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.
Year in Review: 2020
2020 proved to be a relatively mild year for rockfalls in Yosemite. Thirty-four rockfalls were documented in 2020, with a cumulative volume of about 3,120 cubic meters (9,285 tons). Both metrics are well below recent averages. The lower numbers may be due in part to below-average precipitation in 2020 (rainfall is a common rockfall trigger) but they more likely result from under-reporting. As a result of varying park closures and the implementation of a day-use reservation system throughout most of 2020 there were fewer people present to witness and report rockfalls.
The two largest rockfalls of 2020 happened in the summer. The first occurred at 7:14 pm on June 20, when a large exfoliation slab fell from the “Porcelain Wall” just west of Half Dome. Hundreds of park visitors around Mirror Lake watched as the slab, about 1,040 cubic meters in volume (nearly 3,100 tons), toppled from the wall and impacted a ledge below, exploding into thousands of fragments and generating a large dust cloud that filled Tenaya Canyon. Although spectacular, the rockfall was fortunately not consequential, as rock debris did not make it as far down as the Mirror Lake loop trail.
Another large rockfall occurred just two weeks later, when about 940 cubic meters (2,80 tons) of rock fell from low on Middle Brother at 8:42 am on July 4. Again, the rockfall was witnessed by hundreds of visitors enjoying the holiday. The rockfall created a dust cloud but did not damage infrastructure in the area. Four smaller rockfalls occurred from the same location later that night. This location has been intermittently active since 2016, demonstrating a progressive pattern that is relatively common in Yosemite. The Porcelain Wall and Middle Brother rockfalls both occurred on hot days, suggesting that heat may have triggered the rockfalls by thermal expansion of partially detached exfoliation slabs.
Other substantial rockfalls in 2020 occurred from El Capitan, Yosemite Falls, Glacier Point, and the Cascade Cliffs above Little Yosemite Valley.
It is very likely that there were additional rockfalls and rockslides in 2020, 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.
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.