El Capitan Area Rockfall Probably Biggest in Yosemite Valley This Year

A follow-up to yesterday's post about the rock fall in Yosemite Valley: the rockfall is still being analyzed, but according to Greg Stock, Yosemite's park geologist, the event yesterday near Horse Tail Falls is probably the largest mass wasting event in the valley this year. They are using a LiDAR scan to calculate the size of the chunk of rock that fell. A preliminary report on the fall on a climber's site has a much better picture of the event itself, and a before/after shot of the release point. There were a total of three rock falls, a small event at about 11:30 AM, a much larger fall about three minutes later, and another smaller fall at about 1:00 PM. Thanks, Greg, for the info.

More updates as they become available! The scar and impact zones can be seen in the photo above.

Update #1:
Greg Stock, park geologist reports:

Several large rock falls occurred from near the East Buttress of El Capitanon October 11, 2010. The first rock fall occurred around 11:30 am, and was followed about three minutes later by a much larger rock fall. A third smaller fall occurred at around 1 pm.

All three rock falls detached about halfway up the far eastern side of the southeast face of El Capitan, roughly along the path that Horsetail Falls takes when flowing. Rock debris hit a prominent ledge beneath the cliff and fragmented into smaller boulders, producing substantial dust; the dust cloud produced by the second impact was visible throughout Yosemite Valley. Boulders did reach the base of the cliff, but did not impact any trails or roads. Although there were many climbers on El Capitan at the time, there do not appear to have been any injuries associated with these rock falls. However, climbers are cautioned that future rock falls from this area arepossible.

Geologists are still investigating these events and are mapping the size of the failures in detail, but preliminary estimates suggest the volume exceeded 1000 cubic meters, making this the largest rock fall thus far in 2010...

...More information is also available on the Yosemite NPS website:
href="http://www.nps.gov/yose/naturescience/rockfall.htm">http://www.nps.gov/yose/naturescience/rockfall.htm

Also, an excellent shot of the dust cloud is posted on Planet Mountain: http://www.planetmountain.com/english/News/shownews1.lasso?l=2&keyid=37636#

Rock Fall Near El Capitan in Yosemite Today

Your on-the-spot geological reporter was on the scene and on the job today when a rock fall occurred on the cliff just east of El Capitan in Yosemite Valley. Well, ok, I was almost on the scene, and almost on the job...to be honest, I was near the top of Sentinel Dome on the south rim of the valley, eh, dozing on the smooth granite slope. I was listening to what I thought was an odd-sounding jet off in the distance, but when the sounds abruptly stopped, I thought "rockfall!". I ran to the edge of the cliff just in time to see the dust rising from the tumbling boulders.

There has been little in the way of official information as of yet, but it seems to have originated in the vicinity of the cliff at Horse Tail Falls, the high vertical wall just east of the main cliff of El Capitan. I only had time for a cursory examination, but I think the impact point is the white rock on the cliff just above the tips of the trees.

There are white streaks on the dark gray cliffs above the white rock that may be impact scars(click on the photo below for a larger image), and lots of fresh looking boulders on the slope below. I can't tell if trees were snapped off or not. I would appreciate any reports or additional from climbers, hikers, or anyone who was in the meadow below. I will gladly post any pictures, too!

Once a big chunk of rock breaks off a cliff, nearby rocks may be destabilized, and subsequent rock falls are possible. Clearly the climbers know this...