{"id":24632,"date":"2023-10-12T16:51:24","date_gmt":"2023-10-12T11:06:24","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=24632"},"modified":"2023-10-12T16:54:16","modified_gmt":"2023-10-12T11:09:16","slug":"boom-crackle-pop-sounds-of-earths-crust","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/boom-crackle-pop-sounds-of-earths-crust\/","title":{"rendered":"Boom, crackle, pop: Sounds of Earth\u2019s crust"},"content":{"rendered":"

By Jennifer Chu<\/p><\/div><\/div>\n\n\n

CAMBRIDGE, Mass. — If you could sink through the Earth\u2019s crust, you might hear, with a carefully tuned ear, a cacophany of booms and crackles along the way. The fissures, pores, and defects running through rocks are like strings that resonate when pressed and stressed. And as a team of MIT geologists has found, the rhythm and pace of these sounds can tell you something about the depth and strength of the rocks around you. <\/p>\n\n\n\n

\u201cIf you were listening to the rocks, they would be singing at higher and higher pitches, the deeper you go,\u201d says MIT geologist Mat\u011bj Pe\u010d. <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Pe\u010d and his colleagues are listening to rocks, to see whether any acoustic patterns, or \u201cfingerprints\u201d emerge when subjected to various pressures. In lab studies, they have now shown that samples of marble, when subjected to low pressures, emit low-pitched \u201cbooms,\u201d while at higher pressures, the rocks generate an \u2018avalanche\u2019 of higher-pitched crackles. <\/p>\n\n\n\n

Pe\u010d says these acoustic patterns in rocks can help scientists estimate the types of cracks, fissures, and other defects that the Earth\u2019s crust experiences with depth, which they can then use to identify unstable regions below the surface, where there is potential for earthquakes or eruptions. The team\u2019s results, published in the Proceedings of the National Academy of Sciences<\/a><\/em>, could also help inform surveyors\u2019 efforts to drill for renewable, geothermal energy. <\/p>\n\n\n\n

\u201cIf 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,\u201d says Pe\u010d, who is an assistant professor in MIT\u2019s Department of Earth, Atmospheric and Planetary Sciences (EAPS). \u201cBut overall, this is fundamental science that can help us understand where the lithosphere is strongest.\u201d <\/p>\n\n\n\n

Pe\u010d\u2019s collaborators at MIT are lead author and research scientist Hoagy O. Ghaffari, technical associate Ulrich Mok, graduate student Hilary Chang, and professor emeritus of geophysics Brian Evans. Tushar Mittal, co-author, and former EAPS postdoc, is now an assistant professor at Penn State University.<\/p>\n\n\n\n

Fracture and flow<\/strong><\/p>\n\n\n\n

The Earth\u2019s crust is often compared to the skin of an apple. At its thickest, the crust can be 70 kilometers deep \u2014 a tiny fraction of the globe\u2019s total, 12,700-kilometer diameter. And yet, the rocks that make up the planet\u2019s thin peel vary greatly in their strength and stability. Geologists infer that rocks near the surface are brittle and fracture easily, compared to rocks at greater depths, where immense pressures, and heat from the core, can make rocks flow. <\/p>\n\n\n\n

The fact that rocks are brittle at the surface and more ductile at depth implies there must be an in-between \u2014 a phase in which rocks transition from one to the other and may have properties of both, able to fracture like granite, and flow like honey. This \u201cbrittle-to-ductile transition\u201d is not well understood, though geologists believe it may be where rocks are at their strongest within the crust.\u00a0<\/p>\n\n\n\n

\u201cThis transition state of partly flowing, partly fracturing,\u00a0is really important because that\u2019s where we think the peak of the lithosphere\u2019s strength is and where the largest earthquakes nucleate,\u201d Pe\u010d says. \u201cBut\u00a0we don\u2019t have a good handle on this type of mixed-mode behavior.\u201d<\/p>\n\n\n\n

He and his colleagues are studying how the strength and stability of rocks \u2014 whether brittle, ductile, or somewhere in between \u2014 varies, based on a rock\u2019s microscopic defects. The size, density, and distribution of defects such as microscopic cracks, fissures, and pores can shape how brittle or ductile a rock can be. <\/p>\n\n\n\n

But measuring the microscopic defects in rocks, under conditions that simulate the Earth\u2019s various pressures and depths, is no trivial task. There is, for instance, no visual-imaging technique that allows scientists to see inside rocks to map their microscopic imperfections. So the team turned to ultrasound and the idea that, any sound wave traveling through a rock should bounce, vibrate, and reflect off any microscopic cracks and crevices, in specific ways that should reveal something about the pattern of those defects.\u00a0<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

\u201cThis kind of ultrasound method is analogous to what seismologists do in nature, but at much higher frequencies,\u201d Pe\u010d explains. \u201cThis helps us to understand the physics that occurs at microscopic scales, during the deformation of these rocks.\u201d\u00a0<\/p>\n\n\n\n

A rock in a hard place<\/strong><\/p>\n\n\n\n

In their experiments, the team tested cylinders of Carrara marble. <\/p>\n\n\n\n

\u201cIt\u2019s the same material as what Michaelangelo\u2019s David is made from,\u201d Pe\u010d notes. \u201cIt\u2019s a very well-characterized material, and we know exactly what it should be doing.\u201d<\/p>\n\n\n\n

The team placed each marble cylinder in a vice-like apparatus made from pistons of aluminum, zirconium, and steel, which together can generate extreme stresses. They placed the vice in a pressurized chamber and then subjected each cylinder to pressures similar to what rocks experience throughout the Earth\u2019s crust.\u00a0\u00a0<\/p>\n\n\n\n

As they slowly crushed each rock, the team sent pulses of ultrasound through the top of the sample and recorded the acoustic pattern that exited through the bottom. When the sensors were not pulsing, they were listening to any naturally occurring acoustic emissions.<\/p>\n\n\n\n

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. <\/p>\n\n\n\n

\u201cFor the first time, we have recorded the \u2018noises\u2019 that rocks make when they are deformed across this brittle-to-ductile transition, and we link these noises to the individual microscopic defects that cause them,\u201d Pe\u010d says. \u201cWe found that these defects massively change their size and propagation velocity as they cross this transition. It\u2019s more complicated than people had thought.\u201d<\/p>\n\n\n\n

The team\u2019s characterizations of rocks and their defects at various pressures can help scientists estimate how the Earth\u2019s crust will behave at various depths, such as how rocks might fracture in an earthquake, or flow in an eruption.    <\/p>\n\n\n\n

\u201cWhen rocks are partly fracturing and partly flowing, how does that feed back into the earthquake cycle? And how does that affect the movement of magma through a network of rocks?\u201d Pe\u010d says. \u201cThose are larger scale questions that can be tackled with research like this.\u201d<\/p>\n\n\n\n

This research was supported, in part, by the National Science Foundation.<\/p>\n","protected":false},"excerpt":{"rendered":"

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