Despite the towering walls of granite that rim the Yosemite Valley, the place is decidedly not static. Rather, it's a constantly evolving geologic entity that is both deadly and mesmerizing.
My first visit to Yosemite National Park back in June 1999 coincided with a rockfall that forced me out of a tent cabin in Curry Village and into one in Tuolumne Meadows. On my next trip to the valley, in October 2008, another rockfall thundered down on the village, this one severe enough that it led park officials to permanently close 233 tent cabins, cabins with bath, cabins without bath, or roughly one-third of the village's overnight capacity, due to the threat of more rock peeling off from the cliff that culminates in Glacier Point.
In March 2009, a rockfall from Ahwiyah Point near Half Dome was judged the largest recorded in the park in at least two decades.
With this recent history -- in 2009 alone there were 52 recorded rockfalls in the park -- there can be no doubt that rockfalls are frequent visitors to Yosemite's iconic valley. While the frequency of rockfalls in recent years might be alarming to park visitors, they are calling cards evidencing the active geology that's ongoing in Yosemite.
Here, courtesy of the staff at Yosemite, is a primer on rockfalls in the park:
What Is a Rock Fall?
Rock falls are a natural and dynamic geologic process. Due to its steep, glacier-carved cliffs, Yosemite Valley experiences many rock falls each year. Historical records indicate that at least 600 rock falls have occurred in the park during the past 150 years. Massive piles of “talus” or rock debris at the base of Yosemite Valley’s cliffs are reminders of these dramatic events. Natural processes like rock fall help to create the beautiful and changing scenery in Yosemite National Park, but they also present potential hazards.
What Causes Rock Fall?
A number of geologic processes set the stage for rock fall. Glaciation, weathering, and bedrock fractures all play a role in causing rockfall. Tectonic stresses and erosion cause granite rock to fracture. Rock falls later occur along these fractures. Fractures that develop parallel to the surface are called sheet joints. Sheet joints create large slabs of rock that ultimately fall away in a process known as exfoliation. In Yosemite Valley, Royal Arches, and the face of Half Dome are landforms that have resulted from this process. Over long periods, water flowing through fractures weathers the bedrock, loosening bonds that hold rocks in place. This is called weathering.
Triggering mechanisms like water, ice, earthquakes, and vegetation create the final forces that cause unstable rocks to fall. If water enters fractures in the bedrock, it lubricates surfaces and can build up pressure behind unstable rocks. Water also may seep into fractures in the rock and freeze, causing the fractures in the rock to expand. This process is called “frost wedging” or “freeze-thaw” and can incrementally lever loose rocks away from cliff faces. Ground shaking during earthquakes can also trigger rock falls. Additionally, a variety of vegetation—most notably firs, pines, and canyon live oaks—grow into the sheer rock faces where their roots expand and pry apart joints in the granite. There is still uncertainty about exactly what triggers rock fall; historical records indicate that more than half of all documented rock falls were not associated with a recognizable trigger.
Most rock falls in Yosemite occur in the winter and early spring, during periods of intense rainfall, snow melt, and/or subfreezing temperatures, but large rock falls have also occurred during periods of warm, stable weather.
In 2009, there were 52 documented rock falls in Yosemite, with an approximate cumulative volume of 48,120 cubic meters (142,000 tons). The vast majority of this volume was associated with rock falls occurring in March from Ahwiyah Point near Half Dome. The largest of these had an approximate volume of 45,300 cubic meters (about 134,000 tons), making it the largest rock fall in Yosemite Valley in 22 years. The impact of the falling rock generated ground-shaking similar to a magnitude 2.5 earthquake. Other notable 2009 rock falls occurred in August and September from the Rhombus Wall immediately north of the Ahwahnee Hotel, for a cumulative volume of about 1,200 cubic meters (roughly 3,600 tons).
Predicting actual rock-fall events is not yet possible, but understanding the forces that trigger rock falls is an important step toward this goal.
How Does the Park Address Rock Fall?
The National Park Service is responding to rock fall in a variety of ways. Park scientists, in collaboration with the U.S. Geological Survey (USGS) and academic researchers, are actively studying rock fall through the use of new technology, such as high resolution digital photography and laser mapping of cliffs. These tools offer vast improvement in resolution from previously available data, allowing geologists to accurately map rock-fall zones and to study rock-fall source areas. The park is also actively investigating possible methods for monitoring rock-fall activity. Additionally, new computer modeling technology shows some promise in simulating future rock-fall behavior. Park planning incorporates potential rock-fall hazards, and park rangers have developed emergency plans for rock-fall events. Park managers may also close trails and post warning signs in particularly hazardous areas.
What Should I Do in the Event of a Rock Fall?
* Be aware of your surroundings. Rock-fall hazard zones occur throughout the park near any cliff faces. If you witness a rock fall from the Valley floor, quickly move away from the cliff toward the center of the Valley. If you are near the base of a cliff or talus slope when a rock fall occurs above, immediately seek shelter behind the largest nearby boulder. After rocks have stopped falling, move quickly away from the cliff toward the center of the Valley. Be aware that rock falls are inherently unpredictable and may happen at any time. Pay attention to warning signs, stay off of closed trails, and, if unsure, keep away from the cliffs.
* Inform park staff if you witness a rock fall. If you witness or hear a rock fall of any size, please report it by calling 209/379-1420 or reporting it at one of the park Visitor Centers. This information is useful for assessing rock-fall hazards and adds to the growing knowledge base of rock-fall activity in the park.
* Understand this dynamic natural process. Remember that Yosemite is a wild place. Rock fall is the most powerful geologic agent acting today in Yosemite. The dramatic cliffs of Yosemite are constantly being shaped by this potent natural force.
Learn More about Yosemite National Park's Geology
* Geologists use a 50-foot-wide photo from the Yosemite Panoramic Imaging Project to monitor and analyze rock-fall events.
* Get the details concerning the Ahwiyah Point rock fall on March 28, 2009, near Half Dome in which rocks fell roughly 1,800 feet, knocking down hundreds of trees and burying hundreds of feet of trail on the southern portion of the Mirror Lake Loop Trail. This rock fall is the largest one in Yosemite National Park since the 1987 Middle Brother event.
* Learn about the October 2008 Glacier Point rock fall. An analysis has shown that approximately 6,000 cubic meters of rock were involved in the events.
* Follow a geologic overview of Yosemite National Park
* USGS description and analysis of the 1996 Happy Isles rock fall
* USGS description and analysis of the 1998-1999 Curry Village rock falls
* Rock-fall potential in Yosemite Valley in a USGS article
* Rock-fall hazards in Yosemite Valley in a USGS article
* Rock-fall hazard and risk assessment in Yosemite Valley in a USGS article
* Database of historic rock falls in Yosemite National Park, according to the USGS
* USGS description and analysis of the 2003 and 2007 Staircase Falls rock falls
* "Investigating the El Capitan Rock Avalanche" by Greg Stock in Yosemite magazine 70(4), Fall 2008, Yosemite Association. [585 kb PDF]
* The Geologic Story of Yosemite National Park by Norman King Huber, 1987, U.S. Government Printing Office, Washington, D.C., U.S. Geological Survey Bulletin 1595.