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 Yosemite 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.
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?
Be aware of your surroundings. Rockfall hazard zones occur throughout the park near any cliff faces. If you witness a rockfall 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 rockfall 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 rockfalls 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 rockfall. If you witness or hear a rockfall 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 rockfall hazards and adds to the growing knowledge base of rockfall activity in the park.
Understand this dynamic natural process. Remember that Yosemite is a wild place. Rockfall is the most powerful geologic agent acting today in Yosemite. The dramatic cliffs of Yosemite are constantly being shaped by this potent natural force.
Yosemite Rockfall Year in Review: 2022
2022 proved to be an active year for rockfalls and other slope movements in Yosemite, particularly along lower elevation roadways. Although there weren’t any especially large-volume rockfalls in 2022, the year was a reminder that even relatively small rockfalls can be highly consequential.
The first significant rockfall occurred at 2:37 pm on May 15, originating from just below Union Point. A rock outcrop at the top of the slope there slid over a cliff, breaking into boulders that tumbled down a gully, crossing nine switchbacks of the Four Mile Trail before falling to the valley floor. Two visitors hiking the trail were hit by rocks, sustaining minor injuries.
Rockfall activity increased in the fall as storms swept across Yosemite. The Big Oak Flat Road was damaged by a rockfall on the evening of November 8, and again by a larger rockfall on the early morning of December 12; in both cases the road was closed for a few days while rocks were removed and the road repaired.
The largest rockfall of 2022 originated from midway up Middle Brother on the evening of November 12. A slab of rock of about 1,344 cubic meters in volume (about 4,000 tons) detached from the face, hitting a ledge; that impact then dislodged another 158 cubic meters (446 tons) from two other points lower on the cliff. Most of the resulting rock debris was deposited on the talus slope below the cliff, but some boulders bounced over Northside Drive, causing minor road damage.
The most consequential rockfall of 2022 occurred at 9 am on December 27. About 60 cubic meters (about 180 tons) of rock fell from the north rim of the canyon east of the Arch Rock Entrance Station, fragmenting into dozens of boulders that tumbled down to the El Portal Road. A vehicle traveling toward Yosemite Valley was struck by a boulder, tragically killing the two occupants.
The final days of 2022 corresponded with another period of heavy rain, resulting in at least 15 rockfalls and debris slides within a 24-hour period that again affected park roads.
Other substantial rockfalls in 2022 occurred from El Capitan, Half Dome, near Pulpit Rock, and in the Grand Canyon of the Tuolumne. In all, there were 52 rockfalls documented in 2022, with a cumulative volume of about 2,100 cubic meters (6,250 tons).
If you witness a rockfall of any size or encounter fresh rock debris, 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 (in new interactive map form), enabling long-term evaluation of rockfall activity to improve public safety.
Get the details concerning the Ahwiyah Point rockfall on March 28, 2009, near Half Dome. This rockfall was the largest in Yosemite National Park since an even larger one occurred from Middle Brother in 1987.