If one were to embark on a journey through Earth’s crust, they might discern a symphony of booms and crackles resonating through the rocks. These fissures, pores, and imperfections within the stone are akin to strings that vibrate when subjected to pressure and strain. In a fascinating exploration led by a team of geologists from MIT, it has been revealed that the tempo and pitch of these geological sounds can provide valuable insights into the depth and resilience of the surrounding rocks.
Matěj Peč, a geologist from MIT, explains, “If you were to eavesdrop on these rocks, you’d notice that their tunes ascend in pitch the deeper you venture.”
Peč and his fellow researchers are, indeed, eavesdropping on rocks, endeavoring to unveil acoustic patterns or “fingerprints” that emerge under varying levels of pressure. In laboratory experiments, they have demonstrated that samples of marble emit deep, resonant “booms” when subjected to low pressures. In contrast, higher pressures produce a cascade of higher-pitched crackles within the rocks.
These acoustic signatures in rocks can serve as a valuable tool for scientists to infer the types of cracks, fissures, and imperfections present within the Earth’s crust at different depths. This knowledge, in turn, can be employed to identify regions beneath the surface that are potentially unstable, holding the risk of earthquakes or volcanic eruptions. The results of the MIT team’s research, recently published in the Proceedings of the National Academy of Sciences, could also prove beneficial in guiding efforts to extract renewable geothermal energy through drilling.
Matěj Peč elaborates, “If we aspire to harness these incredibly hot geothermal resources, we must learn how to drill into rocks that exhibit this mixed-mode behavior—where they possess both brittleness and some degree of malleability.” He adds that, fundamentally, this research contributes to a better understanding of the Earth’s lithosphere’s strength.
The team at MIT working alongside Matěj Peč includes lead author and research scientist Hoagy O. Ghaffari, technical associate Ulrich Mok, graduate student Hilary Chang, and geophysics professor emeritus Brian Evans. Co-author Tushar Mittal, formerly a postdoc at EAPS, now serves as an assistant professor at Penn State University.
Fracture and flow
The Earth’s crust is often likened to the skin of an apple, characterized by its relatively shallow depth, which can reach up to 70 kilometers—an insignificant fraction compared to the planet’s immense 12,700-kilometer diameter. Nevertheless, the rocks comprising this thin outer layer exhibit significant disparities in strength and stability. Geologists deduce that surface rocks tend to be brittle and prone to fracturing, while those at greater depths face extreme pressures and heat from the Earth’s core, causing them to flow like a viscous substance.
The intriguing aspect lies in the transition between surface brittleness and deep ductility—a phenomenon known as the “brittle-to-ductile transition.” This transitional phase, where rocks display characteristics of both brittleness, akin to granite, and flowability, like honey, is not fully understood. Geologists believe that this transitional region within the Earth’s crust may harbor the peak strength of lithosphere, making it a critical area where major earthquakes may originate.
Matěj Peč and his team are actively investigating how the strength and stability of rocks, whether they lean towards brittleness, ductility, or an intermediate state, are influenced by microscopic defects within them. Factors such as the size, density, and distribution of tiny flaws like microscopic cracks, fissures, and pores can dictate a rock’s brittleness or ductility.
Nonetheless, assessing these minuscule defects within rocks under conditions mimicking the Earth’s varying pressures and depths poses a formidable challenge. Traditional visual-imaging techniques are inadequate for peering inside rocks to map their microscopic imperfections. As a solution, the team has turned to ultrasound technology, capitalizing on the notion that sound waves traveling through rocks interact with and bounce off microscopic cracks and crevices in distinctive ways. These interactions offer valuable insights into the pattern of these defects.
The intriguing twist is that these microscopic imperfections within rocks generate their own sounds when subjected to stress. Therefore, by actively transmitting and receiving ultrasound waves through the rock, scientists can glean a wealth of information. This innovative ultrasound approach operates at megahertz frequencies, allowing researchers to explore the physics unfolding at microscopic scales during the deformation of rocks.
In essence, this ultrasound method parallels the work of seismologists, but it delves into much higher frequencies, shedding light on the intricate physics occurring during the deformation of these geological formations.
A rock in a hard place
In their experiments, the research team worked with cylindrical samples of Carrara marble, a material famously used for Michelangelo’s masterpiece, David. As Matěj Peč points out, “It’s a material with well-established characteristics, offering us a clear benchmark.”
To subject these marble cylinders to extreme stresses akin to those found within the Earth’s crust, the team ingeniously constructed a vice-like apparatus. This apparatus was composed of pistons crafted from aluminum, zirconium, and steel, which, when combined, could generate formidable pressures. They positioned the vice within a pressurized chamber and gradually applied pressures similar to those encountered by rocks beneath the Earth’s surface.
Throughout the process of slowly compressing each marble sample, the team employed ultrasound pulses, sending them through the top of the cylinder and meticulously recording the resulting acoustic patterns as they exited from the bottom. In addition to these controlled ultrasound pulses, the researchers also listened for any spontaneous acoustic emissions emitted by the rocks.
Their findings unveiled a remarkable acoustic symphony. At the lower end of the pressure spectrum, where rocks exhibit brittleness, the marble indeed underwent sudden fractures, generating sound waves reminiscent of deep, low-frequency booms. In contrast, at higher pressures where rocks display greater ductility, the acoustic waves transformed into a higher-pitched crackling. This crackling was attributed to microscopic defects known as dislocations that propagated and flowed like an avalanche.
Matěj Peč highlights this groundbreaking discovery: “For the first time, we have captured the ‘sounds’ that rocks emit during deformation as they transition from brittleness to ductility, and we’ve connected these sounds to the specific microscopic flaws responsible for them. What we found is that these defects undergo significant changes in size and propagation velocity as they cross this transitional phase. It’s a much more complex process than previously thought.”
These meticulously characterized rocks and their defects at various pressure levels hold the potential to enhance our understanding of how the Earth’s crust behaves at varying depths. This knowledge can inform predictions about rock fracturing during earthquakes or flow patterns during volcanic eruptions.
Matěj Peč poses broader questions, stating, “When rocks are caught in this state of partial fracturing and partial flow, how does it impact the earthquake cycle? And how does it influence the movement of magma through a network of rocks? These are larger-scale inquiries that can be addressed through research like ours.”