Hydrographic Center, a partnership between NOAA and the University of New Hampshire.



 Date: 05/26/2010 Description: Healy breaking ice ahead of the Louis S. St.-Laurent. - State Dept Image
Healy breaking ice ahead of the Louis S. St.-Laurent

One of our most exciting data collection efforts has been our joint expeditions with Canada. This summer will mark the third year the U.S. Coast Guard Cutter Healy and the Canadian Coast Guard Ship Louis S. St.-Laurent collect seismic and bathymetric data in the Arctic Ocean. This cooperative effort, initiated in July 2007, has proven to be remarkably worthwhile given our mutual interest in defining the Arctic continental shelf, which would be a difficult task for either country to do on its own. Both countries can provide data the other needs, saving each other millions of dollars. More specifically, the United States needs seismic reflection data -- a very tricky endeavor in the Arctic -- and Canada has successfully mastered this type of data collection from the Louis S. St.-Laurent. Likewise, Canada prefers to have the multi-beam bathymetric data that can be collected from Healy, as opposed to single-beam data on the Louis S. St.-Laurent. Canada also needs the Healy to clear a path for the seismic data collection work, especially in the heavy ice conditions. In some areas, the bathymetric data are more important, so the Louis S. St.-Laurent takes the lead to clear the ice, while the Healy follows collecting better soundings of the seafloor. Finally, Arctic experts comprise a relatively small group of people, many of whom have worked together for many years, so utilizing one another's expertise is typical in this area of the world.

Date: 05/26/2010 Description: The Healy following behind the Louis S St.-Laurent. - State Dept Image



The Healy following behind the Louis S St.-Laurent


The collection and analysis of the data necessary to support the establishment of the outer limits of the U.S. ECS has also provided interesting scientific discoveries. For instance, the data from the last three cruises to the Arctic revealed scours created by past glaciers scraping along the ocean bottom, and large craters thought to be formed by gas seeps emanating from the ocean floor. One of these missions even discovered a 3,000 meter tall underwater mountain, subsequently named the Healy Seamount, which had never been mapped previously.


Date: 05/26/2010 Description: The newly discovered Healy Seamount in the Arctic Ocean. - State Dept Image

The newly discovered Healy Seamount in the Arctic Ocean


These data also serve a range of other environmental, geologic, engineering, and resource management needs. The United States will gain specific insights related to such areas as climate variability, marine ecosystems, undiscovered or unconventional energy, mineral resources, and hazards resulting from extreme events, such as earthquakes and tsunamis. These data will also provide a better scientific understanding of formation and transformation processes of our continental margins. Finally, exploration of little known areas, particularly in the ice-covered Arctic, will advance our operational capabilities and open new windows on this remote and inaccessible environment. All bathymetric data collected by the United States in support of defining its continental shelf have been released to the public. The bathymetric data are available from the National Geophysical Data Center and the Joint Hydrographic Center.


The work to define the U.S. extended continental shelf is coordinated by the U.S. Extended Continental Shelf Task Force, an interagency body headed by the U.S. Department of State. Participants in this Task Force include the: U.S. Geological Survey, National Oceanic and Atmospheric Administration, Executive Office of the President, Joint Chiefs of Staff, U.S. Navy, U.S. Coast Guard, Department of Energy, National Science Foundation, Environmental Protection Agency, Minerals Management Service, the Arctic Research Commission, and the Marine Mammal Commission. In addition, much of the data collection has been done by the Joint Hydrographic Center, a cooperative office between the University of New Hampshire and NOAA. For more information on the work of the ECS Task Force and the data collection for summer 2010, see our website:


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