El Capitan Rockfall: Alternate Views, and a Realization That This Wasn't the First TIme

El Capitan and the September rockfall from Taft Point on Oct. 29, 2017

Yosemite Valley changed geologically on September 27 and 28, and as I noted in my previous post, the valley will never look the same again. A series of rock falls resulted in a new scar on the east face of the east wall of the El Capitan cliff that can be seen from many points within the valley, including the iconic Tunnel View.

September rockfall from Taft Point on Oct. 29, 2017

I finally got to Yosemite Valley on Saturday, and got a few photos from the Tunnel View that I posted yesterday, but I was with students on a bus. Buses aren't allowed to stop just anywhere in the valley, so I was anxious to get back to the valley in my own car, and Mrs. Geotripper was all for visiting the valley, so we went back up there yesterday. The question, of course, was where to go for a good look? I definitely wanted an aerial view, so we headed up Glacier Point Road, the only route that allows for views from the rim of Yosemite Valley. Glacier Point offers views beyond belief, but only of the upper end of the valley. For a view of El Capitan, the "easy" choices are to hike to the top of Sentinel Dome, or to hike to Taft Point. Both hikes are about a mile and are not overly difficult. Sentinel is farther to the east and doesn't provide as good an angle on the cliff face that fell, so I headed out to Taft Point as the sun approached the western horizon.

The view is striking. The rocks let loose from well over a thousand feet over the valley floor. According to Greg Stock, geologist for Yosemite National Park, the biggest slab (out of seven total), was 394 feet long, 148 feet wide, and between 8 and 28 feet thick. That's bigger than a football field. It had a volume of 10,250 cubic meters, and weighed about 27,675 metric tons. If these numbers and dimensions seem incredibly exact, there's a reason. As Greg Stock explains, in an informative research paper, the walls have been mapped in three dimensions by a form of radar (lidar), and in the aftermath of the rockfall, the walls at the site of the fall were mapped again. The difference in volume could then be calculated.

El Capitan from Taft Point in 2005

I was at Taft Point in 2005, taking pictures of course, and one can compare the appearance of the cliff prior to the event in September.

Horsetail Falls area and future site of rockfall in 2005

The rockfall is also apparent from many spots on the valley floor. We headed to El Capitan Bridge, which offers an excellent straight-on view with the Merced River in the foreground. The white scar is unmistakable.

The Horsetail Falls cliff on Oct. 29, 2017

The difference between before and after couldn't be clearer. It's easy to miss the scale on cliffs this grand...remember, the missing slab of rock was bigger than a football field.

The cliff near Horsetail Falls in 2016

Another way to get a sense of scale is to add the 3,000 foot cliff of El Capitan to the scene...

El Capitan and the Horsetail Falls cliff on October 30, 2017

As I was going through my old pictures, I came to realize that slides have come off this particular cliff in the past. I actually witnessed one of them in 2010 when I was lounging on the granite near the top of Sentinel Dome across the valley. When I say "witnessed" I mean I heard the commotion first, took a moment to realize what I was hearing, jumped up, scanned up and down the valley, and finally thought to turn on the camera and take a few pictures of the rising dust plume.

Dust plume from the 2010 rockfall east of El Capitan

My account of that event can be found here: http://geotripper.blogspot.com/2010/10/rock-fall-near-el-capitan-in-yosemite.html. One can see in the picture below that the 2010 fall was just below the slab of rock that came down in 2017. One can imagine that the stresses on the cliff above were complicated by the event several years earlier.

You Can Never See This View Again. Ever.

The title isn't as ominous as it sounds, but it is true. This is the panorama seen from Tunnel View at the west end of Yosemite Valley, one of the most famous viewpoints in the world, in 2013. But the specific scene above can never be experienced by anyone ever again. Geological forces, instead of acting at a slow incremental pace, moved things tragically fast late last month. A chunk of granitic rock the size of a football field came off the cliff just east of El Capitan (cliff on the left), and the scar will be visible for centuries. The picture below is how it looked today.

The view is fundamentally the same, but the changes are occurring, and the appearance of Yosemite changes with it. See below for a zoom of the cliff near Horsetail Falls, first in 2013, and then how it was this afternoon.

There were a total of seven rockfalls from the site on September 27-28, 2017. One person was killed and two were injured. One report mentions the largest of the falls as weighing 30,000 tons. This wasn't the biggest rockfall in Yosemite's history, not even close. The largest in recorded history was the Middle Brothers slide of 1987, which totaled about 1.4 million tons. Prehistoric rockfalls in the Mirror Lake area were larger still.

Actually, if we were to look at photographs from a decade ago, there would be an additional difference. The 2009 Ahwiyah Point rockfall scar can be seen just right of center, in both of the pictures above (but especially in the upper one). It totaled 115,000 tons, but thankfully injured no one. Check below for a 2009 view of the same spot to compare the difference.

Although partly hidden in shadow, the rock face at the bottom, just right of center (below the snow) is composed of darker weathered rock instead of the white scar seen in the photos above. Geologic change is constant only in that fact that it happens. Sometimes it is imperceptible, and sometimes is rapid and tragic.

National Park Service Rockfall Report for 2010

Dust rises from the El Capitan rock fall on October 11

Courtesy of Park Geologist Greg Stock, a review of mass wasting events in Yosemite National Park this year....

2010 Rock Fall Year in Review

Although not as newsworthy as those of recent years, many significant rock
falls occurred in Yosemite in 2010. The largest occurred on October 11
from the southeast face of El Capitan, midway up the cliff and along the
path of Horsetail Falls. This was actually the largest in a series of rock
falls from that area over several days. There were no associated injuries
despite the fact that these rock falls occurred during the peak of the fall
El Capitan climbing season. The cumulative volume of these failures was
approximately 1,700 cubic meters (5,000 tons).

Another area of notable rock fall in 2010 was the Rhombus Wall immediately
north of the Ahwahnee Hotel. This area was first active in August of 2009,
with subsequent failures in September 2009. Quiet for the early part of
2010, the cliff experienced renewed activity beginning in August, with at
least five rock falls radiating outward from the initial failure point
between August and November 2010. The cumulative rock fall volume over
this interval was 187 cubic meters (550 tons). This area of the Rhombus
Wall is an impressive example of a progressive failure due to stress
redistribution and crack propagation, and presents a unique opportunity to
learn more about this complex process.

Ironically, the most serious rock fall of 2010 was also one of the
smallest. On October 5, 2010, following several days of intense rain,
local children Serra Weber, Carmen Ortiz, and Angel Ortiz were playing on
the talus slope near the Church Bowl Picnic Area when a 1.7 cubic meter (5
ton) rock fell from low on the cliff. The rock fell directly onto Serra,
pinning her and causing life-threatening injuries. Carmen and Angel ran to
the nearby Yosemite Medical Clinic and notified the medical staff, who
responded immediately. Serra was flown to a local hospital where she
remained for several weeks. Serra recently returned to school in Yosemite
Valley, where she, Carmen, and Angel were honored for their bravery during
this event.

Other areas in Yosemite experiencing rock falls in 2010 include the
Porcelain Wall (on the western shoulder of Half Dome), Glacier Point,
Middle Brother, Middle Cathedral, Indian Canyon, and the Merced River
Gorge. In all, there were 59 documented rock falls in 2010, with an
approximate cumulative volume of 2,900 cubic meters (8,500 tons), more than
half of which derives from the El Capitan rock falls. For comparison, the
cumulative volume for 2009 was roughly 17 times greater, at about 48,120
cubic meters (142,000 tons); in that year the large volume was dominated by
the March 2009 Ahwiyah Point rock fall. Small rock falls are much more
likely to occur than large rock falls, but large rock falls represent a far
more important source of rock fall debris. The database of rock falls and
other geologic events in Yosemite, begun in 1857, now documents over 740
events, making it one of the longest and most detailed landslide databases
in the world.

It is very likely that there were additional rockfalls in 2010, 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 at greg_stock@nps.gov, or contact Park
Dispatch. Predicting rock falls is not yet possible, but understanding the
events that do happen is an important step toward this goal. For more
information on rock falls and rock fall research in Yosemite, please see
the Park’s web page: http://www.nps.gov/yose/naturescience/rockfall.htm

Greg Stock, PhD, PG
Geologist
Resources Management and Science
Yosemite National Park

Video of Yosemite Rockfall from...a Unique Angle

When I noted the rockfall off the wall of El Capitan last Monday, I had no idea one of my former students was climbing on the cliff nearby. She and her partner had a video camera and filmed the rockfall from one of the most unique of vantage points, from on the cliff itself. This is a Facebook link, so I don't know if it will work for everyone, but check it out; it is spectacular!

http://www.facebook.com/video/video.php?v=443539259734

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...