Thursday, July 17, 2014

Geo 730: July 16, Day 562: Wood, Ancient and Modern

A final shot of the petrified (permineralized, to be technically correct) tree stump closest to the intersection at the Sweet Home Community Museum. On the left side is an example of much younger wood, holding up the protective roof sheltering the stump. Other than the petrified wood itself, there are two features of interest to me.
In the lower right portion of this crop, some of the surrounding rock matrix can be seen, still clinging to the outside of the stump. It's angular, suggesting it wasn't transported far, and poorly sorted, which suggests a debris flow. You can't tell directly from the photo, but I assure you, the clasts are volcanic in origin. So I'd guess the tree was killed and buried in a lahar. Also, in the above right of this crop, and... the middle of this one, there's some very pretty secondary silica, likely chalcedony, a microfibrous form of quartz. Texturally, in the photo at least, it bears some resemblance to opal, which is amorphous hydrated silica. Opal is significantly softer than quartz/chalcedony, and the two forms are not that difficult to tell apart in the field. However, before making a call, I should field-check the material. I'm not comfortable trying to distinguish the two from photos. (Opal has more of a vitreous luster, while chalcedony has a more waxy to flat luster. The above looks more vitreous to me. But the hardness is a more diagnostic feature, and I don't want to test that on my computer screen.)

Photo unmodified. March 9, 2012. FlashEarth location.

No comments: