USGS: Hot Days Can Trigger Yosemite Rockfalls

Of academic interest - please ignore if less nerdy than I.

usgs.gov/news/hot-days-can-…
dx.doi.org/10.1038/ngeo2686

"The scientists found that daily heating and cooling of the rock surface caused the crack to open and close by nearly half an inch."

"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."

"Our results have important implications for the triggering of rockfalls in exfoliating landscapes. First, our mea- surements indicate that seemingly static bedrock landscapes are, in fact, quite dynamic; that a 20 tonne sheet of rock can deform in and out of a near-vertical cliff face by up to 1 cm on a daily basis demonstrates the inherent instability of sheeted cliffs. Second, the observed cumulative outward deflection highlights a potential posi- tive feedback loop in promoting detachment of exfoliation sheets. Namely, as crack opening occurs, sheet curvature increases, and likewise tensile stresses (equation (6)). These changes will, in turn, promote still higher values of stress intensity and lead to propagation of fracture tips. Further opening may occur as loose blocks become wedged at the bottom of fractures, preventing full return of sheets to their original position43. Both increasing temperature and temper- ature fluctuations may also promote fracture. Thus, we expect that rates of deformation should increase for already partially detached exfoliation sheets, albeit nonlinearly. 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 Japan27, France28 , Brazil31 , Switzerland44 and Yosemite45 . In Yosemite45 , 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."

http://dx.doi.org/10.1038/ngeo2686 usgs.gov/news/hot-days-can-…

Will Mayo mentions similar in this video. vimeo.com/184963897

Pretty warm yesterday. We experienced some rockfall on Ebbetts. 

So Yosemite is not all it's cracked up to be.

Does something similar happen with extremely cold temps as well? I would assume so. Especially with the freeze/thaw cycle as water filters in between sheets of rock. Any scientific studies done on this?

EDIT: Showing off my geologic ignorance here, based on the replies below… 

Personally I have not ever seen increased rockfall in the coldest temperatures. The opposite seems true as the ice bonds things together. It certainly is putting the forces on the rock as it expands when freezing so that when it does warm the rock has become more unstable.

Whole lotta exfoliating going on!.

Even mud and munge seem to hold choss together. As it warms and dries out what once felt sound loosens. 

Geology in action!

If you’re ever climbing on the  Oblisk, South West Face, your first piece of gear is a #2 under a flake about 20 feet up. Placed in the cool morning it’s a perfect placement. When my good buddy Jason gets up to clean it, the morning sun is shining on it and he wants to know why and how I managed to cram it in like that. He tells me that he is going to leave it! Fast forward 8 hours and we are back on the ground. I run down to the base to get my piece back, cursing Jason all the way. I climb up to get it out, it’s crushed down hard and I can’t get it to budge!
Early next morning, we are packing up, I can’t stand to leave it behind. So I go down to the base again, in the cold early morning. I climb up and it comes right out, EZ PZ.

A demonstration of our living stone in action.