Echoes From A-Blast Provide a Closer Look at Inner Earth

Seismic echoes from a powerful Chinese nuclear test have helped U.S. scientists develop a new process for looking deep within the Earth that has already produced remarkably sharp images of the white-hot layer of gooey rock surrounding the planet’s core.

Using the new process, John E. Vidale and Harley M. Benz of the U.S. Geological Survey in Menlo Park were able to detect an “island” as small as 60 miles across in the layer atop the Earth’s molten iron core. They reported their findings in today’s edition of the journal Nature.

Michael E. Wysession, a scientist at Washington University in St. Louis, called the ability to detect such relatively small features at that depth a “considerable step forward” that could lead to a new understanding of the Earth.

“Before (this) study, the smallest feature we could see was 1,000 kilometers (621 miles) across,” said Wysession. “We could essentially see something as large as, say, a continent or an ocean. Now . . . we cannot only see there are continents, we can see there are mountains and plains on those continents.”

Armed with the more precise images, scientists will be better able to study how events in the mantle influence volcanoes, movement of the continents, spreading of the oceans and other major geophysical processes that shape the Earth’s surface.

The researchers used existing networks of seismometers in the United States and Canada to measure high-frequency seismic waves from a 600-kiloton underground atomic bomb test in China in May, 1992. Vidale said the waves glanced off the Earth’s core, scattering in a way that made it possible to reconstruct, or “image,” conditions at the core-mantle boundary 1,500 miles beneath northern Alaska.

What the new imaging process cannot immediately tell scientists is the nature of the “island” or blob of stiffer, denser rock they found atop the Earth’s core. But Vidale said this new technique ultimately will provide the kind of detailed information needed to infer some answers about such blobs of material.

Scientists have known for some time that large structures lay at the bottom of the mantle, in the core-mantle boundary, and theorists have developed several hypotheses about them. Some suggest these “hot spots” are a mixture of the rocky mantle and molten iron core. Others suspect they are the remnants of Earth’s crust that sunk when tectonic plates on the surface collided.

Many geologists believe these areas are the source of hot plumes of molten rock that shoot up and punch holes through the crust, pushing continents apart and creating certain kinds of volcanoes. Hawaii and other islands may be the product of these “hot spot” plumes.

Wysession said this new imaging method should give scientists a much more accurate picture of the core-mantle boundary. That, he added, will help to refute some theories and refine others.

“This has tremendous implications for history of the surface of Earth,” Wysession said. “It will help us understand why things are as they are.”

The mantle, he explained, is something like a kettle of soup. Scientists know a lot about the surface, where the heat escapes, but understand far less about the bottom of the kettle, nearest the heat source.

“It took centuries of observations of the lithosphere--of people studying mountains and volcanoes and measuring the thickness of the crust--before we were able to develop a theory of plate tectonics,” he said, referring to the idea that earthquakes are caused by collisions between huge chunks of crust floating on the viscous mantle.

“I expect we could do the same thing by studying the other side of this, the core-mantle boundary, and this (new process) makes that much easier.”