5 January 2008

Friday Field Foto!……Well, almost.

Posted by Jessica Ball

Yes, it’s Saturday. I’m going by the reasoning that it’s Friday somewhere in the world, because I think this is a cool tradition and I want to join in.

Here it is:

This one hails from the wilds of the High Plateaus of Utah, which hold a special place in my heart (and probably feet, knees, thighs and calves as well, considering how much time I spent tramping around out there). Long story short, this shows the fantastic weathering patterns and some of the flow banding in one of the more extensive ash flow tuff deposits out there. Most of them had origins in the Marysvale Volcanic Field in one of multiple calderas; this particular highly-welded deposit was formed from the caldera-collapse eruption of one of the largest calderas. (Yes, I am being deliberately vague; some of this will eventually end up in publication and I don’t want to spoil the fun! Doubtless someone will guess what I’m talking about, but I’m going to continue using the carefully edited version of the commentary.)

Anyway, one of the cooler features of this unit, in addition to the flow banding, is the fiamme. (The similarly-oriented light gray streaks in the photo below.)

Traditionally, fiamme are formed from fragments of pumice that are compacted into little glassy “flames” by the heat and pressure of overlying material in an ash flow. These are a little different: it turns out that they’re not pumice at all, but appear in outcrop to be a very fine-grained version of the darker gray matrix. The actual mineralogy/geochemistry is a bit more complicated, and hints at an interesting history of magma mixing in the melt that formed the unit, but again, not published yet!

I suppose I ought to include this: All photos and research copyright (C) Tuff Cookie. No filching, please!