When Exfoliation Becomes a Hazard (Updated)
There is a really cool video of an exfoliation event at Twain Harte Lake in the Sierra near the Sonora Pass road that went on (or might still be ongoing) in August 2014. (Copied to Ars Technica and Gary Hayes Geotripper blog; a shorter wider view is on YouTube). Exfoliation is the creation of joints nearly paralleling the earth’s surface. While these have been the subject of discussion in the Sierra for a long time, actually seeing one form (let alone videoing it) is one surprise, and that there seems to be a whole swarm of exfoliation events going on. Steve Martel at the University of Hawaii has taken a major interest in the creation of exfoliation (or sheeting) joints; his 2011 paper in GRL suggests that you get these by the application of lateral compressive stresses (note these are not deviatoric but total stresses) to certain topographic geometries (basically ridge lines and saddles). He makes a good case that it is not simply unloading of the rock that creates these fractures, which is the explanation typically put forward.
The cartoon from his paper illustrates the idea: if the net sum of the compressive stresses exceeds the weight of the rock and the tensile stress of the rock, then the rock will fail in tension at the base. This actually seems like a reasonable explanation for what is seen at Twain Harte Lake, where the dam abutment where the problem arose is a small dome; the lake being in a broad topographic depression means that there is plenty of regional topographic stress near the surface. It would be interesting to know if the failures tended to be at the hottest parts of the day (we might expect expansion of the rock to add a bit more to the compressive stresses).
It still seems we are missing some pieces. For instance, profound exfoliation joints are seen on the upper parts of Half Dome in Yosemite. Just where the compressive stresses are coming from on Half Dome is a mystery to GG; about the only source would seem to be gravity at this location. (It is interesting that Martel cites an unpublished 1970 thesis as having measured surprisingly high horizontal stresses at Tuolumne Meadows; the orientation is consistent with the N60W direction of many extensional markers to the east, suggesting some connection to the regional stresses at depth).
Update 9/29/14: Kerry Leith at Munich has pointed out his own blog post about similar events in Finland this year and provides links to a somewhat different take on how these kinds of events work mechanically. In the Finnish case, the rock being deformed was still glacially striated, making this the first such event there is about 10,000 years.