Earth's Hot Spots Keep Continents Afloat, Study Says

Richard A. Lovett
for National Geographic News
June 26, 2007
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].)

In general, cold spots signal low elevation and hot spots are high.

"Cold" and "hot" are relative terms, Hasterok noted. Cold would be about 900 degrees Fahrenheit (500 to 600 degrees Celsius). Hot would be 1,832 degrees Fahrenheit (1000 degrees Celsius).

Swimming With the Fishes

If the entire continent were "cold" under his definition, Hasterok said, New York City would be 1,427 feet (435 meters) beneath the Atlantic Ocean.

New Orleans would be 2,416 feet down (736 meters), and Los Angeles would be deeper yet, at 3,756 feet (1,145 meters). Denver would be 727 feet (222 meters) below sea level.

In fact, most of the continent would be below sea level, except the high Rocky Mountains, the Sierra Nevada, and the Pacific Northwest west of the Cascade Range, according to Hasterok.

No one should worry that a cooling Earth will force us to move to mountaintops, however—that would take a couple of billion years, Hasterok said.

Intriguing Findings

Other scientists find the study, which appeared in the Journal of Geophysical Research, intriguing.

The hot regions seen by the study, said Norm Sleep, a geophysicist at Stanford University who was not involved in the analysis, come from a variety of sources.

These include upwelling plumes of hot rock from the deep mantle and tectonic forces that stretch the lithosphere or cause underlying layers to pull away, opening a path for hotter rock from below.

(See related: "Earth's Core Spins Faster Than Surface, Study Confirms" [August 25, 2005].)

Although Sleep believes that there may be some errors in basing estimates of Earth's lithosphere thickness solely on seismic analyses, he said the study methodology works well.

Gillian Foulger, a seismologist from Britain's Durham University, also finds the study important.

"I look forward to further development and scrutiny of this new technique and applications to other areas," Foulger said by email.

So far the research has only been applied to North America.

"We've been working on extending this to the rest of the world," said lead author Hasterok, "but we're not quite there yet."

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