28 October 2010

Spooky Deskcrop – Accretionary Wedge #28

Posted by Jessica Ball

The latest Accretionary Wedge is being hosted at Research at a Snail’s Pace by Matt Kuchta…and the theme is deskcrops (spooky if possible, for Halloween!) My favorite deskcrop is one that I acquired fairly recently on Montserrat, from the February 2010 dome collapse deposits of the Soufriere Hills volcano. This isn’t a particularly small sample, but it was one of the easiest to collect – already pre-rounded, and of course it didn’t add much mass to my luggage, because it’s a chunk of pumice! (Sorry for the color variations in the next few photos – I was using the cheap camera and the color balance always goes wonky on that one.)

What I find really spooky about this particular sample is that it has a story to tell, recorded in that big black mark. It’s suspiciously femur-shaped.
Femur figure courtesy of Gray’s Anatomy, via Wikimedia Commons.

But don’t worry! That mark has nothing to do with human remains (no one was killed in that dome collapse, and I would never disturb remains if I came across them). It’s actually a scorch mark from a tree limb that I found the pumice block resting against. Remember this photo?

When this pumice flow lobe came into contact with the vegetation in this area, it was hot enough to scorch it, and in a lot of cases the pumice clasts are still resting on the tree limbs they burned. This particular piece of pumice records the shape of one of those limbs.The scorch mark itself is pretty cool – it’s sooty from the carbonized wood, but it also has little patches of remelted glass.

And, for all you mineralogists out there, here’s a really nifty little shattered hornblende phenocryst. (The predominant composition of the eruptive products at Soufriere Hills is andesitic, so there are also quite a few large plagioclase phenocrysts in there as well.)