Rockfall

Map of Yosemite Valley showing rockfall locations and sizes, with most
Mapped above are documented rockfalls in Yosemite Valley, from 1857 to 2020, for which location and seasonal timing are known.
 

On This Page Navigation

 

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.

 
Rockfall in progress
A rockfall tumbles down the face of Half Dome on July 27, 2006.

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.

 
Computer simulation of rock fall
Computer modeling can help to identify potentially hazardous rockfall areas.

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.

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.
 
Rockfall from the "Porcelain Wall" just west of Half Dome on June 20, 2020, was 1,040 cubic meters in volume (nearly 3,100 tons).
This rockfall from the "Porcelain Wall" (just west of Half Dome) on June 20, 2020, was 1,040 cubic meters in volume (nearly 3,100 tons).

Morgan Newport

Yosemite Rockfall
Year in Review: 2021

2021 proved to be a relatively mild year for rockfalls and other slope movements in Yosemite. Forty-seven rockfalls were documented in 2021, with a cumulative volume of about 1,570 cubic meters (4,670 tons). Both metrics are below recent averages, perhaps related to below-average precipitation in 2021.

The two largest rockfalls in 2021 occurred at El Capitan. The largest was an approximately 1,000 cubic meter (nearly 3,000 ton) slab that fell from low on the west face; this event was not directly observed but likely happened during intense rain in October. The other, a 55 cubic meter (160 ton) slab, fell from the southeast face on the afternoon of November 16.

Other substantial rockfalls in 2021 occurred from Parkline Slab, Sentinel Rock, Sunnyside Bench, Glacier Point, Porcelain Wall, and Royal Arches.

The most consequential event of 2021 was a series of debris flows (water-laden sediment flows) that damaged portions of the John Muir and Mist trails. At 4 pm on June 30 a localized thunderstorm dropped heavy rain over eastern Yosemite Valley, triggering several debris flows between Happy Isles and Vernal Fall. The largest debris flow cut a 2-meter-deep gully through the John Muir Trail, depositing about 50 cubic meters (150 tons) of sand, gravel, and boulders onto the Mist Trail below. Smaller debris flows totaling about 30 cubic meters (90 tons) damaged the stock trail between Happy Isles and the Vernal Fall footbridge.

It is very likely that there were additional rockfalls and rockslides in 2021, 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

References

 

Historical Rockfalls and Case Studies

 

Prehistorical Rockfalls

 

Rockfall Hazard and Risk

 

Methods of Rockfall Characterization

Last updated: January 31, 2022

Park footer

Contact Info

Phone:

209/372-0200

Contact Us

Stay Connected