Foy
Resident Geologist
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
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