Scientists believe that climate change may be the culprit behind the mysterious rockfalls in Yosemite.
The occurrence of spontaneous rockfalls often does not have a specific cause. From time to time, slabs of rocks can suddenly fall down. However, scientists did a close monitoring of a granitic cliff and noted that cyclical changes in temperature causes the hard rocks to accumulate damage until the rocks crack.
In the Yosemite National Park in California, rockfalls seems to be ordinary with about hundreds occurring periodically. About 15 percent of these rockfalls do not have any trigger, such as earthquakes or freeze-thaw cycles that can trap water in a fissure that can in turn cause a crack.
The Brian Collins, a geological engineer from the U.S. Geological Survey (USGS) and a mountain climber, worked with Yosemite National geologist Greg Stock to identify the cause of the frequent rockfalls.
The experts installed strain gauges and crackmeters to measure the overall length changes at three spots present in a 19-meter (62-feet) long and 4-meter (13-feet) long slab. The slab has its top and bottom edges barely attached to a south facing cliff.
From May 2010 to October 2013, the scientists identified and measured, every 5 minutes, the deformations present in the near vertical 20 metric ton (293.49 cubic feet) layer of granite slab, which is about 10 centimeters (0.3 inch) thick.
The scientists also monitored the slab movements along the directions it was splitting. Weather conditions including sunlight intensity and air conditions on site were also taken into consideration, even getting the air temperature and humidity from the slab surface and slab gaps that are only about 4.7 inches wide. To gain an independent measurement of the rock motions in an 18-hour period, the experts scanned the slabs from 30 meters away (98 feet) using a laser device.
The study found that in an average day, the slab would have periodic bulging and shrinking of about 8 millimeters (0.31 inch), which is largely due to the temperature variations. During the highest temperature of the day, between 1 p.m. and 4 p.m., the maximum bulges were noted. On the other hand, minimum bulges occurred in the morning (7 a.m. to 9 a.m.) where temperature is at its lowest. The range of deformations was also noted to be greatly affected by temperature changes as biggest deformations would occur during spring and fall.
Collins said the material expands as the material heats up but because it has edges that are still attached to a rock, the slab would only bulge and shrink. The periodic bulging causes cracks and fissure at the top and bottom of the slab to open. These open cracks generate stresses, grow and eventually cause rockfalls.
"Over time, the cracks are going to become bigger and bigger and bigger and ultimately result in a rockfall," Collins said. He added that intensifying changes in temperature brought about by climate change may aggravate the process.
Collins and Stock reported that about 15 percent of the national park's rockfalls, which may be linked to thermal stresses, happen during the warmest time of the day (from noon time to 6 p.m.) and during the hottest months (July to September) of the year. The scientists surmised that if the event happened at random, the number of rockfalls at those times would only amount to about 6 percent.
Geologists believe the study is an eye opener that offers new information about rocky landscapes. Jeffrey Moore, a University of Utah geologist, said daily temperature variation can also cause similar stress in layered sandstones that could also lead to cracking and trigger rockfalls.
Stephen Martel, a geologist from the University of Hawaii, also said thermal stresses, previously ignored, gave them a different perspective. He noted that studies about these phenomena are important as they could help in disaster management.
The scientists of the new study said their research cannot help in rockfall predication, but it does give an understanding of how such events can occur. The study does not only offer hazard assessment for Yosemite alone, but also in other rock formations around the world, as climate change persists.
The study was published in Nature Geoscience.
Due to Yosemite's steep, glacier-carved cliffs, rockfalls are quite common. In the past 150 years, the park had about 1,000 rockfalls. In 2015 alone, the park had 66 documented rockfalls that had about 8,700 cubic meters of rocks.
Yosemite National Park is a favorite among rock climbers. In 2014, two rock climbers braved the Dawn Wall of the El Capitan mountain, which is 3,000 feet above the park using only their bare hands and feet. The El Capitan mountain was previously tagged as the greatest cliff for rock climbing.
Photo: Amit Patel | Flickr
