Twain Harte rock in California is in a period of active exfoliation, sloughing sheets of granite in creaking, cracking, rock-flaking, dust-shedding, exhilarating glory.
Explosive exfoliation! Image credit: dotysan/Mika McKinnon
Geologic processes happen on inconvenient timescales, either so slowly it's hard to notice, or catastrophically quickly in a manner decidedly hazardous to observers. But sometimes we get lucky, and are in just the right place at just the right time to watch something incredible without dying in the process.
Exfoliation is a form of mechanical weathering, breaking intact rock into smaller pieces.
The classical explanation is that exfoliation is a form of pressure-release. Rocks can transition from being under pressure to no longer being under pressure by being uplifted, or having overburden removed through erosion, excavation, or even glacial retreat. When the pressure is released, the previously-confined rocks expand.
Pressure release exfoliation. Image modified from J. Merck.
Alternately, exfoliation may be the consequence of gravity acting on a curved surface, creating tension and compression beneath the dome. Smaller-scale exfoliation events can also be attributed to diurnal temperature changes causing stress from uneven expansion and contraction.
The end effect is the same either way: rocks are hard, not stretchy, so a change in shape causes brittle outer layers crack in sheets subparallel to the surface. Exfoliation joints shed as sheets, with layers of rock peeling off like onion skins. As the curved slabs spall off, they leave behind a spherical core.
Half Dome is shaped by exfoliation weathering and glacial scour. Image credit: HylgeriaK
Over time, exfoliation leads to beautiful, smooth rounded surfaces. One of the most photogenic examples of exfoliation is Half Dome in Yosemite. But a new location is quickly displacing it as a new favourite case-study in exfoliation: Twain Harte rock. Twain Harte is pretty, but certainly not spectacular enough to displace Yosemite. What makes it special is that we have video of exfoliation in all its creaking, cracking, dust-spewing, rock-breaking unstable glory.
Twain Harte rock is in the Sierra Nevada foothills of California. It's a beautifully exposed slab of granite, and is the nice, solid dome of bedrock adjacent to the concrete and metal Twain Harte Lake dam. At least it was solid, until it started shifting and cracking in early August.
Weathering is typically described as a "slow, unspectacular process," so when word spread that Twain Harte rock was being decidedly quick and exciting with its exfoliation, the geo-curious swarmed to see. Geologist Garry Hayes headed out for an unofficial field trip. He posted photos and his observations on Geotripper, describing freshly loosened slabs, chipped edges, and newly-deformed surfaces.
At least three periods of active exfoliation have occurred: August 3rd, August 6th, and August 20th. The initial event on August 3rd triggered a fury of consternation and a flash flood warning. Local officials quickly drained the lake after leaks were accompanied by decidedly unnerving pops, cracks, and bangs from the adjacent bedrock. Staff from Condor Earth Technologies were investigating how the situation was impacting the dam during the second event, catching it in video.
Exfoliation is startling enough to make anyone back up rapidly. Image credit: Condor Earth Technologies/Mika McKinnon
The gentleman rapidly back-peddling from the bursting rock in the Conor Earth Technologies video has posted four clips from his viewpoint of the escalating activity. It starts off slowly with our videographer revelling in gentle curiosity and reacting to slowly-widening cracks with a friendly, "Hello there, fella!" The next video in the sequence zooms in on the tiny flakes bursting free, popping like those first few kernels of popcorn. The third provides more context views, showing some of the massive layers that have broken free over the previous few days. It's also the calm before the storm as stress builds up. Finally, the moment rupture (have your sound on!):
As stress builds up, the rock first buckles and bends under elastic deformation, then finally breaks free in brittle failure. The sudden release in cracks and joints relieves the stress at that point in the rock, flattening the buckled rock while simultaneously increasing stress even farther in any place that has not yet moved. That stress overcomes both rock strength and friction, breaking bigger cracks and slabs free.
The last observed exfoliation event was on August 20th. It is uncertain if the rock has reached its new equilibrium state, or has more slabs of rock to shed before settling down. For now, the reservoir remains drained, and will continue to be empty until the situation stabilizes and funds are raised to repair the new cracks.
I've learned about exfoliation weathering more times than I can count, and given more than a few lectures on the topic. I capture photographs of exfoliated rocks I stumble across in the wild, and occasionally go out deliberately seeking scenic vistas with beautifully rounded domes. But I never thought I'd see exfoliation happening in real-time, hearing the ground crack and rumble as it stretches out of too-tight rock layers. I had no idea it'd be so explosive! This is amazing.
For more startling geology caught on video that didn't kill anyone, did you see the shockwaves from the Mount Tavurvur eruption?