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gstock
climber
Yosemite Valley
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Topic Author's Original Post - Jun 8, 2010 - 07:00pm PT
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Ever had a loosely slotted nut become hopelessly stuck behind a flake? Or wondered why that previously wedged pin fell right out when you clipped it?
The National Park Service and the U.S. Geological Survey are investigating how rock flakes deform in respond to changes in ambient temperature and/or solar radiation. The main goal is not to replace the nut tool, but rather to investigate crack propagation and rockfall triggering. Many rockfalls occur in the absence of “traditional” triggers such as precipitation, freeze-thaw, earthquakes etc.; in Yosemite, the July 2006 Half Dome and August 2009 Rhombus Wall rockfalls are just two examples. Geologists have long suspected that thermal stresses may destabilize partially detached flakes, but so far little work has been done to confirm this or quantify the effects.
We have instrumented a large flake on a south-facing cliff in Yosemite Valley and are measuring changes in the crack width, as well as air temperatures and solar radiation on the flake surface. If you encounter these instruments while climbing in the Valley, please do your best not to disturb them! Thanks. We are collecting some really interesting data and would like to avoid any confounding signals (and would obviously also like to avoid vandalism of the instruments). The instruments will be up for one year, after which everything will be cleanly removed. In the meantime, I would be interested to hear any anecdotal evidence for expansion or contraction of flakes.
Although our data are preliminary - and each flake is different - the next time you get that nut stuck, you might consider returning to retrieve it in the early evening.
Greg Stock
Park Geologist
(209) 379-1420
greg_stock@nps.gov
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Mungeclimber
Trad climber
sorry, just posting out loud.
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Sweet
what about differing aspects with greater thermal variance?
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Buju
Big Wall climber
the range of light
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Congratulations for getting it up and running!
-Roger Putnam
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Jun 8, 2010 - 07:47pm PT
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Great link, thanks. That is exactly why we are undertaking this project.
Greg
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Tork
climber
Yosemite
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Thanks Greg
Been to long again
Jeff
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EdBannister
Mountain climber
CA
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what about unloading and convex surface exfoliation, isn't that the main driver of rockfall on domes?
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Jun 9, 2010 - 03:33pm PT
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We do hope to instrument another flake with a different aspect in the future. As we were initially unsure whether we would measure any change at all, we started with this one, which appears to be quite sensitive to temperaturate/solar radiation changes. The question now is whether all flakes behave like this one.
Exfoliation joints (also known as sheeting joints) are indeed one of the primary contributors to rock falls from domes and other cliffs. Whether these joints are formed by removal of overburden or by tensile stresses acting perpendicular to regional compressional stresses is a topic of debate amoung geologists; see this article for a new perspective:
http://www.soest.hawaii.edu/martel/Martel.pubs.pdf/Martel_2006.pdf
Flakes represent rock slabs that have been partially detached by sheeting joints, but have not yet fallen. We are interested in the process by which these partially detached flakes become fully detached and fall, and suspect that the diurnal and seasonal deformation observed by climbers plays an important role.
Greg
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Joe Metz
Trad climber
Bay Area
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Thermal expansion seems to happen pretty quickly. Climbing Dome Rock (Sequoia National Monument), in the morning, a #5 hex fit easily into an exfoliation crack while leading. The sunlight reached the crack while the second was climbing. By the time he got to the hex, it was wedged in tight.
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Mighty Hiker
climber
Vancouver, B.C.
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Thanks, Greg - interesting stuff.
How (if at all) will you measure the other forces that may be at work, such as freeze/thaw, rainfall, earth tremors, isostatic rebound, wind pressure, and vegetation? All are causes of rockfall, at least in popular mythology, and may be contributing causes. Also, how can you isolate the effect (the rockfall) from the cause(s) - the rockfall may happen sometime after its main cause.
Some at Squamish are paranoid about the effects of trees on flakes, and indeed about trees and shrubs generally. It's always seemed to me that other factors, especially freeze/thaw, must be the main contributing cause of rockfall, although earth tremors must contribute, such as the magnitude 7.3 earthquake on Vancouver Island in 1946. Gives everything a big shake. It would be interesting to get the rockfall areas below the Chief dated, to learn about the incidence.
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JEleazarian
Trad climber
Fresno CA
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Bump for an interesting and important thread.
John
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gstock
climber
Yosemite Valley
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Topic Author's Reply - Oct 3, 2010 - 11:40am PT
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For those interested in this topic, here is an abstract relating our preliminary results from the instrumented flake, to be presented at the Geological Society of America annual meeting in Denver next month:
QUANTIFYING THERMALLY INDUCED ROCK FLEXURE AS A POTENTIAL ROCK-FALL TRIGGER
COLLINS, Brian D., U.S. Geological Survey, Geology and Geophysics Science Center, 345 Middlefield Rd, MS-973, Menlo Park, CA 94025, bcollins@usgs.gov and STOCK, Greg M., National Park Service, Yosemite National Park, El Portal, CA 95389
Among rock fall triggering mechanisms, thermally induced flexure is likely the least understood. The mechanism, where solar radiation and temperature variation drives deformation of partially detached rock flakes, has been postulated for some rock falls, but has yet to be adequately quantified. For example, in Yosemite Valley, California, rock falls have occurred on hot summer days in the absence of any meteorologic, seismic, or other recognized trigger, but the late afternoon timing of the majority of events, when rock temperatures are maximum suggest that thermally driven rock flexure may be the cause. However, little is known about the magnitude of rock deformation during thermal stressing or whether this is sufficient for crack propagation. To address these questions, we are monitoring the deformation of a near-vertical granitic rock flake in Yosemite Valley. The flake, 14 m tall, 4 m wide and 12 cm thick, faces south and receives direct sunlight. Whereas the flake is attached to the cliff face at its bottom and top, the sides are detached from the cliff by a 10 cm wide crack on one side, tapering to a 1 cm wide crack on the opposite side. Instrumentation consists of three custom-designed crackmeters placed between the flake and the adjacent cliff face, three air temperature sensors located behind the flake, and three dual air temperature-solar radiation sensors located on the outside surface of the flake.
Five-minute interval data from summer 2010 indicates the flake undergoes maximum deformation at mid-span between attachment points and that it deforms from both diurnal and climatic temperature fluctuations. Recorded maximum deformations are 1 cm diurnally and nearly 1.5 cm (including diurnal effect) over a 5-day period of cooler temperatures. Diurnal fluctuations reach peak contraction (crack closing) in mid-morning, synchronous with low solar radiation and air temperature, and peak expansion (crack opening) in late afternoon when temperatures are maximum. These measurements demonstrate that thermally driven rock flexure is capable of deforming large rock slabs. Cumulative outward deformation with moment-inducing tensile stresses and crack tip propagation may also be occurring in this and other partially detached rock flakes in Yosemite Valley, thereby providing a potential trigger for many rock falls.
Also note that Brian Collins (first author above, and an avid Yosemite boulderer, climber, and big waller) will be giving a one-hour talk on this and other rock fall topics at the Auditorium in Yosemite Valley at noon on Tuesday, October 12.
Greg Stock
Yosemite Park Geologist
(209) 379-1420
greg_stock@nps.gov
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mongrel
Trad climber
Truckee, CA
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Wow, that's a lot of deformation! Kind of makes you think twice about gear-based bivouac anchors to make sure they're in straight-in and not flake cracks. Very interesting.
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Conrad
climber
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Thanks Greg. Great to hear about your work. The Loma Prieta Earthquake of '89 was felt in the Valley - we were enjoying a sandwich at Degnan's after an ascent of Horse Chute. We were lucky to be off the wall when it shook. There used to be a squashed Hex part way up DNB that might have been a result of this event. Is it still there?
I'll be at the GSA meeting and have noted your presentation. See you there.
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neebee
Social climber
calif/texas
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hey there greg.... say, thanks for sharing all this...
this is really interesting stuff ....
when i was just a kid, i wanted to learn all about rocks...
course, no one ever encouraged me to do so... :O
so---i learned all about babysitting, and cooking, and quite young...
:)
now, chappy sure learned about rocks, but not in the way, that
i wanted to.... :))
thanks for the neat shares... :)
god bless...
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Wade Icey
Trad climber
www.alohashirtrescue.com
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I think the technical term for this thread is Bitchen.
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Ed Hartouni
Trad climber
Livermore, CA
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very cool to see the results of your work
looking forward to reading the full article
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Jingy
climber
Somewhere out there
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if you grab a flake
and it falls off in your hand
it's deformed beyond repair
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Mungeclimber
Trad climber
sorry, just posting out loud.
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crackmeters
hehehe, he said crackmeters.
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gstock
climber
Yosemite Valley
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Topic Author's Reply - May 2, 2011 - 03:51pm PT
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Thanks Del Cross for being careful with the datalogger, and sorry that it created a second crux! I don't expect that your climbing by the logger will affect the data, but I'll make a note of it all the same.
We intend to have all of the equipment removed from the cliff by the end of this summer. I'll post up our final results when we have them. The quick summary is that the deformation of this flake is really impressive over both daily and annual cycles, and the temporal pattern of deformation might provide an explanation for a number of summertime/evening rockfalls that have otherwise lacked good explanations.
Greg
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