Without it, mile-high Denver would be 727 feet below sea level, the scientists calculate, and New York City, more than a quarter-mile below. Los Angeles would be almost three-quarters of a mile beneath the Pacific.
In fact most of the United States would disappear, except for some major Western mountain ranges, according to research at the University of Utah.
"Researchers have failed to appreciate how heat makes rock in the continental crust and upper mantle expand to become less dense and more buoyant," said Derrick Hasterok, a graduate student in geology and geophysics.
Hasterok and his professor, David Chapman, published their findings in the June online issue of Journal of Geophysical Research-Solid Earth.
In what they said was the first calculation of its kind, the researchers said heat inside the planet accounts for half the reason land rises above sea level or higher to form mountains.
Scientists previously gave other factors greater weight in explaining elevation differences, such as the density and makeup of rocks and tectonic forces.
The Utah team calculated how much of North America would sink if the engine of heat was taken away, leaving regions as relatively cold as the bottom of the vast Canadian shield
-- bedrock that hasn't changed for billions of years.
They did it by estimating temperatures under the North American plate based on previous experiments that bounced seismic waves deep underground. The waves travel faster through colder, denser rock. That data allowed the researchers to calculate how much of an area's elevation is due to the thickness and composition of its rock and how much is due to the heating and expansion of rock.
Their measurements showed that among coastal cities, New York would drop to 1,427 feet below the Atlantic ocean, Boston and Miami even deeper. Los Angeles would rest 3,756 feet below the surface of the Pacific ocean.
[to top of second column]
New Orleans, still recovering from Hurricane Katrina's 2005 storm surges, wouldn't have a chance without planetary heat. No levee could protect the city, which would sit 2,426 feet deep in the Gulf of Mexico.
The country's midsection wouldn't be spared, either. Chicago would sink 2,229 feet below sea level. Most of the country, in fact, would disappear, leaving only ridges of the Rocky Mountains and the Sierra-Nevada Range and the the area west of the Cascade Range in the Pacific Northwest.
The Colorado plateau, a major uplift of land driven by 1,200-degree underground heat, consists of much the same layers of rock found deep under the Great Plains, where the base of the Earth's crust is relatively cooler, 930 degrees, the researchers estimated.
Their scenario actually lifts Seattle and the Pacific Northwest. The region sits on a cold slab of oceanic crust that is diving under the continent, insulating the land mass from the Earth's heat. It would rise if the crust was warmed to a temperature equal to the warmer bottom of the Canadian shield.
The Seattle scenario is puzzling but emphasizes a region that's on a different tectonic plate than the rest of the West Coast, said Barbara EchoHawk of Geological Society of America.
The researchers used the Canadian shield -- "a special, stable and cool area of rock"
-- as their statistical baseline for the effect of removing heat from under the continent. The slab under Seattle, however, is colder than the Canadian shield, so it would be the only U.S. region to rise under this analysis, she said.
Hasterok said heat from Earth's deep interior and from radioactive decay of uranium, thorium and potassium in Earth's crust will stay around for a long time to come.
Even if the planet's interior cooled, it would take billions of years for continents to sink. Coastal areas face a more immediate threat from global warming, which could raise sea levels and flood cities, he said.