Rockfall

Mapped above are documented rockfalls in Yosemite Valley, from 1857 to 2020, for which location and seasonal timing are known.

 

 

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.

 

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 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.

 

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: 2024

2024 was a relatively quiet year for rockfalls in Yosemite, with 42 documented rockfalls and a cumulative volume of 1,397 cubic meters (4,158 tons), both are which are below 20-year averages. Although there wasn’t an obvious “standout” rockfall in 2024, several deserve mention.

The largest rockfall of 2024 occurred at 10:23 pm on April 15. Visitors sleeping in the Pines campgrounds were awoken by distant thundering sounds indicative of a rockfall, but the location was unclear. Three days later a wilderness Ranger hiking the seasonally closed section of the John Muir Trail between Nevada Fall and Clark Point encountered literal tons of rock debris on the trail. Approximately 562 cubic meters (1,672 tons) of rock slid from just below the south rim of the canyon, fragmenting into boulders that tumbled across three switchbacks. Hundreds of boulders came to rest on the trail, two areas of the trail sustained deep impact craters, and several fir trees were snapped or toppled by rock impacts. Although the weather was clear at the time, water was observed seeping from the source area, suggesting a snowmelt trigger. The trail was closed for several weeks as boulders were drilled, blasted, and cleared.

Rockfalls alter Yosemite’s landscape, sometimes dramatically. At 11:20 am on May 14, a small rockfall occurred from LeConte Gully below Glacier Point, sending boulders down the path of Staircase Falls. The rockfall originated from a narrow granite fin forming the east wall of LeConte Gully that was the source of rockfalls in 2003 and 2007. Part of the fin collapsed, allowing Staircase Creek a more direct path over the cliff, instantly forming a new 150-foot-tall waterfall at the top of Staircase Falls.

Two rockfalls affected the Upper Yosemite Falls Trail in 2024. The first occurred at 7:14 am on October 11, when approximately 28 cubic meters (84 tons) of rock fell from the cliff above the traversing section of the trail between Columbia Point and the base of the Upper Falls. Boulders swept across the trail and fell into the Lower Falls Amphitheater below. Fortunately, the early morning timing meant that both that section of trail and the amphitheater were devoid of people. The second rockfall occurred at 2:58 pm on December 26. Approximately 33 cubic meters (98 tons) of rock fell from high on the “Forbidden Wall” above the upper switchbacks, with rock debris tumbling over 20 switchbacks of the trail. One visitor sustained leg injuries but was able to walk out with assistance. In both cases, the trail was closed pending geological assessment.

Other significant rockfalls in 2024 occurred from Grizzly Peak, Castle Cliffs, Sentinel Creek, and the Big Oak Flat Road.

Also, of note in 2024 was the continued widening of the crack in the cliff west of Royal Arches and north of The Ahwahnee, adjacent to the climbing route “Super Slide.” This crack was about one inch wide when first discovered in August 2023. Monitoring showed that the crack widened in the fall of 2023, narrowed over the winter of 2023-2024, and then widened again to nearly four inches in August 2024. The crack is being monitored closely, and local closures remain in place.

If you witness a rockfall of any size or encounter fresh rock debris, please contact park geologist Greg Stock at 209/768-1028 or by email, or contact park dispatch by calling or texting 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

• Quantitative Rock Fall Hazard and Risk Assessment for Yosemite Valley (September 2014)
  Analysis of rockfall hazard and risk prepared by the National Park Service, US Geological Survey, and other academic geologists.
• 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.

References

 

References: Historical Rockfalls and Case Studies

Historical Rockfalls and Case Studies

• "Historical Rock Falls in Yosemite National Park, California (1857-2011)" by Greg M. Stock et al, 2013, U.S. Geological Survey Data Series Report 746
• "Rock fall dynamics and deposition: an integrated analysis of the 2009 Ahwiyah Point rock fall, Yosemite National Park, USA" by Valerie L. Zimmer et al, 2012, Earth Surface Processes and Landforms, Volume 37, pages 680-691.
• "Progressive failure of sheeted rock slopes: the 2009-2010 Rhombus Wall rock falls in Yosemite Valley, California, USA" by Greg M. Stock et al, 2012, Earth Surface Processes and Landforms, Volume 37, pages 546-561.
• "Evaluation of hypothesized water-system triggers for rock falls from Glacier Point, Yosemite National Park, California, USA" by Greg M. Stock et al, 2012, in Landslides and Engineered Slopes: Protecting Society through Improved Understanding, pages 1165-1171. 
• "Investigation and hazard assessment of the 2003 and 2007 Staircase Falls rock falls, Yosemite National Park, California, USA" by Gerald F. Wieczorek et al, 2008, Natural Hazards and Earth System Sciences, Volume 8, pages 421-432.
• "Unusual July 10, 1996 rock fall at Happy Isles, Yosemite National Park, California" by Gerald F. Wieczorek et al, 2000, Geological Society of America Bulletin, Volume 112, pages 75-85. 
• "Rock falls from Glacier Point above Camp Curry, Yosemite National Park, California" Gerald F. Wieczorek and James B. Snyder,1999, US Geological Survey Open-File Report 99-385.
• "Rock Slides in Yosemite" C.C. Jensen, 1933, Yosemite Nature Notes, v. XII, pages 9-11. [260 kb PDF]

 

References: Prehistorical Rockfalls

Prehistorical Rockfalls

• “Magnitude and timing of the Tiltill Rockslide in Yosemite National Park, California” by Christopher Pluhar et al, 2021, v. 27, p. 395-407.
• "Surface exposure dating and geophysical tomography of the Royal Arches Meadow rock avalanche, Yosemite Valley, California" by Marcus Pacheco et al, 2020, Frontiers in Earth Science, v. 8, 372.
• "Near-surface geophysical imaging of a talus deposit in Yosemite Valley, California" by Anna G. Brody et al., 2015, Environmental and Engineering Geoscience, v. 21, p. 111-127. [3.2 MB PDF]
• "Supporting evidence for a 9.6 +/- 1 ka rock fall originating from Glacier Point in Yosemite Valley, California" by Shaun E. Cordes et al, 2013, Environmental and Engineering Geoscience, v. 19, p. 345-361. [11.7 MB PDF]
• "Catastrophic Rock Avalanche 3600 Years BP from El Capitan, Yosemite Valley, California" by Greg M. Stock and Robert A. Uhrhammer, 2010, in Earth Surface Processes and Landforms, Volume 35, pages 941-951.

 

References: Rockfall Hazard and Risk

Rockfall Hazard and Risk

• "Reducing rockfall risk in Yosemite National Park" by Greg M. Stock and Brian D. Collins, EOS, v. 95, p. 261 & 263.
• "Quantitative rock-fall hazard and risk assessment for Yosemite Valley, Yosemite National Park, California" by Greg M. Stock et al, 2014, U.S. Geological Survey Scientific Investigations Report 2014–5129.
• "Rockfall hazard and risk assessment in Yosemite Valley, California, USA" by Fausto Guzzetti et al, 2003, Natural Hazards and Earth System Sciences, Volume 3, pages 491-503.
• "Rock-fall Potential in the Yosemite Valley, California" by Gerald F. Wieczorek et al, 1999, US Geological Survey Open-File Report 99-578.
• "Rock-fall Hazards in the Yosemite Valley" by Gerald F. Wieczorek et al, 1998, US Geological Survey Open-File Report 98-467.
• "Triggering mechanisms and depositional rates of postglacial slope-movement processes in the Yosemite Valley, California" by Gerald F. Wieczorek and Stefan Jager, 1996, Geomorphology, Volume 15, pages 17-31. [1.2 MB PDF]
• "Analysis of rock falls in the Yosemite Valley, California" by Gerald F. Wieczorek et al, 1995, in Rock Mechanics: Proceedings of the 35th U.S. Symposium, p. 85-89.

 

References: Methods of Rockfall Characterization

Methods of Rockfall Characterization

• “Assessing rockfall susceptibility in steep and overhanging slopes using three-dimensional analysis of failure mechanisms” by Battista Matasci et al, 2018, Landslides v. 15, p. 859-878.
• "Quantifying 40 years of rockfall activity in Yosemite Valley with historical Structure-from-Motion photogrammetry and terrestrial laser scanning" by Antoine Guerin et al, 2020, Geomorphology, Volume 356, pages [2 MB PDF]
• "Detection of rock bridges by infrared thermal imaging and modeling" by Antoine Guerin et al, 2019, Scientific Reports, 9:13138
• "Rapid 3-D analysis of rockfalls" by Greg M. Stock et al, 2018, GSA Today, v. 28, p. 28-29. [1.9 MB PDF]
• "Rockfall triggering by cyclic thermal stressing of exfoliation fractures" by Brian D. Collins and Greg M. Stock, 2016, Nature Geoscience, p. 395-401. [1.2 MB PDF]
• "Lidar-based rock-fall hazard characterization of cliffs" by Brian D. Collins and Greg M. Stock, 2012, Proceedings of the 2012 American Society of Civil Engineers GeoCongress, p. 3021-3030. [4.8 MB PDF]
• "High-resolution three-dimensional imaging and analysis of rock falls in Yosemite Valley, California" by Greg M. Stock et al, 2011, Geosphere, Volume 7, pages 573-581.
• "Structural characterization of rockfall sources in Yosemite Valley from remote sensing data: toward more accurate susceptibility assessment" by Battista Matasci et al, 2011, Proceedings of the 2011 Pan-Am CGS Geotechnical Conference. [750 kb PDF]

 

Videos

 

• 

  Yosemite Nature Notes 10: Rock Fall

  Since the glaciers retreated around 15,000 years ago, rock fall has been the major force of change in Yosemite Valley. Geologists work to understand this force of nature in order to protect the millions of visitors who come here each year.

  Duration:

  8 minutes, 30 seconds

 

• 

  Clearing a Rockslide from the Big Oak Flat Road

  In this video, our roads and trails crew work to clear one of many rockslides that occurred in spring 2023 along the Big Oak Flat Road with drilling, blasting, and heavy equipment.

  Duration:

  1 minute, 18 seconds

 

• 

  Aftermath of the 1996 Happy Isles Rockfall

  Footage taken after the 1996 rockfall at Happy Isles. On July 10, two blocks with a combined volume of about 30,000 cubic meters and weighing about 90,000 tons broke loose from near Glacier Point, hitting the valley floor near Happy Isles. The impacts generated an airblast with wind speeds up to 280 miles per hr. The wind uprooted and snapped about one thousand trees, which damaged buildings and caused one fatality and several serious injuries. A dense cloud of pulverized rock blanketed the area with dust.

  Duration:

  4 minutes, 31 seconds