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Twain Harte area rock exfoliation
#40236 09/12/14 05:25 PM
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The rock IS alive. Gets going at about 30 seconds. I'd be running the other way.

Rock exfoliation LIVE

A dam near the area is cracking, too.
Maybe our SN knows about that.

Re: Twain Harte area rock exfoliation
Harvey Lankford #40265 09/15/14 08:30 PM
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Very cool video, Harvey. Yes, this is one of my projects. I was trying to explain exfoliation to a journalist the other day. It's hard to describe this natural phenomena without sounding crazy. Rock just cracks by itself, yeah right, you're hiding something that's really wrong with the dam...or you wouldn't drain the reservoir...

I'm going to use this video to help explain it to the media and public. thanks

There's ample evidence of exfoliation in Yosemite and other glaciated granite areas, but I've never seen video of the process. What's happening is the internal stress in the rock is being released, causing the rock to peel off in shell layers like an onion. So what caused the internal stress? The internal stress was built into the rock when it formed by cooling deep in the ground. Then the granite was uplifted and the overlying rock was eroded away over time. This mountain building process ends up exposing the buried granite to "normal" surface pressure, but the huge stresses are still locked in. Witnesses report this sounding like gunshots when the rock cracks. The cracks we are concerned with are actually a lot wider than you see in this video.

Re: Twain Harte area rock exfoliation
SierraNevada #40335 09/23/14 03:02 PM
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this was awesome. Sorry, Salty.

Re: Twain Harte area rock exfoliation
Brent N #40378 09/25/14 08:52 PM
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I finally visited the dam last Friday. It's amazing. The cracks in that video are minor compared to others on the top and side of the dome. There's a lifeguard platform on a 1-ft thick slab that's uplifted about a foot forming a bridge you can look under. I haven't downloaded my pictures yet, I've been at a conference in San Diego, but I probably have some good ones to upload.



Here's another video from a distance that gives a better sense of the magnitude of the big events - there have been 4 big cracking events so far, and lots of minor events including one while I was there.

Skip forward to 1:09 where it sounds like thunder going off.


Re: Twain Harte area rock exfoliation
SierraNevada #44376 09/27/15 07:38 AM
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The Weather Channel will be showing a segment on this tonight (Sunday September 27). Episode "Big Bangs" "Exploding Rocks" on their series "Strangest Weather on Earth" - as if this geologic event had anything to do with weather. That aside, it should be cool to see it described in a professional production. I'm not in this one, but the geologists probably did a great job on camera. These episodes tend to get replayed a lot when there's no hurricane or tornado to cover 24/7.

Dam Update:
We were able to design and construct the repair in record time last spring. We approved it for filling and closed the outlet valves just days before a rare spring storm hit and filled it back up. This reservoir is critical to the town's economy during the summer. The local businesses were ecstatic having it fixed and full by Memorial weekend.

If anyone cares, it was repaired by isolating the foundation under the dam from the larger exfoliation slabs by drilling closely spaced holes into the granite - creating a perforated edge. So when the next event happens, the uplifting slab will break off at the "tear line" before going under the dam. Also added anchor bars into the foundation and filled in the arches with six feet of mass concrete for weight. The design checked out nicely on a finite element computer model.

Last edited by SierraNevada; 09/27/15 08:28 AM.
Re: Twain Harte area rock exfoliation
SierraNevada #44377 09/27/15 09:31 AM
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Originally Posted By: SierraNevada
The design checked out nicely on a finite element computer model.

We will not have to wait in typical slow geological time to see what Mother Nature thinks about that. :-)

SN, your PM folder is full. You and I might be the only ones following this subject. Thanks for the update.

Re: Twain Harte area rock exfoliation
Harvey Lankford #44379 09/27/15 03:08 PM
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Originally Posted By: Harvey Lankford
Originally Posted By: SierraNevada
The design checked out nicely on a finite element computer model.

We will not have to wait in typical slow geological time to see what Mother Nature thinks about that. :-)

You're right Harvey, Mother Nature could damage it again at any time. But the risk of a dam failure from an exfoliation event is extremely low and meets public safety guidelines for dams. Just like you deal with in medicine, risk from infrastructure can only be minimized, it never goes away. Living in a cave vs a developed society still carries risk, they're just different risks.


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