Rocks are unmoving. They are solid and stoic, massive and stolid; forever dependable as an essential part of the Earth’s crust. Promontories and peaks, cliffs and crags, are all rocks. Even the very word rock implies a brand of strength, sturdiness and stability, unchanging over the millennia.
That’s the conventional view, anyway.
But rocks do wiggle. And a 400-foot-tall rock in Utah is wobbling ever so slightly. Castleton Tower, a red-rock formation that is about 265 miles northeast of Bryce Canyon National Park, rises from the desert like a turret. It’s also pulsating at about the rate of a human heartbeat—and we can hear it.
Castleton Tower is vibrating constantly but mostly imperceptibly, swayed by the energy produced by aviation noise, cities, ocean waves, road traffic, trains, wind and even tiny earthquakes that rumble in the distance. And now, for the first time, scientists have measured Castleton’s beating pulse and have recorded its “voice.”
A study in stoicism
One of the world’s largest freestanding rock towers, Castleton is a spire of Wingate Sandstone, located in Utah’s Castle Valley. The formation dates back to the late Triassic Period, around 200 million years ago. First climbed in 1961, Castleton Tower became a widely renowned destination after appearing as one of two Utah sites in the 1979 book Fifty Classic Climbs of North America. The stoic power in its appearance continues to draw rock climbers and nature photographers today.
Jeffrey Moore, a geologist in the Department of Geology and Geophysics at the University of Utah, studies the vibrations of rock structures, including arches and bridges, to understand what natural forces act on them. He also measures the resonance of rocks, or the way the structures amplify the energy that passes through them.
Moore and his colleagues had always been curious about the vibrations of Castleton Tower, but the structure is only scalable by skilled climbers. So, when two professional climbers, Kathryn Vollinger and Natan Richman, volunteered their time, the scientists jumped at the chance to train the climbers to unpack and use seismometers, instruments that can measure miniscule movements in three dimensions.
To get the needed data, the climbers trekked to the base of the tower, carrying two, hefty seismometers in a protective box that was about the size of a suitcase. There, they placed one of the seismometers to serve as a reference. They then carried the other to the top of Castleton Tower and ran measurements for three hours before returning both instruments to the research team.
The seismometers picked up two primary frequencies, 0.8 and 1.0 hertz, which means the tower sways around once per second—roughly the same frequency as a human heartbeat. While the sound has ebbs and flows to it, it largely makes a droning, humming noise, meaning that the tower is always vibrating as energy comes up through the Earth.
In a paper that was recently published in the Bulletin of the Seismological Society of America, the researchers stated that the recording confirmed what they had originally thought: that Castleton Tower behaves as one slab of intact rock, connected from top to bottom. Smaller rock formations tend to vibrate at higher frequencies, but Castleton vibrates at a very low one, probably due to its enormity. This makes Castleton less sensitive to accumulating damage over time compared to structures that are more susceptible to transferred energy.
Luckily, that means that Castleton Tower is relatively stable, unlike some arches and hoodoos. For example, in 2008, the well-known Wall Arch—the 12th largest arch in Arches National Park—collapsed. So, being able to listen to the sounds of rocks is a way to noninvasively assess the health of such features in order to identify any precursors before there’s a dangerous rockfall.
In order to make the three-hour recording of Castleton Tower audible to humans, Moore’s team amplified and sped up the low-frequency seismic data—allowing you to listen to the voice of a rock.
An exercise in erosion
The research team is still collecting baseline measurements about rock movements to see if such repeated data can help them assess damage to the structures and how vibrational energy—both from natural and human sources—may impact the integrity of formations such as Castleton Tower over time. While some of the pressures that humans create might appear minor, we may find out that the long-term effects are something quite different.
Of course, on a geological time scale of millions of years, arches and rock towers eventually crumble from the twin, natural forces of erosion and gravity. But I think now being able to “hear” the red rocks of Utah could make us see the Earth in a new light; that even the parts of it that we think are immutable—such as mountains and rocks—are dynamic, energetic and fragile, subtly responding to changes in their surrounding environments.
Put another way, in the words of one of my favorite singer/songwriters, Beth Nielsen Chapman, in her song titled “Sand and Water”:
Solid stone is just sand and water, baby;
Sand and water, and a million years gone by.
Here’s to finding your true places and natural habitats,