Tuesday, November 11, 2014

Geo 730: November 11, Day 680: Wood, Quartz, Agate and Ash

Above is a closer shot of the same general area on the petrified log at the Cascade Locks Visitor Center as in the previous post. Looking even more closely in a crop from this photo, there's some very pretty agate with a botryoidal (grape-like) texture.
I've been meaning to discuss just why these ash-rich sedimentary environments tend to have such rich and well-preserved fossils. This is not just a feature of the Cascades; many renowned fossil localities are in ash-rich sediments, including Petrified Forest National Park in Arizona. Volcanic ash is, in large part, simply broken glass. Glass is geologically unstable- it would "rather" be in the form of quartz and feldspars. In other words, those minerals are more stable. If there is organic material nearby, SiO2 (silica) will dissolve from the glass and re-precipitate as quartz in and on the organic material. I'm not familiar enough with the geochemistry of silica and organic matter to say why the latter is a preferred nucleation site for the former, but my petrology professor said that silica is more soluble in bases, while organic matter tend to decay to acidic end products. This would mean silica tends to dissolve in areas away from the organic material, and precipitate nearby. I can say from experience that in ash-rich environments, preservation appears to be rapid, thorough, and with exquisite retention of cellular-level detail.

Photo unmodified. October 10, 2012. FlashEarth Location.

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