Yosemite rock falls

The cover photo is a great one of the October 11, 2010 rock fall and huge cloud of dust rising, from the face of El Capitan.

From the Yosemite Daily Report:

Yosemite Rockfall Year in Review: 2013

Following the trend of the previous two years, 2013 was a quiet year for rockfalls in Yosemite.  In all, there were 34 documented rockfalls, which is well below the recent (2006-2012) average of about 55 documented rockfalls per year.  The cumulative rockfall volume was about 1500 cubic meters (about 4460 tons), also well below average.  Given that 35% of rockfalls occurring since 1857 were associated in some way with precipitation (rain or snowmelt), the fact that 2013 was arguably the driest year on record for California may explain the relatively low level of rockfall activity.

The two most consequential rockfalls in Yosemite in 2013 impacted trails.  The first occurred early on the morning of May 11 when a block about 320 cubic meters in volume (about 950 tons) fell from the top of the Panorama Cliff and landed on the John Muir Trail below Clark’s Point.  Rock debris covered about 500 meters (1600 feet) of switchbacks, and the trail was closed for several weeks while damaged walls were repaired and the trail cleared. 

The second consequential rockfall occurred at about 5 am on the morning of October 5 when a block of about 735 cubic meters (about 2200 tons) fell from Ahwiyah Point north of Half Dome; this was the largest rockfall of 2013.  Ahwiyah Point experienced a very large rockfall in 2009 and has had intermittent activity since then.  The rockfall on October 5 occurred from the same location as the 2009 event, and also curiously occurred at the same time of day.  This early morning timing, combined with the fact that it occurred during the government shutdown, ensured that there were no injuries associated with the rockfall.  The Mirror Lake Loop trail, recently reopened after extensive repairs following the 2009 event, experienced some minor damage from boulder impacts.  This trail was also closed for several weeks while the source area was monitored and the trail cleared.

Other areas in Yosemite experiencing rockfalls in 2013 include Middle and Higher Cathedral Rocks, the Porcelain Wall west of Half Dome, Glacier Point, Royal Arches, and El Capitan. 

It is very likely that there were additional rockfalls in 2013, 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 by email at greg_stock@nps.gov, or contact Park Dispatch by dialing 911 within the park. Documented rockfalls are added to the park database (http://pubs.usgs.gov/ds/746/), which enables evaluation of rockfall activity to help improve public safety. (G. Stock)

Yosemite Rockfall Year in Review: 2012

2012 was a relatively quiet year for rockfalls in Yosemite. The most consequential event occurred at about 11:00 pm on January 22 above the Big Oak Flat Road. A boulder some 250 cubic meters in volume (about 745 tons) fell from a cliff above the road, slid down the slope below the cliff, and struck the Big Oak Flat Road, punching a hole in the eastbound lane, tearing out some of the retaining wall, and severely damaging nearly 30 m (100 feet) of roadway. The road was closed for about six weeks for repairs.

Consequential rockfalls also occurred from the Church Bowl area, located between Yosemite Village and the Ahwahnee Hotel. On April 3 and 4, two nighttime rockfalls occurred from the cliff directly above the popular climbing route “Bishop’s Terrace”. The area beneath the climb was devastated by rock debris, with dozens of trees snapped or toppled by the impacts. Small rocks tumbled all the way to the Valley Loop Trail. Climbing routes in this area were temporarily closed until the cliff stabilized.

Other areas in Yosemite experiencing rockfalls in 2012 include Mirror Lake, El Capitan, Ahwiyah Point, Half Dome, Glacier Point, Elephant Rock, Washington Column, and Hetch Hetchy.

In all, there were 43 documented rockfalls in 2012, with an approximate cumulative volume of about 700 cubic meters (about 2,100 tons). Both the number of rockfalls and the cumulative volume for all rockfalls in 2012 are substantially less than that documented in recent years. Although it is not clear exactly why rockfall activity was reduced from previous years, it may relate to the exceptionally dry winter of 2011/2012.

It is very likely that there were additional rockfalls in 2012, 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 email him at greg_stock@nps.gov , or contact Park Dispatch by dialing 911 within the park. Predicting rockfalls is not yet possible, but understanding the events that do happen is an important step toward this goal. (G. Stock)

Yosemite Rock Fall Year in Review: 2011

Although not as newsworthy as those of recent years, many significant rock falls occurred in Yosemite in 2011. Probably the most spectacular rock falls originated from the north face of Middle Cathedral rock in July. Three distinct rock falls occurred over a period of several weeks, with a total volume of about 120 cubic meters (about 325 tons). Witnesses in El Capitan Meadow captured impressive photos of the falls and subsequent dust clouds. Fortunately rock debris was contained to the talus slope beneath the cliff and did not affect nearby trails or roads. The Middle Cathedral rock falls occurred during very warm conditions and may have been triggered by thermal stresses. Ongoing monitoring by the National Park Service and U.S. Geological Survey indicates that air temperatures can cause partially-detached rock flakes to deform substantially on both daily and annual timescales.

Another area of notable rock falls in 2011 was Yosemite Falls. On 10 September 2011 a rectangular boulder 2 cubic meters in volume (6 tons) fell from below the upper Yosemite Falls Trail, shattered on a bedrock slab, and fell into the Lower Yosemite Falls amphitheater. Small rock fragments landed in the vicinity of the Lower Yosemite Falls footbridge, but there were no reported injuries. A second and much larger rock fall occurred on 28 October 2011, originating from near the lip of Upper Yosemite Falls. This block, about 20 cubic meters in volume (55 tons), fell from beneath an overhanging roof, skimmed the cliff, and then shattered on impact with the bedrock at the base of the upper falls. Off-trail hikers were present at the base of the falls but avoided injury.

The two most consequential rock falls in 2011 affected park roads. The largest event was not a true rock fall but rather a mass of broken rock and soil that slid onto Foresta Road between Old El Portal and Rancheria during the heavy rain and snow in late March 2011. The volume of this slide was about 1,200 cubic meters (about 3,200 tons). Foresta Road was closed for several weeks as the slide was cleared, and is presently closed again to accommodate engineering of the slope to reduce future slide potential. In the early morning of 7 November 2011, a large boulder (about 18 cubic meters, or 50 tons) and several smaller boulders tumbled from the north wall of the Merced Gorge just east of Arch Rock and embedded themselves in the El Portal Road. Following assessment of the source area, the boulders were removed and the road repaired in time for the morning commute into Yosemite Valley.

Other areas in Yosemite experiencing rock falls in 2011 include Royal Arches, Ahwiyah Point, Half Dome, Glacier Point, the Rockslides, and Hetch Hetchy. In all, there were 53 documented rock falls in 2011, with an approximate cumulative volume of 2,200 cubic meters (6,000 tons). This volume is comparable to the volume that fell in 2010 (2,900 cubic meters) but much smaller than what fell in 2009 (some 50,000 cubic meters); the volume in 2009 was dominated by the large Ahwiyah Point rock fall. Our database of rock falls and other geologic events in Yosemite, begun in 1857, now documents nearly 900 events, making it one of the longest and most detailed landslide databases in the world.

It is very likely that there were additional rock falls in 2011, 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 by dialing 911 within the park. 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. (G. Stick - 1/23)

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  (G. Stock - 01/10/11)

At the above webpage you will find causes of rockfall, what to do in the event of a rockfall, pictures and a map of 154 years of Yosemite valley rock falls (from 1857 to 2009)

2009 Rockfall Year in Review:

Several notable rockfalls occurred in
Yosemite Valley in 2009.  By far the largest event was the Ahwiyah Point
rockfall on March 28, which originated near the summit of Ahwiyah Point
northeast of Half Dome.  This rockfall had an approximate volume of 45,300
cubic meters (about 134,000 tons), making it the largest rockfall in
Yosemite Valley in 22 years (considerably larger than the 1996 Happy Isles
rockfall).  The impact of the falling rock on the floor of Tenaya Canyon
destroyed hundreds of trees and generated ground shaking similar to a
magnitude 2.5 earthquake.  Rocks continued to fall from this area for many
weeks after the initial failure.  The southern portion of the Mirror Lake
loop trail, buried by rock debris, remains closed.

The other area of notable rockfall in 2009 was the Rhombus Wall immediately
north of the Ahwahnee Hotel.  On August 26 a series of five rockfalls fell
from midway up the Rhombus Wall, with the largest occurring at
approximately 1:30 pm.  Large boulders reached the edge of the talus slope,
and several vehicles in the Ahwahnee parking lot were damaged by smaller
rock fragments.  The hotel was evacuated for 48 hours to allow for
geological assessment.  Over the next two weeks, loud cracking and popping
sounds were heard emanating from the cliff, suggesting propagation of
cracks within the rockfall source area.  This activity culminated in
another rockfall from the source area on September 14, roughly three weeks
after the initial failure.  The cumulative volume of the numerous rockfalls
is estimated to be about 1,200 cubic meters, or roughly 3,600 tons.
Twenty-nine parking spaces in the Ahwahnee parking lot were permanently
closed as a result of these rockfalls and the subsequent hazard assessment.

Other areas in Yosemite experiencing rockfalls in 2009 included El Capitan,
Glacier Point, Half Dome, Royal Arches, Cathedral Rocks, and Wapama Falls
at Hetch Hetchy.  In all, there were 52 documented rockfalls in 2009, with
an approximate cumulative volume of 48,120 cubic meters (142,000 tons); the
vast majority of this volume was associated with the Ahwiyah Point
rockfalls.

It is very likely that there were additional rockfalls in 2009, but these
events either were not witnessed or went unreported.  If you witness a
rockfall of any size, or if you 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
rockfalls is not yet possible, but understanding the events that do happen
is an important step toward this goal. (G Stock 1/25/10)

Rockfall Year in Review:  2008

Rockfall dramatically influenced human activities in Yosemite Valley in
2008.  By far the largest and most consequential rockfalls of 2008 occurred
on October 7 and 8 from above the Ledge Trail near Glacier Point.  Repeat
laser mapping revealed the total volume of these rockfalls to be 5738 cubic
meters (17,080 tons).  Though not particularly large by Yosemite standards
(for comparison, the July 10, 1996 Happy Isles rockfall was about 5 times
larger, and the March 10, 1987 Middle Brother rockfall was more than 100
times larger), the October 7 and 8 rockfalls were consequential because of
their proximity to Curry Village.  Occupied cabins were extensively damaged
by rockfall debris and three minor injuries resulted.  A comprehensive
geologic assessment led to the permanent closure of nearly 300 buildings in
the Curry Village area.  The other area of notable rockfall in 2008 was El
Capitan, where a ~35 cubic meter (104 ton) rockfall occurred from low on
the southeast face on July 7, and a ~80 cubic meter (238 ton) rockfall
occurred from high on the southwest face on November 4.  Other areas
experiencing rockfalls in 2008 included Yosemite Falls, Middle Cathedral,
the Four Mile Trail, Middle Brother, Washington Column, and Basket Dome.
In all, there were 37 documented rockfalls in 2008, with an approximate
cumulative volume of 6385 cubic meters (19,003 tons).

It is very likely that there were additional rockfalls in 2008, but these
events either were not witnessed or went unreported.  If you witness a
rockfall of any size, please contact park geologist Greg Stock at (209)
379-1420, or at greg_stock@nps.gov. ; Predicting rockfalls is not yet
possible, but understanding the events that do happen is an important step
toward this goal. (G. Stock - 1/12/09)

Here are rockfall safety tips from the Wilderness Safety Action Team.

This text is from the official wording on the rock fall closures posted at trail barricades, but applies to all of us, all the time.

ROCKFALL POTENTIAL IN THE PARK

Rocks and/or ice can fall at any time. While hiking trails, avoid
lingering near talus or steeper slopes, particularly in winter. If you
observe a rockfall in the area, call 911 or 209/379-1992 with information
on the time, location, and duration of the event.

BE RESPONSIBLE—BE SAFE

Rockfalls are a dynamic—and dramatic—natural process. But it is impossible
for the park to monitor for every potential rockfall. In Yosemite, and in
any natural area, it is up to visitors and employees alike to be aware of
their surroundings and enjoy the park safely.

Rockfalls are dangerous and can cause injury or death. Use caution when
entering any area where rockfall activity may occur, such as Valley walls,
climbing areas, or talus slopes.

LEARN MORE

Winter is one of the most active periods for rockfall activity in Yosemite
National Park. As temperatures fall in the evening and warm up during the
day, cracks in the granite can widen, eventually causing rocks to separate
and fall.

To learn more about rockfalls and geology in Yosemite National Park, stop
by any visitor center for an information sheet or visit online at
www.nps.gov/yose. (L.Boyers)

From the Daily Report - Yosemite National Park Tuesday, March 6, 2012

25th Anniversary of the Middle Brother Rockfall

Saturday, March 10, marks the 25th anniversary of the 1987 Middle Brother rockfall, the largest rockfall in Yosemite's documented history.

On the afternoon of March 10, 1987, Ranger Jim Tucker received a call from Road Crew Foreman Ralph Parker, who reported that a road crew patching potholes on Northside Drive below the southeast face of Middle Brother had witnessed several small rockfalls and had heard loud cracking noises coming from the cliff face. Upon arrival, Tucker and Ranger Kim Aufhauser witnessed another small rockfall. Tucker, sensing a growing hazard, instructed Parker and his crew to leave the area, and ordered westbound traffic stopped at Camp 4. Ranger Phil Hibbs located Trail Crew Foreman Jim Snyder and the two set up a spotting scope in Leidig Meadow to observe the release point. As the cars piled up behind Camp 4, Tucker received a radio call from Chief Ranger Roger Rudolph, who suggested that Tucker strongly consider reopening Northside Drive. As Tucker and Aufhauser returned to Camp 4 to discuss the situation with Rudolph, they turned back many visitors, including a pair of skeptical hikers who were ordered to leave the area. At they proceeded toward Camp 4, Tucker and Aufhauser heard a tremendous thundering sound.

At 2:47 pm, a huge slab of rock broke loose from the top of Middle Brother. Snyder later described the cliff “unfolding like the stairs of an escalator”. The giant slab disaggregated into a rock avalanche as it hit a prominent ledge and cascaded to the talus slope below. As boulders smashed into the talus slope and sent dust and debris outward, Hibbs yelled, “Run!” Snyder, wanting to run but also wanting to watch the event unfold, tried to do both until he and Hibbs were enveloped in dust and small rock fragments. Stumbling blindly, they made their way to back to Tucker, who was preparing to send out a search party. When the huge dust cloud finally cleared, approximately 180 feet of Northside Drive was covered in rock debris up to 12 feet deep. Several boulders had landed on the far side of the Merced River. There were no injuries, and, aside from a patrol car dented by flyrock, no property damage.

Another large rock fall occurred later that day at 5:10 pm, and smaller rockfalls went on for weeks. The total estimated volume of the rockfalls was 600,000 cubic meters, or 1.4 million tons. Northside Drive was closed for months, and traffic was rerouted to Southside Drive; in anticipation of future rockfalls, reversible detour signs are now posted at key intersections to quickly establish two-way traffic. The 1987 Middle Brother slide provided impetus for Valley Rangers and Search and Rescue to plan and train for large rockfall events, and also initiated a scientific process for documenting and monitoring rockfalls. A triggering mechanism for this rockfall was never identified. (Thanks to former NPS employees Jim Snyder and Jim Tucker for sharing their recollections of this event.) (G. Stock)