Earthquakes Help Take Deep Earth's Temperature

Christine Dell'Amore
for National Geographic News
March 29, 2007
New high-resolution seismic images have produced the best estimate to date of the temperature of Earth's extremely deep interior, researchers report.

Using a method initially developed for oil and gas exploration, the scientists studied the core-mantle boundary, a region that lies about 1,860 miles (3,000 kilometers) below the planet's surface.

This technique allowed the team to piece together images based on seismic waves bounced off materials around the boundary.

The resulting 3-D map of the region revealed minerals and pressure levels that indicated the surrounding temperature.

Reporting in tomorrow's issue of the journal Science, the team calculated that the temperature of the region is 3,950 Kelvin, plus or minus 200.

This translates to a fiery 6,650 degrees Fahrenheit (3,677 degrees Celsius)—which is actually lower than previous predictions.

Robert van der Hilst is an earth science professor at the Massachusetts Institute of Technology and lead author of the new study.

"These findings are exciting," he said, "because they demonstrate these techniques adapted from the oil industry actually work" for geologic research.

"My group and I take this as an enormously encouraging development."

Seismic Map

Van der Hilst's team studied data from earthquake-prone areas of Central America.

The regions are among the few in the world where a large number of quakes occur close enough to seismographic stations for scientists to record earthquakes' seismic waves bounced back from the core.

The researchers compiled data from thousands of earthquakes recorded at more than a thousand stations to create a detailed 3-D map of the core-mantle boundary.

Using this map, the team then estimated the temperature based on two key factors: pressure and mineral content.

For instance, scientists already knew the temperature at which a mineral in the mantle called pervoskite transforms into a high-pressure material called post-pervoskite.

The transition takes place in the lowermost mantle, just above the core, the new data suggested. This in turn provided a correlating temperature for the core-mantle boundary.

Such findings could shed light on how heat flows from Earth's core into the mantle, said Donald Helmberger, a professor of geological and planetary sciences at the California Institute of Technology.

This heat flow drives the planet's magnetic field and is still poorly understood (related: "North Magnetic Pole Is Shifting Rapidly Toward Russia" [December 15, 2005]).

"Every technique we can bring to bear to study these things is a step forward," Helmberger said.

Edward Garnero is a professor of geological sciences at Arizona State University who also uses seismology to study the deep interior.

This kind of research could demystify how the planet evolved, he said (related: early-Earth interactive).

Knowing Earth's temperature at such depths can tell scientists how much the core has cooled over time and how fast it is cooling now.

"We live on this amazing planet, and we still don't know for certain the processes that go on inside it," Garnero said.

"This paper represents another little piece that brings it into a sharper focus."

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