for National Geographic News
Were it not for hot spots in the top layer of Earth's surface, most of us would be swimming, according to a new study.
It has long been known that the continents float atop the denser rock of the underlying mantle, like foam on a hot tub.
This was simply because the land masses are made of lighter types of rock, scientists had presumed. However, it turns out that about half of the buoyancy on solid Earth is provided by hot spots.
In these warm regions rock expands from the heat, making it even less dense than would otherwise be the case.
Without the heat, scientists say, even Denver, Colorado—the Mile High City—would be below sea level.
A Difficult Process
For the new study, scientists first accounted for differences in the thickness of the planet's top layer, the lithosphere, which affects the buoyancy of the continents.
Warm or cold, thick lithosphere will float higher than thin lithosphere.
"It's a difficult process, which is part of why nobody's done it," said the study's lead author, Derrick Hasterok, a Ph.D. student in geology and geophysics at the University of Utah.
In a complex analysis based on seismic data, Hasterok and study co-author David Chapman, also of the University of Utah, found that these differences in thickness accounted for about 10,000 feet (3,000 meters) of the elevation range in North America.
The scientists then correlated the remaining variations to what is known of the temperature of the lithosphere and upper mantle of Earth.
(See related: "Earthquakes Help Take Deep Earth's Temperature" [March 29, 2007].)
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