Dec. 8, 2003 Like doctors taking a sonogram of a human body, Princeton geoscientists have captured images of the interior of the Earth and revealed structures that help explain how the planet changes and ages.
The scientists used tremors from earthquakes to probe the inside of the planet just as sound waves allow doctors to look inside a mother's womb. The technique, a greatly refined version of earlier efforts, produced a surprisingly sharp image and yielded the first direct measurements of giant spouts of heat, called mantle plumes, that emanate from deep within the planet.
Mantle plumes are believed to cause island chains, such as the Hawaiian Islands and Iceland, when the Earth's crust passes over the column of heat. Although accepted by most scientists, the existence of mantle plumes has been fiercely contested by a minority of researchers in recent years.
"This is the first visual evidence that mantle plumes exist," said Raffaella Montelli, a Princeton geoscientist and the lead author of a paper published online by the journal Science on Dec. 4. "There is still a very open debate, but we are saying 'Look, here they are; you can see them.'"
Montelli, who received a Ph.D. from Princeton this year and is now a postdoctoral fellow, conducted the study in collaboration with Princeton professors Guust Nolet and Tony Dahlen as well as Guy Masters of the University of California-San Diego, Robert Engdahl of the University of Colorado and Shu-Huei Hung of National Taiwan University.
The scientists used data from more than 3,000 seismographic stations around the world. The stations monitored tremors from more than 86,000 earthquakes since 1964. The seismic waves change speed slightly when they encounter different temperatures and materials in the Earth, said Nolet. In particular, the waves slow down when they encounter warm spots where the rock is very slightly softer than in cooler spots.
"If we can find out if waves are being slowed down or speeded up, then we know whether the Earth is locally hotter or colder," Nolet said. The researchers analyzed these changes in speed and assembled their data into a three-dimensional temperature map. They immediately noticed broad columns of warm material rising out of the Earth's mantle, which is the layer nearly 2,000 miles thick just under the crust.
"We started the research without any thought of mantle plumes," said Nolet. Their goal was to improve on a century-old theory of how seismic waves travel through the Earth, taking into account how the waves interact with varying temperatures and materials. In addition to developing a better theory, the researchers selected only the highest quality data from millions of measurements that were available, Nolet said.
When the heat columns appeared in their map, the researchers compared their locations to those of suspected mantle plumes around the globe and found close correlations. They identified 32 plumes, most of which are located beneath known hot spots that had been assumed to result from plumes. A few are entirely new and were not associated with known hot spots. At the same time, some expected plumes, such as one believed to be under Yellowstone National Park, did not show up.
The results are an important step in understanding mantle plumes and also raise a host of new questions, said Princeton geophysicist Jason Morgan, who first proposed the existence of mantle plumes in 1971 and, in December, won the National Medal of Science in part for his work on the subject.
The results, for example, do not show all the plumes extending from the bottom of the mantle as he and others anticipated, Morgan said. Some seem to begin in shallower parts of the mantle, he said, noting that understanding the reason for this is likely to provide valuable insights into the dynamics within the Earth. "Some plumes may be gaining strength and others may be fading. I don't know what will come of that but it will be something interesting I am sure," he said.
Montelli said she plans to continue refining the imaging technique and will repeat the analysis with a different type of seismic wave, which should yield an even sharper image of the plumes.
The research was funded by grants from the National Science Foundation.
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