What Is a Rockfall?
Rockfalls are a natural and dynamic geologic process involving the detachment and rapid downard movement of rock. Due to its steep, glacier-carved cliffs, Yosemite Valley experiences many rockfalls each year. Historical records indicate that more than 1,000 rockfalls 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 rockfall help to create the beautiful and changing scenery in Yosemite National Park, but they also present potential hazards.
Photo by Amanda Nolan
What Causes Rockfall?
A number of geologic processes set the stage for rockfalls, including glaciation, weathering, and bedrock fractures. Tectonic stresses and erosion cause granite rock to fracture. Rockfalls later occur along these fractures. Fractures that develop parallel to the surface are called sheeting joints. Sheeting 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 examples of landforms that have resulted from this process. Over long periods, water flowing through fractures decomposes the bedrock in a process called weathering. Weathering loosens bonds that hold rocks in place.
Triggering mechanisms like water, ice, earthquakes, and vegetation growth are among the final forces that cause unstable rocks to fall. If water enters fractures in the bedrock, it can build up pressure behind unstable rocks. Water also may seep into cracks in the rock and freeze, causing those cracks to grow. This process is called "frost wedging" or "freeze-thaw" and can incrementally lever loose rocks away from cliff faces. Recent research suggests that daily temperature variations and extreme heat can also cause rock slabs to become unstable. Ground shaking during earthquakes often triggers rockfalls. 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. Most rockfalls in Yosemite occur in the winter and early spring, during periods of intense rainfall, snow melt, and/or subfreezing temperatures, but many large rockfalls have also occurred during periods of warm, stable weather.
For any given rockfall, there is always a large degree of uncertainty about what exactly triggered it; historical records indicate that more than half of all documented rockfalls in Yosemite were not associated with a recognizable trigger.
Predicting actual rockfall events is not yet possible, but understanding the forces that trigger rockfalls is an important step toward this goal.
How Does the Park Address Rockfall?
The National Park Service in Yosemite is responding to rockfall in a variety of ways. Park scientists, in collaboration with the U.S. Geological Survey (USGS) and academic researchers, are actively studying rockfalls through the use of new technology, such as high resolution digital photography, laser mapping of cliffs, and exposure dating of boulders. These tools offer vast improvement in resolution from previously available data, allowing geologists to accurately map rockfall zones and to study rockfall source areas. Additionally, new computer modeling technology shows promise in simulating future rockfall behavior. The park is also actively investigating methods for monitoring rockfall activity.
The National Park Service has produced a report quantifying rockfall hazard and risk in Yosemite Valley to help improve safety and guide future park-planning efforts: Quantitative rock-fall hazard and risk assessment for Yosemite Valley, Yosemite National Park, California. Based on the results of this study, the National Park Service reduced rockfall-related risk in Yosemite Valley by removing or repurposing high-risk buildings within hazardous areas. These actions reduced rockfall-related risk by 95%.
Additionally, park rangers have developed emergency plans for rockfall events, and may close trails and post warning signs in particularly hazardous areas.
What Should I Do in the Event of a Rockfall?
Photo by Tom Evans
Yosemite Rockfall Year in Review: 2014Despite near-record dry conditions, 2014 was an active year for rockfalls in Yosemite. In all, there were 77 documented rockfalls, which is considerably more than the recent (2006-2013) average of about 45 rockfalls per year. Most of the rockfalls in 2014 were relatively small, with a total volume of about 6,500 cubic meters (about 19,350 tons).
The most consequential rockfall of 2014 occurred at 8:15 pm on June 29, when a rock slab of about 215 cubic meters (about 640 tons) fell from the east wall of Indian Canyon. Although nobody was in this remote area at the time, a segment of the communications cable running up Indian Canyon was destroyed, cutting off all phone communications to White Wolf and Tuolumne Meadows. The cable was replaced by a microwave transmitter.
Perhaps the most important rockfall of 2014 was also one of the smallest. At 3:22 am on the morning of February 11, a rockfall of approximately 15 cubic meters (about 45 tons) occurred in the vicinity of Staircase Falls above Curry Village. Previous rockfalls in 2003 and 2007 damaged cabins, caused injuries, and prompted evacuations. In contrast, the February 11, 2014 rockfall was hardly noticed because cabins there were removed in late 2013 following a comprehensive assessment of rockfall hazard and risk (http://pubs.usgs.gov/sir/2014/5129/). One boulder landed within the footprint of a former cabin. This event, described in a short publication, (http://onlinelibrary.wiley.com/doi/10.1002/2014EO290002/pdf [2.7 MB PDF]), demonstrated the merit of removing buildings from hazardous areas.
Other areas in Yosemite experiencing rockfalls in 2014 include Yosemite Falls, Royal Arches, LeConte Gully, and the Merced River Gorge; these latter rockfalls were related to the Dog Rock Fire in early October.
It is very likely that there were additional rockfalls in 2014, but these events either were not witnessed or went unreported. However, the significant increase in the number of small rockfalls in 2014 suggests more thorough reporting of rockfalls (and likely not a real increase in small rockfalls). If you witness a rockfall of any size, encounter fresh rock debris, or hear cracking or popping sounds emanating from the cliffs, please contact park geologist Greg Stock at 209/379-1420 or by email, or contact park dispatch by dialing 911 within the park. Documented rockfalls are added to the park database (http://pubs.usgs.gov/ds/746/), enabling evaluation of rockfall activity to improve public safety.
Learn More about Rockfalls in Yosemite
Historical Rockfalls and Case Studies