Archive for the ‘Geology’ Category

Half Dome Geology

August 30, 2011

Half Dome – Yosemite Musing

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  I just bought the latest book by the Park geologist, Greg StockGEOLOGY Underfoot in Yosemite National Park. It’s a great layperson’s presentation of the geology of Yosemite. He really makes this subject clear and interesting. I was “kinda’ disappointed at the meager coverage of Half Dome’s origins. So I called him.
   First, I asked where the USGS marker is on Half Dome. I (and many of you) have not seen it. Most every other peak there has one. He said he didn’t know but would ask his USGS buddies. Next, I asked several questions about the formation of Half Dome and that it would be really neat to see a cartoon video of its formation with the granite plutons, erosion, glaciers and exfoliation. He politely replied that there is not a lot of firm scientific data to bank on. Seems there are just several theories. My version is that molten magma rose from below and stopped miles below the surface. It hardened slowly and chemically was transformed into granite under the high pressures underground. As it cooled, it expanded and resembled a deck of cards that you bend up in the middle. Then eons of erosion brought it to the surface. The “card deck” of plates became vertically oriented at the edges of the “dome” that fractured along a vertical joint in the larger field of granite that runs through to Glacier Point. This could have been accelerated by the freezing of rain that expanded and grew to push the fragmentation along.

   Greg said that version is as accurate as any other guess. He did say that erratics are found in the area of the “Diving Board” – the formation that Ansel Adams took his classic Half Dome pictures from. He said the rock on material of the erratic is found far away at Cathedral Peak and it’s proof that the glaciers only extended that high. About 700 feet below the summit. Glaciers certainly did erode the base of Half Dome, with the debris long ago carried down river.
    Another fun thing he told me was that scientists don’t know how the jumble of rocks at the Visor came to be. You may have seen or even gone inside the “rock cave” (where lightning struck fatally in 1985). It could be that lightning strikes with enough energy moved the slabs. Or perhaps icy, freezing snows could have picked  them up and moved them, Fascinating. So, is it all clear as mud?

     A tip of the hat to Greg Stock! He’s one of just a couple full time geologists at National Parks. 

Unrelated thought worth quoting:  “People try to put us d-down Just because we g-g-get aroun. Things they do look awful c-c-cold.  I hope I die before I get old.” – The Who

*MrHalfDome™ – Rick Deutsch –