What Is a Rock Fall?

Rock falls are a natural and dynamic geologic process. Due to its steep, glacier-carved cliffs, Yosemite Valley experiences many rock falls each year. Historical records indicate that at least 600 rock falls 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 rock fall help to create the beautiful and changing scenery in Yosemite National Park, but they also present potential hazards.

Amanda Nolan

A rock fall occurred on July 27, 2006, at Half Dome (see more pictures below).

What Causes Rock Fall?

A number of geologic processes set the stage for rock fall. Glaciation, weathering, and bedrock fractures all play a role in causing rockfall. Tectonic stresses and erosion cause granite rock to fracture. Rock falls 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 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 rock falls. 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. There is still uncertainty about exactly what triggers rock fall; historical records indicate that more than half of all documented rock falls were not associated with a recognizable trigger.

Most rock falls in Yosemite occur in the winter and early spring, during periods of intense rainfall, snow melt, and/or subfreezing temperatures, but large rock falls have also occurred during periods of warm, stable weather.

In 2010, there were 59 documented rock falls in Yosemite, with an approximate cumulative volume of 2,900 cubic meters (7,700 tons). This is much less than in 2009 when 52 documented rock falls with an approximate cumulative volume of 48,120 cubic meters (142,000 tons). The vast majority of this 2009 volume was associated with rock falls occurring in March from Ahwiyah Point near Half Dome. The largest of these had an approximate volume of 45,300 cubic meters (about 134,000 tons), making it the largest rock fall in Yosemite Valley in 22 years. The impact of the falling rock generated ground-shaking similar to a magnitude 2.5 earthquake. Other notable 2009 rock falls occurred in August and September from the Rhombus Wall immediately north of the Ahwahnee Hotel, for a cumulative volume of about 1,200 cubic meters (roughly 3,600 tons).

Predicting actual rock-fall events is not yet possible, but understanding the forces that trigger rock falls is an important step toward this goal.

Computer modeling shows promise in simulating potential rock-fall zones.

How Does the Park Address Rock Fall?

The National Park Service is responding to rock fall in a variety of ways. Park scientists, in collaboration with the U.S. Geological Survey (USGS) and academic researchers, are actively studying rock fall 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 rock-fall zones and to study rock-fall source areas. The park is also actively investigating possible methods for monitoring rock-fall activity. Additionally, new computer modeling technology shows some promise in simulating future rock-fall behavior. Park planning incorporates potential rock-fall hazards, and park rangers have developed emergency plans for rock-fall events. Park managers may also close trails and post warning signs in particularly hazardous areas.

What Should I Do in the Event of a Rock Fall?

• Be aware of your surroundings. Rock-fall hazard zones occur throughout the park near any cliff faces. If you witness a rock fall 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 rock fall 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 rock falls 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 rock fall. If you witness or hear a rock fall 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 rock-fall hazards and adds to the growing knowledge base of rock-fall activity in the park.
• Understand this dynamic natural process. Remember that Yosemite is a wild place. Rock fall is the most powerful geologic agent acting today in Yosemite. The dramatic cliffs of Yosemite are constantly being shaped by this potent natural force.

Learn More about Yosemite National Park's Geology

• Film: By climbing with the park's geologist, get a different view of a rock slide. Watch the Yosemite Nature Notes "Rock Fall" episode. (8 minutes 30 seconds)
  
• Geologists use a 50-foot-wide photo from the Yosemite Panoramic Imaging Project to monitor and analyze rock-fall events.
• Get the details concerning the Ahwiyah Point rock fall on March 28, 2009, near Half Dome in which rocks fell roughly 1,800 feet, knocking down hundreds of trees and burying hundreds of feet of trail on the southern portion of the Mirror Lake Loop Trail. This rock fall is the largest one in Yosemite National Park since the 1987 Middle Brother event.
• Learn about the October 2008 Glacier Point rock fall. An analysis has shown that approximately 6,000 cubic meters of rock were involved in the events. 
• Follow a geologic overview of Yosemite National Park

Sources

• USGS description and analysis of the 1996 Happy Isles rock fall
• USGS description and analysis of the 1998-1999 Curry Village rock falls
• Rock-fall potential in Yosemite Valley in a USGS article
• Rock-fall hazards in Yosemite Valley in a USGS article
• Rock-fall hazard and risk assessment in Yosemite Valley in a USGS article
• Database of historic rock falls in Yosemite National Park, according to the USGS
• USGS description and analysis of the 2003 and 2007 Staircase Falls rock falls
• "Investigating the El Capitan Rock Avalanche" by Greg Stock in Yosemite magazine 70(4), Fall 2008, Yosemite Association. [585 kb PDF]
• "Catastrophic Rock Avalanches 3600 Years BP from El Capitan, Yosemite Valley, California" by Greg Stock and Robert A. Uhrhammer in Earth Surface Processes and Landforms, Volume 35, 2010, pages 941-951. DOI: 10.1002/esp.1982. [2.84 MB PDF]
• The Geologic Story of Yosemite National Park by Norman King Huber, 1987, U.S. Government Printing Office, Washington, D.C., U.S. Geological Survey Bulletin 1595. 

Photo by Tom Evans

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

Yosemite Rock Fall Year in Review: 2011
Although not as newsworthy as those of recent years, many significant rock falls occurred in Yosemite in 2011. Probably the most spectacular rock falls originated from the north face of Middle Cathedral rock in July. Three distinct rock falls occurred over a period of several weeks, with a total volume of about 120 cubic meters (about 325 tons). Witnesses in El Capitan Meadow captured impressive photos of the falls and subsequent dust clouds. Fortunately rock debris was contained to the talus slope beneath the cliff and did not affect nearby trails or roads. The Middle Cathedral rock falls occurred during very warm conditions and may have been triggered by thermal stresses. Ongoing monitoring by the National Park Service and U.S. Geological Survey indicates that air temperatures can cause partially-detached rock flakes to deform substantially on both daily and annual timescales.

Another area of notable rock falls in 2011 was Yosemite Falls. On 10 September 2011 a rectangular boulder 2 cubic meters in volume (6 tons) fell from below the upper Yosemite Falls Trail, shattered on a bedrock slab, and fell into the Lower Yosemite Falls amphitheater. Small rock fragments landed in the vicinity of the Lower Yosemite Falls footbridge, but there were no reported injuries. A second and much larger rock fall occurred on 28 October 2011, originating from near the lip of Upper Yosemite Falls. This block, about 20 cubic meters in volume (55 tons), fell from beneath an overhanging roof, skimmed the cliff, and then shattered on impact with the bedrock at the base of the upper falls. Off-trail hikers were present at the base of the falls but avoided injury.
 
The two most consequential rock falls in 2011 affected park roads. The largest event was not a true rock fall but rather a mass of broken rock and soil that slid onto Foresta Road between Old El Portal and Rancheria during the heavy rain and snow in late March 2011. The volume of this slide was about 1,200 cubic meters (about 3,200 tons). Foresta Road was closed for several weeks as the slide was cleared, and is presently closed again to accommodate engineering of the slope to reduce future slide potential. In the early morning of 7 November 2011, a large boulder (about 18 cubic meters, or 50 tons) and several smaller boulders tumbled from the north wall of the Merced Gorge just east of Arch Rock and embedded themselves in the El Portal Road. Following assessment of the source area, the boulders were removed and the road repaired in time for the morning commute into Yosemite Valley.
 
Other areas in Yosemite experiencing rock falls in 2011 include Royal Arches, Ahwiyah Point, Half Dome, Glacier Point, the Rockslides, and Hetch Hetchy. In all, there were 53 documented rock falls in 2011, with an approximate cumulative volume of 2,200 cubic meters (6,000 tons). This volume is comparable to the volume that fell in 2010 (2,900 cubic meters) but much smaller than what fell in 2009 (some 50,000 cubic meters); the volume in 2009 was dominated by the large Ahwiyah Point rock fall. Our database of rock falls and other geologic events in Yosemite, begun in 1857, now documents nearly 900 events, making it one of the longest and most detailed landslide databases in the world.
 
It is very likely that there were additional rock falls in 2011, 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 at greg_stock@nps.gov, or contact Park Dispatch by dialing 911 within the park. Predicting rock falls is not yet possible, but understanding the events that do happen is an important step toward this goal.