Sunday, April 5, 2009

A Final Look (for now): The Ahwiyah Point Slide in Yosemite


When the Ahwiyah Point Rockfall impacted after a tumble of 1,800 feet, the slabs and boulders of granitic rock exploded, producing an airblast that is reported to have flattened hundreds of trees (and also producing a magnitude 2.4 earthquake). It also pulverized the rock. Completely. As I walked on the north side of the Mirror Lake Loop Trail, I didn't see anything right away, but then I nearly slipped and fell on a granite boulder that would normally provide fine traction. I looked and realized the boulders around me were covered with a fine powder up to a quarter inch thick. It looked like volcanic ash, and it occurred to me and my students that the dust would actually be chemically indistinguishable from rhyolite tuff (granite and rhyolite are coarse- and fine-grained varieties of silica-rich igneous rock). It really didn't even feel gritty, and it got all over our shoes and pants.

We were on the far outer periphery of the impact zone, so only a few weakened tree trunks were knocked over. Several are shown in the second photo.

That's all I have on the slide for now. I hope to be able to get closer to the impact zone on my next trip in the area.

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