July 14, 2015
The recent rockfalls on the Regular Route and the summit area on Half Dome highlighted a basic Yosemite truth: Climbers in Yosemite need to know that rockfall is commonplace and that it happens every week, not every once-in-a-while. While geologists are not yet able to predict when rocks will fall, they can give us helpful tips about how to climb as safely as possible. In this post, Roger Putnam, professor of geology at Columbia College and co-author of Yosemite Bigwalls: The Complete Guide, shares his wisdom on the subject. Contact Roger directly at: Roger@yosemitebigwall.com.
When the glaciers retreated from Yosemite Valley 15,000 years ago, they left something that climbers adore, but physics dislikes: big steep cliffs. Large cliffs, even when made of nearly flawless granite, are inherently unstable. Until recently, it was believed that, on average, one significant (greater than 1 m3) rockfall happens each week in Yosemite.
However, this number may be much higher. A recent study by geologists from the University of Lausanne, Switzerland, took a LiDAR scan (similar to radar or sonar, but using a laser scanner) of El Capitan, in 2012 and another in 2013. The researchers overlaid the surfaces and found that 5 rockfalls had occurred on El Capitan during that year, the largest of which was a staggering, approximately 136 m3 in volume, sourced left of El Cap Tree. Interestingly, only one smaller rockfall was reported during this time, an approximately 48 m3 fall that erased part of the Waterfall Route. This raises the possibility that we are only noticing 1/5th of the rockfalls that happen in Yosemite, despite all of the attention its walls receive.
Individual rockfalls can be triggered by a number of different means. Water can get into a crack and lubricate it, causing the sides to slip, and making a rockfall happen during or after a storm. Water can also get into a crack, freeze, expand and wedge it apart. Even the action of plant roots can force apart rock. Recent research suggests that cracks propagate during the cyclical expansion and contraction, that a mass of rock experiences as it heats up during the day and cools off during the night. This mechanism is hypothesized to be responsible for the many rockfalls that occur during the heat of the summer.
Exfoliation is another process by which a cliff weathers away. This process seems to occur because, in a mass of rock, the largest forces usually run parallel to the erosional surface. (Imagine a piece of rock located in the face of El Capitan. The greatest forces acting upon it come from the weight of the cliff above it.) In a uniform material, cracks form parallel to the greatest force. This physical process results in cracks forming more-or-less parallel to the erosional surface in places with geology similar to Yosemite. Places where you can see exfoliation cracks in Yosemite are the Hollow Flake on the Salathe Wall, the Central Pillar of Frenzy, and the entire Royal Arches cliff.
Many features on the Regular Northwest Face (RNWF) of Half Dome are the result of ongoing exfoliation. This was demonstrated when pitches 11 and 12 fell off of the route during a storm on July 2nd. This event erased the pitch after the Robbins Traverse and the lower portions of the chimneys. The whole RNWF climbs up exfoliation cracks from the Robbins Traverse to Thank God Ledge. The rockfall happened at the base of one of these cracks. Often, when you see rock falls coming from underneath a roof at the base of an exfoliation slab, more will follow. That is why we do not suggest going up there for a while.
Thankfully, nobody was injured during the recent Half Dome rockfall. Furthermore, most of the significant rock falls in Yosemite during the last several decades occurred on cliffs unfrequented by climbers – the exception being a major rockfall off the Glacier Point Apron in 1999 that resulted in one climber’s death. Follow these tips to reduce the risk of being caught in a rockfall:
- Minimize the time you spend in the areas that have experienced recent or recurring rockfalls. If you see fresh talus or broken trees, you are likely in an area that has recently experienced a rockfall. Places to avoid are the base of Half Dome, section between High Plains Dripper and Lost in Translation on El Capitan, the east side of the Rhombus Wall, the east side of the Glacier Point Apron, and the 9 O’Clock Wall.
- Cracking and popping noises often precede a rockfall. If you hear these noises, get out of the area.
- Wearing a helmet can save your life in the event of a small rockfall or protect you from the shrapnel that often flies around during a larger event.
As immortal as the landscape of Yosemite Valley appears to be, in a geologic sense, the place is in constant change. Rockfall is a reminder of that beautiful impermanence.