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: 2015
Despite a fourth consecutive year of drought, rockfall activity in 2015 was about average, with 66 documented events (rockfalls, rockslides, and debris flows). The cumulative volume of all events was about 8,700 cubic meters (roughly 25,000 tons).
Surprisingly, the largest and most notable rockfall of 2015 was not directly observed. On July 5, two rock climbers attempting the Northwest Face of Half Dome found themselves stymied by a new expanse of blank rock. Sometime in the previous days, a rock slab totaling some 1,800 cubic meters (about 5,200 tons) parted from the cliff in a classic case of exfoliation, taking with it two pitches of one of the world’s most famous climbing routes. Although the rockfall happened at the height of the summer tourist season—and also the Half Dome climbing season—the stormy weather that apparently triggered the rockfall ensured that no one was in the immediate vicinity to witness the event. Another rockfall from Half Dome on July 15 originated from a different location near “The Visor” and was apparently unrelated to the earlier event.
Other substantial rockfalls in 2015 occurred from Middle Brother west of Camp 4, Washington Column, Clouds Rest, Glacier Point, and the north wall of Hetch Hetchy Valley.
In a notable departure from past years, more than half (54%) of the cumulative volume for 2015 was related to debris flows triggered by intense rainstorms. In particular, two thunderstorms in July and October—the former a remnant of Hurricane Dolores—generated substantial runoff and debris from within the burned areas of the Dog Rock and El Portal fires. The El Portal Road was closed for three days as debris from the July event was cleared from the road. Although burned areas proved susceptible to debris flows, unburned areas also experienced large debris flows, indicating that localized weather plays the primary role in triggering these events.
It is very likely that there were additional rockfalls in 2015, 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, enabling long-term evaluation of rockfall activity to improve public safety.
Learn More about Rockfalls in Yosemite
Historical Rockfalls and Case Studies