What Is a Rockfall?

Rockfalls are a natural and dynamic geologic process. Due to its steep, glacier-carved cliffs, Yosemite Valley experiences many rockfalls each year. Historical records indicate that more than 900 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 occurred on July 27, 2006, at Half Dome.

Photo by Amanda Nolan

What Causes Rockfall?

A number of geologic processes set the stage for rockfall. Glaciation, weathering, and bedrock fractures all play a role in causing rockfall. Tectonic stresses and erosion cause granite rock to fracture. Rockfalls later occur along these fractures. Fractures that develop parallel to the surface are called sheet joints. Sheet 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 weathers the bedrock, loosening bonds that hold rocks in place. This is called weathering.

Triggering mechanisms like water, ice, earthquakes, and vegetation create the final forces that cause unstable rocks to fall. If water enters fractures in the bedrock, it lubricates surfaces and can build up pressure behind unstable rocks. Water also may seep into fractures in the rock and freeze, causing the fractures in the rock to expand. This process is called "frost wedging" or "freeze-thaw" and can incrementally lever loose rocks away from cliff faces. Ground shaking during earthquakes can also trigger 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. Recent research suggests that daily temperature variations can cause rock slabs to become unstable. There is still uncertainty about exactly what triggers rockfall; historical records indicate that more than half of all documented rockfalls were not associated with a recognizable trigger.

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.

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 the Park Address Rockfall?

The National Park Service 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 rockfall through the use of new technology, such as high resolution digital photography and laser mapping of cliffs. 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 possible methods for monitoring rockfall activity.

In 2012, the National Park Service produced a report quantifying rockfall hazard and risk in Yosemite Valley to help improve safety and guide future park planning efforts. You can read the report, Quantitative rock-fall hazard and risk assessment for Yosemite Valley, Yosemite National Park, California [8 MB PDF], or watch an hour-long presentation by Greg Stock, the park geologist.

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.

On Oct. 11, 2010, a rockfall from El Capitan occurred that is estimated to be the largest fall in 2010.

Photo by Tom Evans

Yosemite Rockfall Year in Review: 2012

2012 was a relatively quiet year for rockfalls in Yosemite. The most consequential event occurred at about 11:00 pm on January 22 above the Big Oak Flat Road. A boulder some 250 cubic meters in volume (about 745 tons) fell from a cliff above the road, slid down the slope below the cliff, and struck the Big Oak Flat Road, punching a hole in the eastbound lane, tearing out some of the retaining wall, and severely damaging nearly 30 m (100 feet) of roadway. The road was closed for about six weeks for repairs.

Consequential rockfalls also occurred from the Church Bowl area, located between Yosemite Village and the Ahwahnee Hotel. On April 3 and 4, two nighttime rockfalls occurred from the cliff directly above the popular climbing route "Bishop's Terrace." The area beneath the climb was devastated by rock debris, with dozens of trees snapped or toppled by the impacts. Small rocks tumbled all the way to the Valley Loop Trail. Climbing routes in this area were temporarily closed until the cliff stabilized.

Other areas in Yosemite experiencing rockfalls in 2012 include Mirror Lake, El Capitan, Ahwiyah Point, Half Dome, Glacier Point, Elephant Rock, Washington Column, and Hetch Hetchy.

In all, there were 43 documented rockfalls in 2012, with an approximate cumulative volume of about 700 cubic meters (about 2,100 tons). Both the number of rockfalls and the cumulative volume for all rockfalls in 2012 are substantially less than that documented in recent years. Although it is not clear exactly why rockfall activity was reduced from previous years, it may relate to the exceptionally dry winter of 2011/2012.

It is very likely that there were additional rockfalls in 2012, 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 email him, or contact park dispatch by dialing 911 within the park. Predicting rockfalls is not yet possible, but understanding the events that do happen is an important step toward this goal.

Learn More about Yosemite National Park's Geology

• Quantitative Rock Fall Hazard and Risk Assessment for Yosemite Valley (April, 2012) [8 MB PDF]
  A new analysis of rockfall hazard and risk prepared by the National Park Service, US Geological Survey, and other academic geologists.
• Film: Watch the Yosemite Nature Notes "Rock Fall" episode. (8 minutes 30 seconds)
• Geologists use high-resolution photographs from the Yosemite Panoramic Imaging Project to monitor and analyze rockfall events.
• Get the details concerning the Ahwiyah Point rockfall on March 28, 2009, near Half Dome. This rockfall is the largest one in Yosemite National Park since the 1987 Middle Brother event.
• Learn about the October 2008 Glacier Point rockfall.
• Follow a geologic overview of Yosemite National Park

Publications

• "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. [500 kb PDF]
• "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. [820 kb PDF]
• "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. [2.1 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. [2.8 MB PDF]
• "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]
  
• "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. [2.84 MB PDF]
• "Investigating the El Capitan Rock Avalanche" by Greg Stock, 2008, Yosemite, Volume 70, Yosemite Association. [585 kb PDF]
• "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. [6 MB PDF]
• "Historical Rock Falls in Yosemite National Park, California" by Gerald F. Wiezcorek and James B. Snyder, 2004, US Geological Survey Open-File Report 03-491.
• "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. [7.5 MB PDF]
  
• "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. [2.3 MB PDF]
• "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-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]