Geo 1095: March 24, Day 813: Dim, Not Bright

...is how this rock made me feel. I recognized the parts, but had no idea of how it could have happened.

Now, to non-geology types, that may not make a whole lot of sense. Here's the thing: all geology is about change. Something changed, or that rock (or land form, or what have you) wouldn't be there, or in that form. What geology struggles to do is to tell stories about what changed, how, and maybe why ("why" in terms of causal mechanisms). Just eyeballing this rock, I roughly estimate the components at pumice (the light, somewhat rounded pebbles about half a centimeter in diameter- the lens cap is 52 mm diameter), 50%, ash (the slightly darker fine material between the pebbles), 40%, and quite a range of mafic to intermediate fragments (the much darker bits) at 10%. Again, that's just a quick and dirty guess. I can tell you what the pieces are, but the point is, what's the story? How was this deposited? It's too-well sorted to be a lahar, but too poorly sorted to be a simple ash fall. What's the deal with those dark pebbles and grit? Where did they come from, and how did they get in there?

The fact of the matter is, I should have been able to recognize the answers to those questions, but there were some features here (notably the dark bits) that I had never seen in such abundance, and some others (notably the well-sorted pumice) I hadn't seen in this form. So when I finally got a look at a similar rock on Twitter, the link clicked immediately. Not knowing "the answer" troubled me, but finding the answer made me understand why I didn't see it to begin with. Learning can be a frustrating process, but reaching a satisfactory conclusion is satisfying, even when accompanied by a mental facepalm. "Oh, of course! Duh!"

Photo unmodified. October 9, 2012. FlashEarth Location.