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.
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.
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.
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: Quantitative rock-fall hazard and risk assessment for Yosemite Valley, Yosemite National Park, California. Based on the results of this study, in 2013 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% compared to 2008 levels.
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?
Photo by Tom Evans
Yosemite Rockfall Year in Review: 2013Following the trend of the previous two years, 2013 was a quiet year for rockfalls in Yosemite. In all, there were 34 documented rockfalls, which is well below the recent (2006-2012) average of about 55 documented rockfalls per year. The cumulative rockfall volume was about 1500 cubic meters (about 4460 tons), also well below average. Given that 35% of rockfalls occurring since 1857 were associated in some way with precipitation (rain or snowmelt), the fact that 2013 was arguably the driest year on record for California may explain the relatively low level of rockfall activity.
The two most consequential rockfalls in Yosemite in 2013 impacted trails. The first occurred early on the morning of May 11 when a block about 320 cubic meters in volume (about 950 tons) fell from the top of the Panorama Cliff and landed on the John Muir Trail below Clark Point. Rock debris covered about 500 meters (1600 feet) of switchbacks, and the trail was closed for several weeks while damaged walls were repaired and the trail cleared.
The second consequential rockfall occurred at about 5 am on the morning of October 5 when a block of about 735 cubic meters (about 2200 tons) fell from Ahwiyah Point north of Half Dome; this was the largest rockfall of 2013. Ahwiyah Point experienced a very large rockfall in 2009 and has had intermittent activity since then. The rockfall on October 5 occurred from the same location as the 2009 event, and also curiously occurred at the same time of day. This early morning timing, combined with the fact that it occurred during the government shutdown, ensured that there were no injuries associated with the rockfall. The Mirror Lake Loop trail, recently reopened after extensive repairs following the 2009 event, experienced some minor damage from boulder impacts. This trail was also closed for several weeks while the source area was monitored and the trail cleared.
Other areas in Yosemite experiencing rockfalls in 2013 include Middle and Higher Cathedral Rocks, the Porcelain Wall west of Half Dome, Glacier Point, Royal Arches, and El Capitan.
It is very likely that there were additional rockfalls in 2013, 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 (http://pubs.usgs.gov/ds/746/), which enables evaluation of rockfall activity to help improve public safety.
Learn More about Rockfalls in Yosemite
Historical Rockfalls and Case Studies
Did You Know?
In Yosemite Valley, dropping over 594-foot Nevada Fall and then 317-foot Vernal Fall, the Merced River creates what is known as the “Giant Staircase.” Such exemplary stair-step river morphology is characterized by a large variability in river movement and flow, from quiet pools to the dramatic drops of the waterfalls themselves.