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Spectrophotometer Observes Radiation from Rocks

Photonics Spectra
Jan 2004
Brent D. Johnson

A few years ago, Friedemann Freund, a professor at San Jose State University in California, embarked on the study of rock deformation. When you squeeze a rock very hard, asked the physicist, what are the physical processes that take place? The data he is collecting with an infrared spectrophotometer promise to provide insights into the geological phenomena that precede earthquakes.

Spectrophotometer Observes Radiation from Rocks

Infrared analysis of stressed samples of rock may offer a better understanding of geological phenomena associated with earthquakes. It is believed that charge carriers become activated when rocks are subjected to short, intense stresses and that the recombination of these carriers produces infrared radiation.

Freund said that images taken with National Oceanographic and Atmospheric Administration and NASA satellites reveal a strange phenomenon: Prior to a major earthquake, the areas where stresses build up deep in the underground often seem to heat up for a few days. He had the idea that what these satellites record as thermal anomalies may in fact be caused by a form of IR luminescence involving electronic charge carriers. In this model, the charge carriers become activated when rocks are stressed to the point of plastic deformation.

To test this, Freund applied sudden, short stresses to rocks by shooting them with a crossbow and with NASA's Ames Vertical Gun Range, a two-stage, light-gas gun that can accelerate 0.25-in. projectiles to velocities of approximately 6.5 km/s. The impacts with the crossbow projectiles created small but intense deformations at the impact points and freed a cloud of electric charges that had existed in the rocks in an electrically inactive, dormant state. The charge clouds propagated at several hundred meters per second, even speeding through portions of the rocks that had not experienced stress.

There were also some high-frequency components, in the range of radio frequencies, which probably arise from the buildup of electric fields at the rock surface; in particular, at the corners and edges. These local fields are so high that they can produce corona discharges accompanied by visible light.

Intrigued by these observations, Freund squeezed rock samples in a hydraulic press until they failed. At first, he used slabs of granite, which is common in the Earth's crust. The stone contains large amounts of quartz, which develops an electric charge when squeezed. Next, he used quartz-free anorthosite, a rock composed almost entirely of labradorite, a feldspar known for its iridescence. Only the central part of each block was compressed, leaving the periphery largely stress-free. During the experiments, he measured the emissions from the rock surface with an IR spectrophotometer from ABB Inc. of Quebec.

Freund recorded a specific IR emission from the rock surface, 10 to 20 cm away from the point where the rock was being squeezed. The emission occurs so rapidly, he explained, that it cannot be propagating heat. Its spectrum has several components, including a narrowband one that occurs where theory predicts the emission band should be from the recombination of the charge carriers.

Freund plans more experiments to study the phenomenon in greater detail. Although he is unwilling to make any firm statement at this time, he believes that the discovery of the dormant electronic charge carriers in rocks, which can be awakened by stress, opens the door to a better understanding of geological phenomena from the thermal anomalies in satellite images to the "earthquake lights" photographed in Japan.


GLOSSARY
infrared spectrophotometer
A spectrophotometer having a prism or, more frequently, a grating for the study and recording of infrared spectra. It usually consists of a radiation source such as a Nernst glower, a monochromator, a detector, an amplifier and a recorder. The most commonly used window and prism materials are rock salt and potassium bromide (KBr).  
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