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Earth Notes: Vibes at Rainbow Bridge

NPS/B. Lee

Rainbow Bridge in southeast Utah is one of the largest natural bridges in the world. And though the immense sandstone span looks solid and unmoving, scientists are finding out otherwise. 


The rock of Rainbow Bridge expands slightly when warmed, and contracts when it cools. When fierce winds blow, it sways ever so slightly. These slow, subtle vibrations strum Rainbow Bridge — and other stone arches — like giant guitar strings.  But that “music” is too low for the human ear to detect unaided.

University of Utah scientist Jeff Moore and his team are measuring these vibrational frequencies to determine the stability of Rainbow Bridge and other natural arches. Using an instrument the size of a coffee cup, they record the frequencies.  Later, scientists can take another set of measurements for comparison. A major change in frequency could mean an arch is weakening or developing a dangerous crack.

Moore also wonders if vibrations from overflying aircraft might cause damage. To find out, he compared the frequencies of the pressure waves produced by helicopter rotors to the natural frequencies measured at the bridge. At lower frequencies, there was no overlap. But, sophisticated computer models do show overlaps at higher frequencies that could potentially harm Rainbow Bridge if a helicopter flies too close.   

The monitoring will continue, and the National Park Service is interested in the results.

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