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“If we want to tap these very hot geothermal sources, we will have to learn how to drill into rocks that are in this mixed-mode condition, where they are not purely brittle, but also flow a bit,” says Peč, who is an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences . “But overall, this is fundamental science that can help us understand where the lithosphere is strongest.
“This transition state of partly flowing, partly fracturing, is really important, because that’s where we think the peak of the lithosphere’s strength is and where the largest earthquakes nucleate,” Peč says. “But we don’t have a good handle on this type of mixed-mode behavior.” All these defects will also generate their own sounds when they move under stress and therefore both actively sounding through the rock as well as listening to it should give them a great deal of information. They found that the idea should work with ultrasound waves, at megahertz frequencies.This kind of ultrasound method is analogous to what seismologists do in nature, but at much higher frequencies,” Peč explains.
They found that at the lower end of the pressure range, where rocks are brittle, the marble indeed formed sudden fractures in response, and the sound waves resembled large, low-frequency booms. At the highest pressures, where rocks are more ductile, the acoustic waves resembled a higher-pitched crackling. The team believes this crackling was produced by microscopic defects called dislocations that then spread and flow like an avalanche.