South Louisiana Is Sliding Into the Sea, Study Says

Craig Guillot
for National Geographic News

January 8, 2007

A large part of southeast Louisiana is slowing sliding into the Gulf of Mexico, according to a recent report from the American Geophysical Union.

Researchers have known for decades that the southern part of the state is sinking (related news: "New Orleans Sinking Faster Than Thought, Satellites Find" [June 1, 2006]).

But the latest findings show that a large egg-shape area about 250 miles (400 kilometers) long and 180 miles (290 kilometers) wide is also gradually oozing into the Gulf.

"This whole section is moving south-southeast and pulling apart from the rest of the country up in the Lake Pontchartrain region," said lead study author Roy Dokka, executive director of the Center for Geoinformatics at Louisiana State University in Baton Rouge.

"It's actually crumpling down at its toe, down south at the bottom of the Gulf of Mexico." (See a Louisiana map.)

Reporting in the December 2006 issue of the journal Geophysical Research Letters, Dokka and colleagues note that the speed at which the region moves is equivalent to that of a glacier—little more than a couple millimeters a year.

Timothy Dixon, a study co-author and professor of geology and geophysics at the University of Miami in Coral Gables, Florida, says there is no need for panic.

"Based on current rates, it would take thousands of years for that process to have any serious effects," Dixon said.

"It just means that New Orleans will [have an elevated] risk of flooding."

Gooey Bedrock

Much of southern Louisiana—which sits on the Mississippi River Delta—was built up over thousands of years by sediments from the river.

Unlike solid bedrock in other parts of the country, this area of the state sits on what Louisiana State's Dokka calls "goo" that is very unstable.

This goo, he said, surges and pulsates, then slides until it is stopped by friction. The last big round of movement was in 1970.

Dixon, of the University of Miami, said that south Louisiana's movement is a slow and natural process, but one that has been increased by human activities such as building levees and exploring for oil.

Authur Berman, a petroleum geologist from Houston, Texas, who has studied subsidence in the Gulf Coast, agreed, adding that the sliding phenomenon is commonly seen around the Gulf of Mexico.

Berman said that over thousands of years the area that extends 150 miles (240 kilometers) inland around much of the Gulf Coast has been continually subsiding.

Quakes and Floods

Large earthquakes that have been occurring south of New Orleans over the past 20 or 30 years are likely tied to the region's sliding movement, Dokka said.

The quakes usually happen far south of New Orleans, he said (related news: "Rare Earthquake Shakes Gulf of Mexico" [September 11, 2006]).

"But they're all connected to this stuff that is right below south Louisiana about five miles [eight kilometers] deep. It's the bottom of this thing that is sliding out."

According to Dokka, there is a slight potential for a seriously damaging earthquake, but nothing that could cause south Louisiana to suddenly break away from the rest of the state.

What's more, study co-author Dixon added, the region that is sliding sits over a layer of salt that may prevent earthquake-type stresses from building up enough to trigger a large quake.

The report authors all agree that flooding remains Louisiana's main concern and that the state can help protect itself by monitoring coastal subsidence through a global positioning system network that is already in place.

With proper monitoring, levees can be raised and flood protection can be altered as the land sinks and shifts.

"What's important," Dokka said, "is that we've been able to recognize one of the important processes that makes south Louisiana vulnerable."

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