At about the same rate that your heart beats, a Utah rock formation called Castleton Tower gently vibrates, keeping time and keeping watch over the sandstone desert. Swaying like a skyscraper, the red rock tower taps into the deep vibrations in the earth—wind, waves and far-off earthquakes.
New research from University of Utah geologists details the natural vibration of the tower, measured with the help of two skilled rock climbers. Understanding how this and other natural rock forms vibrate, they say, helps us keep an eye (or ear) on their structural health and helps us understand how human-made vibrations affect seemingly unmovable rocks. The results are published in the Bulletin of the Seismological Society of America.
“We often view such grand and prominent landforms as permanent features of our landscape, when in reality, they are continuously moving and evolving,” says Riley Finnegan, a graduate student and co-author on the paper.
“A stoic power”
Castleton Tower is a spire of Wingate Sandstone nearly 400 feet (120 m) tall that stands over Utah’s Castle Valley. First climbed in 1961, Castleton Tower became a widely renowned classic destination after appearing as one of two Utah climbs in the 1979 book “Fifty Classic Climbs of North America.” It’s one of the largest freestanding rock towers.
“Most people are in awe of its static stability, in its dramatic freestanding nature perched at the end of a ridge overlooking Castle Valley,” says geologist Jeff Moore, who led the study. “It has a kind of stoic power in its appearance.”
Moore and his colleagues study the vibrations of rock structures, including arches and bridges, to understand what natural forces act on these structures. They also measure the rocks’ resonance, or the way the structures amplify the energy that passes through them. Sources of this energy can be as local as wind gusts or traffic on a nearby road or as distant as far-off earthquakes and even ocean waves. “Because nothing is truly static, there is always energy propagating throughout the earth, which serves as a constant vibration source for the rock,” Finnegan says.
Moore, Finnegan and graduate student Paul Geimer have been developing and refining their methods of measuring rock structures as they’ve surveyed arches, bridges and hoodoos, which are small spire-like formations—towers on a smaller scale. They use seismometers to measure even the slightest movement in three dimensions. For some of their measurements, they’ve sped up the low-frequency seismic data into audible sound—allowing you to listen to the voice of a rock.
Hear Castleton Tower: