Friday, December 14, 2007

My deskcrop has dizzy feldspars

I had a bit of a problem with this month's Accretionary Wedge (which is why I'm late posting). I don't actually have any rocks on my desk at the moment - I've just turned in grades, which means my desk is currently covered with textbooks, unread issues of GSA Today, unopened mail, scribbled notes from phone calls, a slinky, a pair of fault blocks, a pile of scrap paper that was meant to be used for a discussion about triple junctions, and possibly my missing coffee cup.

So I had to dig through the drawers of a rock cabinet, instead. (I rescued a lovely old wooden rock cabinet last time one of my colleagues was getting rid of stuff, and now my teaching collection is organized. Which is a good thing, because I can't find anything on my desk.)

I found this rock in an overgrown field in central Massachusetts. My field partner and I were following an old field trip guide, looking for the Pelham dome. I'm still not quite sure if we found it or not. But we did find this: a perfect example of a shear-sense indicator for class.

Metamorphic rocks usually do a rather imperfect job of telling about their deformational history. The minerals recrystallize as the rock changes shape, and it is possible that, several hundred million years later, it will be impossible to tell if the rock was squished or smeared. Squishing (otherwise known as pure shear) is what happens if you flatten bread dough by pushing down on it. Smearing (otherwise known as simple shear) involves deforming an object between two sides slipping past one another - imagine what would happen to the bread dough if it were caught within a fault zone (other than just getting really dirty, I mean, and possibly making an earthquake rupture less likely). A combination of the two is also possible - a rolling pin flattens dough by a combination of pure shear and simple shear.

If you want to know just how a rock changed shape, it helps to have a way to tell apart pure shear and simple shear. Generally, you need to find some kind of object that has tracked the movement - and this rock has one.

That feldspar has done a bit of spinning.

Objects that are caught in a shear zone should rotate. But they rotate at different rates depending on their shapes. Spherical objects will roll forever at the same rate - try rolling a marble between your hands. But flat objects will rotate fast when they're at a high angle to the shear zone, but will slow down and get stuck parallel to the shear zone boundaries. (Try rolling a poker chip between your hands.) In this rock, the white feldspar has grown a pair of tails (which also tell a story about deformation), and then the round center kept spinning while the tails stayed in place.

That kind of grain is called a "delta porphyroclast," because it looks a bit like the Greek letter delta, and because... well, I don't really know the etymology of "porphyroclast," to be honest. But it's a good word, however it got into geologic terminology.

Dizzy feldspars may not know where they've been, but they can tell you a bit about how they got there.

(The observant reader may note the red pen on top of the sample. That's not just there because my grading wasn't quite finished. The pen shows the direction of the stretching lineation - the direction that the rock was stretched during deformation. If you want to see shear sense indicators, you need to look at the side of the rock that's parallel to the lineation, and also perpendicular to the foliation, which in this rock is defined by the dark- and light-colored layers.)

1 comment:

jrepka said...

My background is nowhere near metamorphic structures, but I could read your descriptions of them all day. I've been fascinated by those rotating clasts since field camp in the Palm Canyon deformation zone.

No idea from whence comes "porphyroclast" (or the related "porphyroblast") but I know that the term "porphyry" comes from the Imperial Porphyry, a maroon andesite with cm-sized plagioclase phenocrysts, that was quarried in Egypt and was used for columns and other sculpted items by the imperial Romans.

The name originally referred to the color of the rock (porphyry being either Greek or Latin for purple, the royal color).