Thursday, May 14, 2015

Geo 1095: May 14, Day 864: Not What It Looks Like

After our brief stop up in the Lost Creek Cirque, we continued back down to the west until we got to the Proxy Falls trail head. The area south of Route 242 and the parking lot (FlashEarth location) is dominated by a steep berm of the Collier Cone Lava Flow. Most of the walk (maybe a mile or a bit more)  is upon that flow, the most recent in the area, at about 1600 years. The blocky Collier flow is basaltic andesite, with an oddly smooth, glass-like (but definitely not glassy) texture and feel to it. Given that it came from a cinder cone, there were certainly portions of the eruption that were gassy, but whether this was a later, less gas-rich portion of the original magma, or perhaps in this distal portion of the flow, the lava had almost entirely lost its volatile components, the result is rock almost if not entirely free of gas bubbles (in geologese, "non-vesicular.")

So I was sort of surprised to see what looked like breadcrust texture on this block. The first of those two links describes what "breadcrust texture" means, how it forms, and therefore what it implies. With little exsolving gas, breadcrust texture shouldn't form. As it turns out, it didn't. This is actually a coating of moss and lichen, nearly an inch thick, that I'd guess has shrunk with dessication at the end of a long dry summer. In the case of breadcrust texture, the crackles are the result of an expanding interior breaking up a solid surface. This is a case of a shrinking exterior breaking up over a solid interior. It pays to look twice.

Photos unmodified. October 9, 2014. FlashEarth location.

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