By the Numbers: Yosemite Rockfall

The stats and science behind the Valley’s constantly exfoliating granite.
By Ula Chrobak,
Rockfall on the right side of El Cap on October 11, 2010; there were multiple slides here that day.

Yosemite is famed for its perfect granite, but the geologic reality is that the Valley is a dynamic landscape in which the cliffs are constantly sloughing. Take the Regular Northwest Face of Half Dome: In July 2015, it shed a massive amount of stone, changing the character of the classic forever. Here, Park Geologist Greg Stock and others help us understand the forces at play—forces, given the danger of rockfall, that are of great importance to climbers.

For a more in-depth look at Yosemite's rockfall and the geologists who study it, see The Science of Rockfall: What We Know—and What We Don’t.


Number of rockfalls of all sizes recorded each week in Yosemite, on average. Many small rockfalls go undetected, however. 


Years ago that Yosemite’s glaciers retreated, revealing kilometer-tall unstable cliffs. Since then, the granite domes have continuously shed their top layers of rock. The falling rock becomes talus and the boulders that climbers now frequent, including the blocks in Camp 4 and Half Dome Village (formerly Curry Village). 


The first year a Yosemite rockfall was documented. James Mason Hutchings, businessman and early promoter of Yosemite, wrote in his Hutchings’ California Magazine that the debris was “said to cover over thirty acres.” Stock in 2011 updated the historic entry, stating that the fall may have occurred on the Lost Brother formation of Profile Cliff at Taft Point, and was 50,000–500,000 cubic meters in volume. The writings of famed naturalist John Muir also contributed to numerous early records of rockfalls. Since then, geologists have documented over 1,000 rockfalls. Now, the Yosemite rockfall database is used to look for patterns and trends.


Number of washing machines (volume: 1 cubic meter) needed to equal the Valley’s major rockfall events, of which there is roughly one annually.  


Number of people killed in a November 1980 rockfall that crashed down upon the upper Yosemite Falls Trail. At least 19 were injured. Rockfalls in Yosemite cause relatively few fatalities compared to the Merced River or traffic accidents, but a big rockfall in a frequented area can be catastrophic. 

10-20 Percent

Estimated percentage of rockfalls documented in the database (the rest either occur at night and/or in low-traffic areas). Of these, geologists attribute 29 percent to water leaking into cracks and building up pressure behind the outer flakes. Other causes—including earthquakes, freeze-thaw cycles, and snowmelt—make up 18 percent. Causes for the rest remain elusive: Either there’s too little info or scientists can’t determine a cause. 


Number of people killed by natural rockfall in Yosemite since recordkeeping began. Of these, only a small portion were climbers. Most infamously, in June 1999, rock shards showering down from 1,000 feet up Glacier Point Apron killed Peter Terbush as he belayed his partner on Apron Jam (5.9; read the full story). However, human-caused rockfalls kill far more people.


Number of buildings destroyed or damaged by rocks breaking off below Glacier Point and tumbling into Half Dome Village on October 8, 2008. Fortunately, there were only minor injuries.


Number of climbing rangers and volunteer climbing stewards at the Yosemite Climbing Management office. The rangers team up
with geologists like Stock to help build a better understanding of rockfalls on Yosemite’s complicated cliffs. (Read the full story of their efforts.)


Tons of rock that peeled off the fifth pitch of Kali Yuga on Half Dome during three rockfalls in June 2003. One climber at the base of the cliff tripped and broke an ankle while running away; two others returned to Camp 4 “pale as ghosts”—perhaps both from shock and the dust that covered them as a result of the rockfall.


The year Stock and fellow geologist Brian Collins discover that heat can trigger rockfall. Every day—but most dramatically in summer—flakes of rock expand outward during warm hours and contract at night. This causes the flakes to gradually crack and split away from the rock, until they break off. The research may explain why a surprising amount of rockfalls occur, seemingly spontaneously, on sunny days.


The number of pitches lost from the Regular Northwest Face of Half Dome during a July 2015 rockfall. The sheet of rock was about 200 feet tall and 100 feet wide, and ripped off from pitches 11 and 12, above the Robbins Traverse. Now, climbers circumnavigate the blank scar by using a bolt ladder to move over onto Arcturus, from where they either toss a knot to pull themselves into the chimney systems of pitch 13 or climb higher on Arcturus and pendulum back onto the RNF. The upper Death Slabs approach and bivy areas at the base of the climb are directly below the scar, which is framed by flakes of loose rock—Climbing Ranger Eric Bissell warns that these areas may be hit by falling rock. 


The year the United States Geologic Survey published their “Quantitative rock-fall hazard and risk assessment for Yosemite Valley, Yosemite National Park, California” (Greg M. Stock, et al.). The study mapped out a “hazard zone” most prone to rockfall, allowing the National Park Service to move structures out of harm’s way. A row of campsites at Camp 4 was relocated farther from the cliff, and several cabins in Half Dome Village were also closed, freeing up climbing potential on previously inaccessible boulders. Two times since these and other preventative actions, boulders have crashed into the old footprints of cabins. 

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