The rockfall on Thursday, September 28 was substantially larger than Wednesday’s series of rockfalls that occurred in the same area on El Capitan. The park geologist has measured the rockfall to have been 10,250 cubic meters in volume, or about 30,500 tons of granite. The dimensions for the source area are about 395 feet tall, 148 feet wide, and up to 26 feet thick. The September 28 rockfall ranks as the 29th largest rockfall on record in Yosemite National Park.
A number of geologic processes set the stage for rockfalls including glaciation, weathering, and bedrock fractures. Triggering mechanisms like water, ice, earthquakes, fluctuation in temperature, and vegetation growth are among the final forces that cause unstable rocks to fall.
More Information From the National Park Service/Yosemite
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?
- 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: 2016
It is very likely that there were additional rockfalls in 2016, 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.
Rockfall activity in 2016 was slightly lower than in previous years, with 58 documented events (rockfalls and rockslides) and a cumulative volume of about 5,000 cubic meters (roughly 15,000 tons). Nevertheless, many of these rockfalls were consequential, impacting park infrastructure and affecting park visitors, employees, and residents.
The year got off to a quick start with a rockfall on January 9 from “Little Windy Point” near Dog Rock. Roughly 90 cubic meters (270 tons) of rock slid onto the El Portal Road, blocking both lanes. The road was closed for several days as the cliff was assessed and the road cleared. (Note: this same location was active again almost exactly one year later.)
Later in the year, another larger rockfall in the Merced River Gorge impacted the El Portal Road on the rainy afternoon of October 31 (Halloween). A huge boulder (approximately 1,000 cubic meters, or nearly 3,000 tons) perched on the canyon rim above and west of Kat Pinnacle slid out along saturated soil and tumbled down the slope toward the river. Fortunately the boulder stopped against a bedrock outcrop midway down the slope, but another 80 cubic meters of associated rock debris landed on the road. The road was closed for a day as the rocks were blasted and cleared.
The most interesting rockfalls of 2016 happened at Middle Brother. Reminiscent of the 2009-2010 rockfalls from the Rhombus Wall, a series of rockfalls occurred from the lower part of the Middle Brother cliff over several months. The first occurred sometime in early February, as a roughly 1,000 cubic meter (3,000 ton) slab of rock exfoliated from the cliff, decimating the live oak forest at the base of the cliff and sending large boulders to the edge of the talus slope near Wahhoga. Smaller slabs fell sporadically throughout the spring and summer. On the afternoon August 3, hundreds of park visitors witnessed two large rockfalls in quick succession that produced large dust clouds. Another rockfall occurred on August 4, and four more occurred on August 5. In all, some 2,000 cubic meters (nearly 6,000 tons) of rock were shed from the cliff in 2016. This “progressive” rockfall behavior is occasionally displayed in exfoliating landscapes and is an area of vigorous scientific research.
Other substantial rockfalls in 2016 occurred from Sunnyside Bench, the East Ledges of El Capitan, Panorama Cliff, Little Yosemite Valley, the Merced River Gorge, and Hetch Hetchy.