Earth Notes: Sandstone Voices
The sandstone pinnacles, arches and natural bridges of the Colorado Plateau may seem like static, silent parts of the landscape. But researchers at the University of Utah have tuned in to a very different reality.
Professor Jeff Moore and a team of scientists are applying methods borrowed from civil engineering to measure vibrational frequencies of sandstone structures. They’re using sensitive seismometers across the state of Utah, including at Arches National Park.
Their data reveals that these seemingly stationery rocks to be in constant motion. Each structure has a dominant set of resonant frequencies, generating a continuous hum, tapping into deep vibrations within the earth.
The base frequency and overtones are determined by each structure’s weight, rigidity and geometry – in effect, each arch, pinnacle and bridge has its own unique voice.
Larger structures vibrate at lower frequencies than smaller, thinner ones. The rocks also resonate in response to local energy sources like wind gusts and nearby traffic, as well as distant triggers like far-off earthquakes.
And if an arch or pinnacle cracks, or a piece falls off, that causes a change in the rock’s vibrational profile, which can be used to help monitor the structure’s integrity.
The Utah researchers have sped up some of these recordings by 25 times to make them audible to human ears. You can hear some of these hidden rock voices at http://geohazards.earth.utah.edu/tones.