Much of the spectacular scenery in Yosemite is a product of glacial erosion followed by rock falls
This illustration is part of a rockfall hazard map for Yosemite National Park. It shows recent, historic and prehistoric rock fall and rock slide areas near some of the busiest sites in the park. The National Park Service, United States Geological Survey and others are working to understand the rock fall and rock slide hazards in the park, educate park visitors, and use what they learn to make the park an even safer place to visit. Map from USGS Open-File Report 98-467. Enlarge map.
Rock Falls and Rock Slides - Yosemite Valley
Yosemite Valley is a deep, glacier-carved valley bounded by steep granite cliffs. These steep cliffs produce numerous
rock falls and rock slides every year. These events range in size from individual boulders to massive falls of several
million cubic meters. Field work in the park area has documented hundreds of large
falls over the past 150 years. Several of these falls have killed people in the valley.
The park receives between three and four million visitors per year. Some of the rock falls and
rock slides have occurrred in parts of the park that are heavily used by visitors. To reduce the potential impact
of future events the National Park Service is working to understand the rock fall and rock slide hazards.
Predicting rock fall events is not possible but information about how, where and why they occur can be used
to improve visitor safety.
Most rock falls in Yosemite are triggered by earthquakes, rainstorms, or rapid snow melt. However, many
rock falls occur without an obvious triggering event. Some are probably associated with gradual stress
release and weathering of granitic rocks in the valley walls. These processes have been active enough
to produce a talus slope over 100 meters thick at the base of some slopes.
"In 2009, there were 52 documented rock falls in Yosemite, with an approximate cumulative volume of 48,120 cubic
meters (142,000 tons). The vast majority of this 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)." Quoted from the National Park Service website (link).
"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." Quoted from the National Park Service website (link).
NPS Response to Rock Fall Studies
The National Park Service is learning from their rock fall research program and using that
information to make the Park a safer place to visit. After studying rock falls in the Curry
Village area in 2008, the Park Service determined that over 200 tent and cabin sites
were within rock fall hazard areas and closed them permanently. These studies continue in
other parts of the park.
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This video by the National Park Service and Yosemite Conservancy demonstrates the rock fall and rock slide hazards in the Yosemite National Park area and explains some of the work being done to learn more about the hazards, educate visitors and reduce future impact.