Heat from the Earth's deep interior helps keep much of North America afloat by warming the continental crust and making it buoyant, scientists say.

If not for this effect, many American coastal cities would lie beneath the sea.

"We have shown for the first time that temperature differences within the Earth's crust and upper mantle explain about half of the elevation of any given place in North America," said study team member David Chapman of the University of Utah.

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Rock composition differences can explain the other half, he added.

Using previously published data of how rock density varies with depth in North America's crust, the researchers created a hypothetical continental crust with a uniform thickness and composition.

"Once we've done that, we can see the thermal effect," Chapman explained.

The researchers could then calculate how much heat flow contributes to elevation in each of the 36 tectonic provinces, or "mini-plates," of North America.

The findings are detailed in two studies published in the Journal of Geophysical-Solid Earth, a publication of the American Geophysical Union.

Cities beneath the sea

The findings show that if North America had a uniform crust, many American cities would be underwater.

New York City, for example, would be dunked 1,427 feet beneath the Atlantic. Boston would be 1,823 below sea level, and Los Angeles would be 3,756 beneath the Pacific.

Other cities would soar to new heights. Seattle, for instance, would reach an elevation of 5,949 feet, up from its current elevation of about 500 feet above sea level.

The rock beneath America's Emerald City is cooler than average for North America; removing the temperature difference would cause the rock to expand and become more buoyant, so Seattle would rise.

Some regions would remain at the same elevation.

"If you subtracted the heat that keeps North American elevations high, most of the continent would be below sea level, except the high Rocky Mountains, the Sierra Nevada and the Pacific Northwest of the Cascade Range," says study team member Derrick Hasterok of the University of Utah.

No immediate threat

According to Chapman, scientists have largely overlooked differences in rock temperature as an explanation of elevations on continents.

Instead, they usually attribute the buoyancy and elevation of various continental areas to variations in the thickness and mineral composition of crustal rocks.

As an example of how rock temperature affects elevation, the researchers point to the Colorado Plateau, which sits 6,000 feet above sea level, and the Great Plains, which is 1,000 feet above sea level, despite having the same rock composition.

"We propose this is because, at the base of the crust, the Colorado Plateau is significantly warmer [1,200 degrees Fahrenheit] than the Great Plains [930 degrees Fahrenheit," Hasterok said.

American cities are in no danger of being submerged any time soon, however, as it will take billions of years for North American rock to cool and become dense enough that it sinks, Chapman said.

If anything, coastal cities face flooding much sooner from sea level rise due to global warming, he added.

Here are other locations, their elevations and how they would sink if the crust had a uniform temperature:

—Atlanta, 1,000 feet above sea level, 1,416 feet below sea level.

— Dallas, 430 feet above sea level, 1,986 feet below sea level.

— Chicago, 586 feet above sea level, 2,229 feet below sea level.

— St. Louis, 465 feet above sea level, 1,499 feet below sea level.

— Las Vegas, 2,001 feet above sea level, 3,512 feet below sea level.

— Phoenix, 1,086 feet above sea level, 4,345 feet below sea level.

— Albuquerque, 5,312 feet above sea level, 48 feet above sea level.

— Mount Whitney, Calif., tallest point in the lower 48 states, 14,496 feet above sea level, 11,877 feet above sea level.

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