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.
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?
Yosemite Rockfall Year in Review: 2018
Yosemite faced a number of challenging events in 2018, including floods and fires, but fortunately serious rockfalls were not among them. Forty-one events (including rockfalls, rockslides, and debris flows) were documented in 2018, below the recent (2006-2017) average of 53 events per year, and less than half of that documented in 2017. Rockfalls in 2018 had a cumulative volume of about 2,220 cubic meters (6,610 tons), also well below the recent average of 10,068 cubic meters (about 30,000 tons).
The largest rockfall of 2018 was not in Yosemite Valley, but instead occurred deep within the Grand Canyon of the Tuolumne, between Muir Gorge and Pate Valley. It likely occurred sometime in March with the first real winter storms, but it wasn’t reported until the trail was accessible in early summer. Approximately 1,500 cubic meters (4,465 tons) of rock slid off the north wall of the canyon, spreading out over the canyon bottom and burying the trail under large boulders and trees. Clearing the trail of this debris was a summertime priority for trail crews.
The most consequential rockfall of 2018 was a relatively small rockfall from the northwest face of Half Dome. On the morning of June 5th, a thin exfoliation slab of about 5 cubic meters (15 tons) fell from the east side of the face, breaking into many fragments that funneled down a steep gully containing the lower pitches of the Regular Northwest Face climbing route. The debris struck a climbing party on the third pitch, causing a neck laceration and possible dislocated shoulder. The climbers' helmets sustained significant damage, suggesting, once again, that wearing helmets probably saved lives. The injured climbers rappelled to the base of the route and were evacuated by helicopter.
Other substantial rockfalls in 2018 occurred at Parkline, El Capitan, Indian Canyon, the south face of Half Dome, and Quarter Domes.
It is very likely that there were additional rockfalls and rockslides in 2018, but these events either were not witnessed or went unreported. 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
Learn More about Rockfalls in Yosemite
Historical Rockfalls and Case Studies
Rockfall Hazard and Risk
Methods of Rockfall Characterization
Last updated: October 25, 2019