It's what you might call very old-school rock music. No, it's not the Rolling Stones — in the deserts of the Southwest it's the rocks themselves that are making timeless ballads.
Wander through places like Arches National Park or Grand-Escalante National Monument and you'll find rock arches of all shapes and sizes. Turns out, those ancient wonders aren't static sculptures; they vibrate and shift throughout the day, creating a variety of sounds as they stretch their aging, eroding "bones," moving up and down, twisting and settling, resonating in a manner that's similar to a guitar string.
The wind makes them vibrate at certain frequencies. So too do distant earthquakes, passing vehicles, aircraft, even the ocean waves from hundreds of miles away. By comparing readings of a specific arch over time, scientists may detect a change in the arch's resonant frequency, something that can indicate cracks or other shifts in the rock's stability. (You can hear a recording in the tweet link below.)
Led by University of Utah geologist Jeff Moore, researchers from the University of Utah place one broadband seismometer on an arch, and another about 328 feet (100 meters) away, on relatively flat, unmoving terrain. By comparing the readings over time, they can isolate and identify resonance from the arch.
"We are currently monitoring about 20 arches in total, some we measure frequently, some rarely, and at one site we monitored ambient vibration and spectral properties continuously over more than one year," says Moore in an email interview.
In researching hundreds of arches in three national parks and two national monuments, Moore's team sometimes uses up to 36 nodal seismometers at one location, all the better to measure the ambient resonance of structures like Musselman Arch. They've learned that each arch moves just a little differently, resonating in response to the environment around them.
Humans can't feel or see the vibrations. But with the right equipment and a bit of audio engineering magic, though, the rock "songs" come to life. The biggest arches create a deep, sinuous rumble; smaller arches are sometimes a little on the squeaky side.
"We've quite enjoyed creating new partnerships with artists sharing vibration recordings as a new medium to experience red rock arches as dynamic natural features," says Moore.
In addition to seismometers, researchers sometimes use tiltmeters and thermometers to track the arches. At high noon, for example, scorching desert heat causes thermal expansion in the rock, imperceptible swelling and tilting; as it cools at night, those joints relax again, sagging in on themselves just like the timbers in an old house.
Because all rock arches have finite lives, and because the Trump administration recently removed about 2 million acres (809,3715 hectares) of protected areas of Bears Ears and Grand-Escalante National Monuments, Moore's team set out to create a digital archive in hopes of preserving the arches should they be affected by any sort of commercial development. The archive includes more than 115 arches, full of pictures, a few interactive 3-D models, and of course, audio recordings for some of the arches.