Political geology: How to find a country's underwater border

On sea, as on land, one country's territory ends where another begins. But what about when there is no border? How far out into the Atlantic does the United States end? Or -- in an increasingly relevant case -- how much of the Arctic Ocean belongs to each of the countries that ring it? How this is determined, it one of the cases where the "geo" in geopolitics becomes particularly relevant.

The U.N. Convention on the Law of the Sea, which went into effect in 1994, gives countries rights to their continental shelf. Geologists define continental shelf as the shallow part of a continent that's submerged underwater, but for legal purposes, it's defined as 200 nautical miles offshore.

However, the all-important Article 76 of the treaty lets countries claim shelf beyond 200 miles in two situations:

1. Countries can claim shelf 60 nautical miles from the foot of the continental slope, specifically "the point of maximum change in the gradient at its base."

2. They can also claim areas where the thickness of the sediment at the sea floor is at least 1 percent of the distance to the foot of the slope. In other words, if you're 100 miles out from where the slope of North American continent hits the Atlantic Ocean floor and the sedimentary layer on the sea floor is at least a mile thick, that's still America!

If those seem like odd criteria, scientists agree with you. "There are two definitions of continental shelf, a geological one and a political one," says Deborah R. Hutchinson of the U.S. Geological Survey. "Article 76 is full of ambiguities, because you're trying to put science in a legal framework."

There are limits on what's called the Extended Continental Shelf (ECS). And it only applies to rights to the sea floor. The water above the ECS is international. And a country can't extend its shelf indefinitely.

Since 2001, more than 50 countries have made continental shelf submissions under the treaty. Hutchinson notes that governments avoid referring to these as "claims" -- "The ECS is yours to define. You're not claiming it. It's yours already." But overlaps do sometimes occur. Britain has competing continental shelf claims with Argentina. Tanzania and the Seychelles are at odds over shelf in the Indian Ocean.

The United States is of course is not party to the Law of the Sea Treaty due to ongoing congressional opposition -- though it has been supported by the last three presidents -- but is nonetheless engaged in a multiagency effort to define the country's ECS, a project that has included dozens of cruises in the Atlantic, Pacific, and is due to wrap up in 2015. (Perhaps by that point, the U.S. will be a party to the treaty and can actually define its claims internationally.)  Hutchinson is on the USGS team tasked with measuring sediment thickness using seismic techniques developed by the oil industry. The National Oceanographic and Atmospheric Administration is mapping the sea floor to determine slope as part of the same effort. According to Hutchison, countries typically use whichever condition is most advantageous.

Hutchison also says that Russia's designs in the Arctic may have been a motivation behind the U.S. push to define its shelf. "Russia was the first country to put a submission in and they included a huge swathe of the Arctic. That was sort of a wake-up call that the U.S. needs to know where our ECS is. We want to make sure that we're in line with what other countries are doing."

The map below shows the area beyond the 200 nautical mile line -- outlined in red -- that Russia claimed in its 2001 submission.


The map below shows the areas around the world (the red numbers) where the U.S. is exploring potential ECS:


The issue of shelf claims continues to heat up. In December, China filed a continental shelf claim that it claims gives it control over disputed islands in the East China Sea -- and nearby gas reserves -- based on the shape of the slope on the sea floor.   "I don't think anybody thinks the law of the sea will help resolve these boundary disputes," she says.*

*Correction: The original version of this post misleadingly implied that this quote referred to all sea boundary disputes. The U.S.G.S. points out that the Law of the Sea has frequently been used to resolve international disputes, particularly among South Pacific nations.  

Images courtesy of continentalshelf.gov and the UN