Geo 730: August 20, Day 597: Altered Andesite?

For reasons I explained earlier, I'm pretty uncertain with respect to the actual field relationships in this quarry, but the gray rock occupying the center of this photo appears to be the host rock into which the rhyolite intruded. It's some kind of altered intermediate volcanic rock (for example, andesite), but it doesn't seem to be as altered as the lighter rhyolite. It's definitely not mineralized with sulfides in a similar manner. As an aside, the pervasive iron staining on the surrounding blocks is due to chemical weathering of the pyrite in the rhyolite, and a close inspection of those surfaces will often reveal small cube-shaped pits where that mineral has weathered out.

Honestly though, the reason for this shot is that I love the moss colonizing what has to be a hostile, barren surface, utterly bone dry for several months each year, and buried in snow for the bulk of the remaining months. (Probably mostly snow covered ~October to ~April)

Photo unaltered. July 9, 2012. FlashEarth Location.