The cracks and pores throughout the rock, from the Earth's crust to the liquid mantle, are like channels and cavities through which sound can resonate. MIT scientists have discovered that the sound beneath our feet has a fairly stable fingerprint. If you could sink down into the earth's crust, just listen carefully and you'd hear roars and crackles along the way. Cracks, pores, and flaws throughout the rock are like strings that resonate when squeezed and stressed.
A team of MIT geologists has discovered that the rhythm and pace of these sounds can tell you the depth and strength of the surrounding rocks.
"If you're listening to the rocks, they sing at a higher and higher pitch, and the deeper you go, the higher their pitch becomes," said MIT geologist Matěj Peč.
Page and his colleagues are listening to the rocks to see if acoustic patterns, or "fingerprints," appear under different pressures. In laboratory studies, they found that marble samples emitted a low-pitched "roar" under low pressure, while under high pressure the rock emitted a high-pitched "avalanche" sound.
These acoustic patterns in rocks can help scientists estimate the types of cracks, fissures and other defects that appear in the Earth's crust as depth increases, Ezoic Peč said. They can then use these acoustic patterns to identify unstable areas below the surface where earthquakes or volcanic eruptions are likely to occur. The team's research results, published in the Proceedings of the National Academy of Sciences on October 9, will also help provide information for surveyors drilling for renewable geothermal energy.
Page is an assistant professor in MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS). Overall, he says, this is basic science that can help us understand where the lithosphere is strongest.
Page's collaborators at MIT include lead author and research scientist Hoagy O. Ghaffari, technical assistant Ulrich Mok, graduate student Hilary Chang and geophysics professor emeritus Brian Evans. Co-author Tushar Mittal, a former EAPS postdoc, is now an assistant professor at Pennsylvania State University.
Fracture and flow
People often compare the crust to the skin of an apple. At its thickest, the Earth's crust is 70 kilometers (45 miles) deep - only a fraction of the Earth's total diameter of 12,700 kilometers (7,900 miles). However, the rocks that make up Earth's thin crust vary greatly in strength and stability. Geologists theorize that rocks near the surface are brittle and break easily, as opposed to rocks deeper down where intense pressure and heat from the Earth's core can cause rocks to flow.
Rocks are brittle at the surface and more ductile at depth, which means there must be an intermediate stage in which the rock transitions from one to the other, possibly having both properties at the same time, breaking like granite and flowing like honey. This "brittle-to-ductile transition" is not well understood, but geologists believe it may be the strongest place in the Earth's crust.
"This transitional state of part flow, part fracture is important because we think this is where the peak of lithospheric strength is and where the largest earthquakes lie," Page said. "But we don't have a good handle on this mixed-mode behavior yet."
He and his colleagues are studying how a rock's strength and stability—whether brittle, ductile or somewhere in between—change based on the rock's microscopic imperfections. The size, density, and distribution of defects such as microscopic cracks, fissures, and pores can determine the brittleness or toughness of a rock.
But measuring microscopic defects in rocks under conditions that simulate various pressures and depths on Earth is no easy task. For example, there is currently no visual imaging technology that allows scientists to see inside rocks and map their microscopic defects. So the team turned to ultrasound, arguing that any sound waves traveling through rock will bounce, vibrate and reflect in any tiny cracks and crevices, revealing the morphology of those flaws in specific ways.
All of these imperfections also produce their own sounds as they are forced to move, so both actively detecting the rocks and listening to the sounds of the rocks can provide them with a wealth of information. They found that this idea should be possible in ultrasonic waves at megahertz frequencies.
"This ultrasonic method is similar to what seismologists do in nature, but at much higher frequencies. This helps us understand the physics that occur at the microscopic scale during the deformation of these rocks," Page explains.
"Carrara marble is the same material as Michelangelo's David. It's a material with very specific properties and we know exactly what it's supposed to do," Page noted.
The team placed each marble cylinder in a vise-like device made of aluminum, zirconium and steel pistons, materials that combine to create extreme stresses. They placed the pliers in a pressurized chamber and then subjected each cylinder to pressures similar to those experienced by rocks throughout the Earth's crust.
While slowly crushing each rock, the team sent ultrasonic pulses through the top of the sample and recorded the sound wave patterns coming out from the bottom. When the sensors are not pulsing, they are listening for any naturally occurring acoustic emissions.
They found that at the low end of the pressure range, where the rock is brittle, the marble does snap suddenly, with sound waves like a huge low-frequency roar. At the highest pressures where the rock is more malleable, the sound waves resemble a higher-pitched crackling sound. The team believes the crackling sound is produced by tiny defects called dislocations, which then spread and flow like an avalanche.
"For the first time, we documented the 'noise' that rocks make when they deform during their brittle-to-ductile transition and linked those noises to the individual microscopic defects that cause the noise," Page said. "We found that these defects change dramatically in size and propagation speed as they cross this transition. It's more complicated than people thought."
The team's characterization of the rocks and their flaws under different pressures can help scientists estimate how the Earth's crust behaves at different depths, such as how rocks might break during an earthquake or flow during an eruption.
"When parts of rock break and parts flow, how does that feed back into the seismic cycle? How does that affect the flow of magma through the rock network?" Page said. "These are larger-scale questions that could be addressed through similar studies."