Monumental rock arches that dot national parks across the Western United States are vibrating. The tremors are minute, but they can add up, eventually causing these formations to collapse, sending hundreds of tons of rocks crashing to the ground. In 2008, an arch collapsed unexpectedly along a popular hiking trail during peak tourist season in Utah’s Arches National Park. Fortunately, the event occurred at night when the park was empty, but the timing of future collapses might not be so lucky.
Jeff Moore is trying to predict these collapses before they happen. A geoscientist at the University of Utah in Salt Lake City, he’s using sensitive instruments to monitor changes in an arch’s vibrations over time.
Moore chatted with Science about what draws him to these erosion-sculpted features and the new technology he’s looking forward to using to survey particularly fragile arches. This interview has been edited for clarity and length.
Q: Why study rock arches? Are you a hiker?
A: Yes, I’ve hiked and climbed all over Arches National Park in Utah, home to over 2000 rock arches. When I lived abroad, I was always dreaming about the red rock landscapes back home. I find arches spectacular—they are such rare and elegant landforms. And they’re simply amazing from a mechanical standpoint. Here in Utah we’re fortunate to be located so close to some of the most iconic arches on the planet, like Rainbow Bridge in the southern part of the state, one of the world’s largest at over 80 meters long.
Q: You've shown that rock arches vibrate. Why do they move?
A: Wind, earthquakes, and even faraway ocean waves crashing on shorelines all cause rock arches to vibrate. Human activity like footsteps, car and truck traffic, and sounds from aircraft also cause arches to move. The Earth is constantly humming.
Q: How are you measuring these vibrations?
A: We use seismometers about the size of a coffee cup that record an arch’s movement in three dimensions. We place one of these instruments on an arch and record data for anywhere from 1 to 24 hours. When we’re finished we simply pick the seismometer up—we don't disturb the rock in any way. We return to the same arches year after year to obtain follow-up measurements and look for changes in vibration patterns, which might indicate that an arch is cracking and potentially more likely to collapse.
Q: How do you study really fragile arches you can't walk on?
A: Certain inaccessible arches simply can’t be measured by physically placing a sensor on the arch, and there are also cases where we wouldn’t dare set foot on an arch for fear of causing additional damage. So we’re currently exploring ways to measure vibrations remotely. One of the most promising methods is laser vibrometry, which involves bouncing a laser beam off the rock to measure very small movements.
Q: What arches are you studying right now?
A: We’re looking at Landscape Arch and Mesa Arch in Arches and Canyonlands national parks, both in Utah. We’re also studying a thin and flat natural bridge—a weak geometry for an arch—in Bryce Canyon in southwestern Utah, and the incomparable Rainbow Bridge. We additionally study less well-known features, including several large and beautiful arches in the area around Moab, Utah. In total, we’re currently monitoring 11 arches and looking to add several new sites this year.
Q: Does your research change where you hike from a safety perspective?
A: No, but it has changed how I think about natural arches and how I approach them as a hiker. I am more than ever struck by their fragility and our sheer luck to observe many of them so beautifully intact.