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: 2019
2019 proved to be a relatively benign year for rockfalls in Yosemite. Despite anxiety about elevated rockfall and debris flow potential in the Ferguson Fire burn scar, abundant snow and generally low-intensity rainfall limited the activity to a few small slides, mostly on the Wawona Road. Sixty-four events (including rockfalls, rockslides, and debris flows) were documented in 2019, with a cumulative volume of about 2,630 cubic meters (7,830 tons).
The largest and most consequential rockfall of 2019 occurred on February 4 at 9:20 am, just after heavy snowfall. Two blocks totaling about 420 cubic meters (1,250 tons) fell in quick succession from midway up the north face of Sentinel Rock, sliding over a ledge and pummeling the base of the cliff. Boulders tumbled across the Four Mile Trail, damaging the power line below and causing a prolonged power outage at Glacier Point.
The second largest rockfall of 2019 occurred from the Panorama Cliff near Happy Isles at 7:15 pm on September 22, when about 300 cubic meters (810 tons) of rock fell from low on the cliff. The rockfall created a dust cloud but did not affect any infrastructure.
The most consequential rockfall for the park geologist occurred on December 5. On that day, a friend and the geologist were conducting field work at the base of the southeast face of El Capitan, near the start of the route “Zodiac.” The wall is overhanging there. At 12:48 pm they heard a scraping sound high above them, followed by the sound of a large object rushing through the air. As this sound increased to deafening levels, they ran to the wall and crouched behind a boulder just as a 15 cubic meter (45 ton) rock exploded near them, pelting them with small rock fragments. A golf ball-sized fragment struck the geologist's helmet, but they emerged from the dust unscathed. This event is a reminder that situational awareness—being vigilant and having a plan for where to shelter in case of rockfall—is essential in rockfall-prone areas. Helmets also help!
Other substantial rockfalls in 2019 occurred in the Merced River Gorge, Glacier Point, Indian Canyon, and the Camp 4 Wall.
It is very likely that there were additional rockfalls and rockslides in 2019, 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: January 14, 2020