What triggers rockfalls? Yosemite study shows it’s about the heat
When rocks get hot, they do what people do: shed a few layers.
That cautionary advice comes from geologists who are measuring the seasonal expansion and contraction of granite cliffs in the Sierra Nevada – showing, in essence, how mountains become molehills.
And, if you happen to be underneath, flatten you.
“Cliffs move in and out, and detach,” said Menlo Park-based U.S. Geological Survey scientist Brian Collins, who uses instrumentation to measure restless rocks at Twain Harte reservoir and Yosemite National Park.
“People look at landscape as static, that it will be there forever,” he said. “But it’s changing all the time.” Think James Franco in “127 Hours” – caught between a rock and a painfully hard place.
Collins and U.S. Park Service geologist Greg Stock looked at the pattern of 925 past rockfalls in Yosemite with no known cause, and found about 15 percent occurred in the hottest hours of day, from noon to 6 p.m., between July and September. That’s more than double the expected rate; if the rockfalls were random, only six percent would have happened during those hours.
Seasonal swings in temperature are also blamed for dangerous cracking in a restless and beautiful dome of granite adjacent to the concrete and metal dam at Twain Harte Lake in the Sierra Nevada foothills, requiring $900,000 in repairs, according to researcher Faye Lynn Moser of the University of North Caroline at Charlotte.
The Sierra Nevada is not the only place where granite domes are sloughing off layers. Other domes include such famed sites as Diamond Rock in South Africa, Rio de Janeiro’s Corcovado Mountain and Stone Mountain, Georgia.
However, what’s going on here is different than the rockfalls recently in southern Big Sur. California’s Coast Range is made of soft sedimentary rock and soil, which becomes saturated and then slides after prolonged winter rains. In contrast, the granite of the Sierra is brittle igneous rock, which cracks when it expands and contracts.
How much do the slabs crack and move? Collins and Stock roped up and climbed a slab of unstable rock – 60 feet tall, 12 feet wide and 4 inches thick – in Yosemite’s Royal Arches, behind the Majestic Hotel, formerly known as the Ahwahnee Hotel.
In the heat of a summer afternoon, the researchers found, an unstable slab in Yosemite bulges and moves off a cliff about 8 millimeters, almost one-third of an inch.
On cool nights, the slab reverses itself, shifting back about seven millimeters.
This means it moves 1 millimeter a year – and, over time, eventually breaks off and crashes into a giant pile of rubble. Their 2016 findings are published in the journal Nature Geosciences.
In human time, that might seem like no big deal. To be sure, it poses a far lesser risk than, say, climbing over guard rails to swim above Vernal Falls or tailgating down Route 120’s Priest Grade Road in a rush to get home.
But this process, called “exfoliation,” is catastrophic if a boulder lands on your head.
“If I hear a big boom, I don’t assume that it’s far away,” Collins said. “I look up to see where it’s coming from.”
Since the glaciers retreated around 15,000 years ago, rock fall has been the major force of change in the Sierra Nevada. While dramatic, rockfalls rarely kill; only about 15 people have died from rocks in Yosemite, for instance, over the past 150 years. The National Park Service has taken steps to reduce deaths still further, permanently closing the most vulnerable cabins in Curry Village.
The Collins-led team measures rock movement by climbing up into deep cracks and installing a long thin wire, which measures strain. (The wire vibrates at different frequencies as it loosens and tightens.) A device records data every five minutes – tracking movement as tiny as 0.001 centimeter.
New research led by UNC’s Moser is now focused on a granite dome of Twain Harte Lake in the Sierra Nevada foothills, as well as 14 other nearby domes.
It has become a sensation with the geo-curious, who come in hopes of spotting what’s called “active exfoliation” – bursting shards of rock.