Discussion Topic |
|
This thread has been locked |
HighTraverse
Trad climber
Bay Area
|
|
A very coarse explanation of summer exfoliation.
First consider the thickness of the slab that apparently fell in the 2nd day rockfall. I don't know how thick it was (I doubt anyone can know for certain) so let's take a guess of 2 feet as this is a thickness I have commonly encountered on El Cap dihedrals.
Now let that slab sit all night so that the temperature on the outside surface goes down and becomes closer to the inner surface temp. It was a clear night so surface radiation from the slab was a maximum.
Now the sun comes up in the Valley and illuminates the south facing slab. It was warm by midday. In Yellow Pines it had been a clear and chilly night. I'd had to pull on a sweater in my summer weight bag. By mid morning I was in shorts and tee shirt. The day/ night temp difference was large.
So now we're late afternoon and the outside of the slab has been warming up for 6 or more hours. This causes the outer surface to expand not only in thickness but also in height and width . The slab is dozens of feet wide and several times that high. So the sun facing side is growing wider and taller. However the inner side of the slab which likely has thin gaps between the flake and the wall, is expanding much more slowly. Because the thermal conductivity of granite is low the inner surface remains much cooler. This differential expansion between the inner and outer surfaces causes the entire slab to warp in all directions! This pulls the entire slab away from the wall. Just a little bit. Every day/night cycle. The warpage being determined by the day/night temperature difference. Day after day after day. This can/will cause micro fractures in the slab and between the slab and the "rock" to get longer every day/night cycle.
It may take thousands of day/night cycles but inevitably the slab will finally detach. Most likely late afternoon in autumn when the day/night temp differences are greatest. My photo of the dust cloud is Sept 28 3:39:54 PM. late afternoon.
This mechanism is least likely in winter as the temperature differences are less and change less rapidly. Of course in winter there is expansion of ice behind the slab but that's a different mechanism.
Autumn is everyone's favorite time to be in the Valley, whether tourist or hiker or climber. Cool nights, warm days, minimal chance of precipitation. Fall colors. What could be finer?
|
|
gstock
climber
Yosemite Valley
|
|
We actually do know the thickness of the slab that fell on the second day (September 28); it was an average of about 3 meters thick, tapering to zero at the edges and with a maximum thickness of 8.4 meters in the center.
A three-year study of an exfoliation flake near the Royal Arches yielded results generally consistent with your idea:
https://www.nps.gov/yose/learn/nature/upload/Collins-Stock-2016-NatureGeoscience.pdf
Here are a few of the key ideas relating to Yosemite rockfalls:
"Exfoliation of rock deteriorates cliffs through the formation and subsequent opening of fractures, which in turn can lead to potentially hazardous rockfalls. Although a number of mechanisms are known to trigger rockfalls, many rockfalls occur during periods when likely triggers such as precipitation, seismic activity and freezing conditions are absent. It has been suggested that these enigmatic rockfalls may occur due to solar heating of rock surfaces, which can cause outward expansion. Here we use data from 3.5 years of field monitoring of an exfoliating granite cliff in Yosemite National Park in California, USA, to assess the magnitude and temporal pattern of thermally induced rock deformation. From a thermodynamic analysis,we find that daily, seasonal and annual temperature variations are sufficient to drive cyclic and cumulative opening of fractures. Application of fracture theory suggests that these changes can lead to further fracture propagation and the consequent detachment of rock. Our data indicate that the warmest times of the day and year are particularly conducive to triggering rockfalls, and that cyclic thermal forcing may enhance the efficacy of other, more typical rockfall triggers."
"Finally, our results offer a potential explanation for rockfalls that have no recognized trigger despite sometimes detailed observation at the time of failure. These include records of spontaneous summertime rockfalls in Japan, France, Brazil, Switzerland, and Yosemite. In Yosemite, a disproportionate number (15%) of rockfalls with either an identified thermal stress trigger or an unrecognized trigger occur during the hottest summer months (July through September) and at the hottest times of the day (12:00 through 18:00 PST) compared to what would be expected under a random distribution (6%). We suggest that cyclic thermal stresses might be the trigger for these rockfalls and potentially many others around the world, highlighting the role of temperature in eroding steep landscapes."
|
|
phylp
Trad climber
Upland, CA
|
|
Wow, what exciting science, Greg!
|
|
HighTraverse
Trad climber
Bay Area
|
|
3 meters......Holy Crap!!!
|
|
looks easy from here
climber
Ben Lomond, CA
|
|
Just reiterating what's already been said, but thanks for sharing all this with us Greg. You do a great job making is simple for us non-geology folks to understand, but without losing too many of the important and interesting details or condescending to us.
|
|
eeyonkee
Trad climber
Golden, CO
|
|
I just knew there was some bigger underlying reason why roofs in Yosemite granite always gave me the willies.
|
|
BLUEBLOCR
Social climber
joshua tree
|
|
Here's a good pic of that second fall
Yeah Thanks for all the great beta Greg! and it was nice to meet you in the meadow.
|
|
c wilmot
climber
|
|
I have always wondered what effect roots/vegetation has on rockfalls.
|
|
im kin
Social climber
one step ahead of ruin
|
|
dude they f*#king grow and that means they take up more volume
and that means they displace any adjacent entity that is of weaker stature.
i could draw some political parallels but let's keep this scientific.
|
|
c wilmot
climber
|
|
Similar things could be said about the role thermal expansion plays in creating rockfalls im kin. Having seen several Yosemite rockfalls mid summer it's a subject I am interested in. Obviously root growth causes displacement. What I was getting at without saying it is that the majority of root growth occurs- mid summer. During the time of said "spontaneous" rockfall
Let's keep this civil...
|
|
Mighty Hiker
climber
Outside the Asylum
|
|
Greg: The photo wasn't from Pete Zed. It was from someone else, of IIRC of the rockfall of September 27th or 28th, from the meadow or such. Somewhere well to the side. All it showed were several quite large rocks that appeared to have been projected horizontally from the cliff, although admittedly perspective can be deceiving. It was posted to Facebook, and I commented about it, and the OP specifically said he had sent it to you.
And, unrelated marvel of marvels, my confuser seems to have learnt Norwegian again. Æ ø Å
|
|
TLP
climber
|
|
What I was getting at without saying it is that the majority of root growth occurs- mid summer.
Not so; and especially not so in the Mediterranean climate (wet in the cold season, dry in the warm season) of California. Roots aren't the culprit in Yosemite rockfalls (or hardly any others, either).
|
|
c wilmot
climber
|
|
You are right tLp- nothing grows mid summer in ca. Everyone knows Yosemite turns green during its lush winter. Realy- it's the ideal light cycle the mid winter months provide that really gets vegetation growing.
And thank you for you tireless research proving that no Yosemite rockfall has ever been caused by root growth
|
|
Contractor
Boulder climber
CA
|
|
We were just across the wall during both rock slides on "New Dawn". I was cold in my sleeping bag during the predawn hours and shortly after the sun hit the wall I was taking my shirt off. I've never been exposed to such rapid temperature changes.
|
|
gstock
climber
Yosemite Valley
|
|
According to Yosemite's rockfall database, root wedging accounts for about 7% of documented rockfalls, although there could be some unidentified root-related events hiding within the "unknown" and "unrecognized" categories. Unknown means that no information about environmental conditions exists (usually rockfalls from the late 1800's), and unrecognized means that there were no obvious environmental triggers. Precipitation is the dominant trigger in terms of the number of rockfalls, but not dominant in terms of the volume of rockfalls.
|
|
limpingcrab
Trad climber
the middle of CA
|
|
That's a cool chart, thanks for sharing.
Just curious, how do the categories work that aren't mutually exclusive? "Rain on Snow" and "Precipitation" for example.
Is it sort of primary cause, rather than specific cause? Or maybe most influential cause?
No worries if you're busy, just a passing question from the data analysis dork side of me. I like stats.
|
|
gstock
climber
Yosemite Valley
|
|
We tried to be as specific as possible as to the environmental conditions present at the time of the rockfall. So "rain on snow" is really a subcategory of precipitation, but it contains more specific information, should that prove useful later on.
The thing to remember is that these trigger assignations are based simply on environmental correlations, and are not necessarily causations. Identifying triggers also requires some judgement, and that can get tricky. For example, what to list as the "trigger" when a brief rain barely wets the rock and a rockfall happens (say, from beneath a tree) as the sun comes out an hour later?
|
|
Big Mike
Trad climber
BC
|
|
Hi Greg, Thanks for all your work. I really enjoy learning about the cliffs where I like to hang.
Cool graph! How does lightning cause rockfall?
|
|
'Pass the Pitons' Pete
Big Wall climber
like Ontario, Canada, eh?
|
|
|
Feb 28, 2018 - 08:47pm PT
|
Well, f*ck me, it's good to still be alive!!
|
|
Mighty Hiker
climber
Outside the Asylum
|
|
|
Apr 30, 2018 - 10:04pm PT
|
It will be interesting to see a photo of the rockfall area from the spring, once it dries out, to see what can be seen, also if there's been further rockfall there.
|
|
|
SuperTopo on the Web
|
Let us know!