Geology in action: Twain Harte Lake, CA

[Foy]

Just last night I saw some video on The Weather Channel which amazed and delighted me: actual footage of exfoliation of a granite body at Twain Harte Lake, CA. I am most certainly not delighted over the problems related to dam safety, etc, but the opportunity to see this process on video is amazing.

The knobby rounded outcrop of many granites is an outgrowth of a weathering process known as exfoliation. It happens all over--there are exfoliated outcrops of the Rolesville Granite pluton within just a few miles of my Raleigh, NC home. An often-visited area featuring weathering involving exfoliation is the Alabama Hills in CA.

Since true granites are plutons which are emplaced beneath the surface and thus do not extrude to become volcanic rocks, their lives begin underground, ordinarily well below the surface. As erosion planes off the rocks above the emplaced granite, huge amounts of weight are removed from the granite, as well. The granite often fractures in response to the unloading, and often the fractures are "onion skin" in shape. Groundwaters and meltwaters can then work in between exfoliated sheets and the main mass of granite and mechanical weathering then occurs as the freeze-thaw cycle occurs.

The video of fracturing occurring at Twain Harte was shot in the morning as the sun really got cranked up and warmed the surface of a large "pavement" outcrop right by the dam. There is even a shot of the "tent" shaped exfoliated piece on which a lifeguard tower is perched.

Absolutely amazing! The video broadcast on the Weather Channel and others can be searched at will.

Foy

 

[Alley-Kat]

And, all of this happens because water (H2O) is the only naturally occurring substance that we know of that "expands" when it get cooler, colder, or freezes. All other naturally occurring substances "contract" when they get cooler, colder or freezes.

 

[Bseek]

Happy to report that the lake is now refilled. Not sure what they did to repair the crack, but everyone seems reassured it is safe enough for those downstream. Filling the lake during a drought was a bit contentious as downstream people objected to the "loss" of water while it filled, but CALFire said they need the water storage in case of wildfire, and the community needs the tourism dollars generated by a full lake. Private Lake + Public water diversion = Crazy Contentious Meetings. Glad it is over.

If you haven't seen the footage, it is pretty amazing to see "Geology in action" since most things happen on a timescale incomprehensible to the average Joe.

 

[Foy]

And, all of this happens because water (H2O) is the only naturally occurring substance that we know of that "expands" when it get cooler, colder, or freezes. All other naturally occurring substances "contract" when they get cooler, colder or freezes.

Correct to the greatest extent. The initial "onion skin" fracturing generally occurs due to unloading. Once groundwaters and surface waters occupy the fractures, freeze-thaw expansion gets to work in climates where such cycles are routine throughout the winters. In climates without extensive long-term freeze-thaw cycles, spherioidal weathering still occurs due to groundwaters and surface waters attacking the granite's crystalline structure and chemically altering the feldspars and dark minerals to clay minerals. So, the Southeastern US granite batholiths, plutons emplaces as far back as the Ordovician, and not exposed to lengthy freeze-thaw cycles for long many tens of thousands of years, exhibit spheriodal weathering.

A really great example of spheroidal weathering outside of California's Alabama Hills and outside of the Southeastern US is the Boulder Batholith in west-central Montana. Where I-90 crosses Homestake Pass just east of Butte, the Boulder Batholith crops out for miles, and it's just a huge jumble of huge rounded boulders. The Humbug Spires within the Highland Range just south of Butte are within a pluton just outside of the outcrop of the Boulder Batholith and likely related to it.

Foy