SALT LAKE CITY (AP) — If it weren't for the hot rocks down below Earth's crust, most of North America would be below sea level, report researchers who say the significance of Earth's internal heat has been overlooked.

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.