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The light side of rock fractures.

The light side of rock fractures

For centuries, miners have dreaded the occasional, mysterious flashes that would erupt from rock faces to light up underground diggings. The air would feel electrified as in a thunderstorm, and bits of paper and straw would jump about. Too often, these effects would presage a major rock collapse.

Although long a part of mining lore, these emissions have only recently been studied in the laboratory. Reporting in the May 29 NATURE, Brian T. Brady and Glen A. Rowell of the Bureau of Mines in Denver suggest that as rocks fracture, they eject electrons, which in turn excite surrounding air molecules to produce light.

"This is the first laboratory investigation of this problem," says Brady. The experiments provide a possible explanation for light and electrical emissions associated with rock fracture not only in mines but also during earthquakes. Reports of "earthquake lights" have a long history, and scientists have proposed several hypotheses to account for this effect (SN:6/5/82, p. 375).

In their experiments, Brady and Rowell compress cylindrical samples of granite, basalt, coal, marble and other rocks. When the pressure is high enough, the rock samples fracture explosively. Within milliseconds, a dust cloud of fine rock particles is ejected and light flashes appear. In general, however, only an observer in darkness with night-adapted vision can see this light.

By fracturing samples in different atmospheres and by examining the spectra of emitted light, the researchers found that the light comes not from the rocks but from the ambient gas. Moreover, the spectra show distinct lines rather than a continuous range of wavelengths. A continuous spectrum would be seen if the effect were due to heat generated by friction within the rock.

Even more surprising is the finding that when the tests are done in water, the water glows and hydrogen is produced. The ejected electrons appear to have enough energy to cause the dissociation of water, says Brady. Thus, the fracturing process, in the presence of water, could promote a variety of chemical reactions.

"We believe that insufficient consideration has been given to the role of rock fracture in fluid- and gas-saturated rock masses in promoting molecular dissociations," the researchers say, "and the role of this process in initiating chemical reactions of geological and biological interest." That may include reactions contributing to the formation of natural-gas deposits or to the origin of life on earth.

Brady and Rowell are now studying the details of these effects. They speculate, for intance, that electron emission within fracturing coal masses may cause the dissociation of methane, leaving pockets of potentially explosive hydrogen gas.

This research also provides the first plausible explanation for the observation of earthquake lights at sea. Previously, says John S. Derr of the U.S. Geological Survey in Denver, "it was not possile to explain earthquake lights at sea, except by invoking the help of legions of excited, phosphorescent plankton."

And, says Derr, the overall results show that under the right conditions, even the smallest earthquakes can produce light. This fits Derr's own observations of luminous phenomena that seem to be associated with very small quakes (SN:12/24 & 31/83, p. 412).

However, there is a great difference in scale between a laboratory sample and a major earthquake. Although the work of Brady and Rowell is a significant step toward finding one possible geological mechanism for light production, Derr says, "... investigations in other areas are still required because we may be looking at several phenomena which sometimes share a common appearance and name."

Also not settled, says electrical engineer Stuart A. Hoenig of the University of Arizona in Tucson, is the question of how the breakup of rock generates free electrons. "Why does the rock give off electrons when it breaks?" he asks. "How do the electrons escape?"

Nevertheless, the fact that electrons are emitted is quite certain, says Hoenig. This electrical activity and its related chemical effects -- possibly causing changes in the air's ion concentration -- may account for the unusual behavior of some animals before an earthquake, he says.

Says Derr, "A new, challenging area of geophysics is just opening."
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Title Annotation:rock fractures emit electrons that produce light
Author:Peterson, Ivars
Publication:Science News
Date:Jun 14, 1986
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