MIT scientists have found that the sounds under our feet are fingerprints that prove the stability of rocks.
If you could dive through the Earth’s crust, you might, with a carefully tuned ear, hear explosions and crackles along the way. The cracks, pores, and faults that run through rocks are like strings that resonate when pressed and pressured. And as a team of Massachusetts Institute of Technology Geologists have found that the rhythm and pace of these sounds can tell you something about the depth and strength of the rocks around you.
“If you listen to rocks, they will sing in higher and higher layers, the deeper you go,” says Matej Pietsch, a geoscientist at the Massachusetts Institute of Technology.
Beach and his colleagues listen to rocks to see if there are any sound patterns or “fingerprints” that appear when they are exposed to different pressures. In laboratory studies, they have now shown that marble samples, when subjected to low pressures, emit low-pitched “pops,” while at higher pressures, the rocks generate an “avalanche” of high-pitched pops.
Beach says these acoustic patterns in rocks can help scientists estimate the types of cracks, fissures and other faults deep within the Earth’s crust, which they can then use to identify unstable areas below the surface, where earthquakes or volcanic eruptions are likely. . The team’s results, published October 9 at Proceedings of the National Academy of Sciencescould also help inform surveyors’ efforts to explore for renewable geothermal energy.
“If we want to tap into very hot geothermal sources, we’re going to have to learn how to drill into rock that’s in this mixed mode, where it’s not quite brittle, but it’s also flowing a little bit,” says Beach, who currently works in geothermal energy. . Assistant Professor in the Department of Earth, Atmospheric, and Planetary Sciences at MIT (EAPS). “But in general, this is basic science that can help us understand where the lithosphere is strongest.”
Peč’s collaborators at MIT are lead author and research scientist Hoji O. Ghafari, technical assistant Ulrich Mock, graduate student Hilary Zhang, and Professor Emeritus of Geophysics Brian Evans. Tushar Mittal, co-author and former EAPS postdoctoral researcher, is now an assistant professor at Pennsylvania State University.
Fraction and flow
The Earth’s crust is often compared to the crust of an apple. At its greatest thickness, the crust can be up to 70 kilometers (45 miles) deep, a small fraction of the Earth’s total diameter of 12,700 kilometers (7,900 miles). However, the rocks that make up the planet’s thin crust vary greatly in their strength and stability. Geologists conclude that rocks near the surface are brittle and break easily, compared to rocks at greater depths, where enormous pressures and heat from the core can make the rocks flow.
The fact that rocks are brittle at the surface and softer at depth means that there must be an intermediate stage – a stage in which rocks transition from one to the other, and may have the properties of both, being able to fracture like granite, and flow. Like honey. This “transition from brittleness to elasticity” is not well understood, although geologists believe it may be where rocks are at their strongest within the Earth’s crust.
“This transition state of partial flow, partial fracturing, is really important, because we think that’s where the strength of the lithosphere peaks, and where the biggest earthquakes nucleate,” Beach says. “But we don’t have a good handle on this kind of mixed behavior.”
He and his colleagues are studying how the strength and stability of rocks—whether brittle, ductile, or somewhere in between—varies based on the rocks’ microscopic defects. The size, density, and distribution of defects such as microscopic cracks, fissures, and pores can shape how brittle or ductile a rock is.
But measuring microscopic defects in rocks, under conditions that mimic different pressures and depths of the Earth, is no easy task. For example, there is no optical imaging technology that allows scientists to see inside rocks to map their microscopic defects. So the team turned to ultrasound, the idea that any sound wave traveling through a rock should bounce back, vibrate and reflect any microscopic cracks and fissures, in specific ways that should reveal something about the pattern of those faults.
All of these faults will also generate their own sounds when they move under pressure, so actively sounding through the rocks as well as listening to them should give them a great deal of information. They found that the idea should work with ultrasound at megahertz frequencies.
“Beach explains that this type of ultrasound method is similar to what seismologists do in nature, but at much higher frequencies. “This helps us understand the physics that occurs at microscopic scales as these rocks deform.”
A rock in a difficult place
In their experiments, the team tested cylinders of Carrara marble.
“It’s the same material that Michelangelo’s David was made from,” Beach notes. “It’s a well-characterized material, and we know exactly what it should do.”
The team placed each marble cylinder in a vice-like device made of aluminum, zirconium and steel pistons, which together can generate extreme pressures. They placed the vice in a pressurized chamber, then subjected each cylinder to pressures similar to those experienced by rocks throughout the Earth’s crust.
As they slowly crushed each rock, the team sent pulses of ultrasound across the top of the sample, recording the sound pattern that emerged from the bottom. When the sensors weren’t pulsing, they listened for any naturally occurring acoustic emissions.
They found that at the lower end of the pressure range, where rocks are brittle, the marble actually formed sudden fractures in response, and the sound waves resembled large low-frequency spikes. At the highest pressures, where the rocks are softer, the sound waves resembled a louder crackle. The team believes this crackling is caused by microscopic faults called turbulence that then spread and flow like an avalanche.
“For the first time, we have recorded the ‘sounds’ that rocks make when they deform through this transition from brittle to ductile, and we have linked these sounds to the individual microscopic defects they cause,” Beach says. “We found that these defects change their size and propagation speed dramatically as they go through this transition. It’s more complicated than people thought.”
The team’s characterizations of rocks and their faults at different pressures can help scientists estimate how the Earth’s crust behaves at different depths, such as how rocks fracture in an earthquake, or flow in a volcanic eruption.
“When rocks partly break and partly flow, how is that reflected in the earthquake cycle? And how does that affect the movement of magma through a network of rocks? These are broad questions that can be addressed with research like this,” Beach says.
Reference: “Dynamics of microstructural defects during the brittle-to-ductile transition” by Hoji Ogavari, Matej Piech, Tushar Mittal, Ulrich Mock, Hilary Zhang and Brian Evans, 9 October 2023, Proceedings of the National Academy of Sciences.
This research was supported in part by the National Science Foundation.
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