A question of scale

When I started in geology, I wanted to think big. Big collisions between continents, big rifts.

So it may be a bit odd to blog about things that are only a few millimeters across. (Well, I supposed it could be worse; I could be doing sub-atomic physics or something.) But I’ve been thinking small lately, because I got to spend time on an electron microprobe after years away, and I had to explain to a student why, exactly, we were spending hours trying to make sense of one crystal and the minerals surrounding it. Why not collect compositions scattered around the thin section? Why spend so much time trying to characterize one tiny part – what if we were missing something by not looking at the complete picture?

My answer was that sometimes there’s a huge story in a tiny texture. Especially when you’re dealing with metamorphic rocks.

I love metamorphic rocks, but I find them very difficult to explain to non-geologists. Ok, yes, there’s the name: metamorphic = changed shape. Changed by high temperatures and pressures. But there’s a world of chemistry hidden in that statement. The changes are chemical reactions that take place between solid minerals, minerals that no longer can coexist when the pressure is too high for their total volume, or the temperature is too high for their low entropy. Thermodynamics provides a theoretical reason why the minerals should tell a story; the difficult kinetics of the solid-solid reactions mean that it can be possible to tease out the early part of the history of the reactions.

And that means that, sometimes, you can see things like this:



This is a metamorphosed mud-rock that was heated by a nearby body of molten granite. The iron, magnesium, calcium, potassium, aluminum, and silicon are now organized into new minerals. Flakes of brown biotite and colorless muscovite. Garnet, too tiny to make jewelry in this case. Staurolite, honey-brown and cross-shaped in rocks, and pale yellow when cut thin like this. Andalusite, aluminum and silicon and oxygen, grown in long, squarish prisms at low pressures and high temperatures, most likely because it was heated by a magma at shallow (for us metamorphic types) depths............

Wait. That’s not andalusite.

Those dark line in the middle of the image... that’s the cleavage of kyanite, pale blue and beautiful in hand sample, harder to recognize in thin section. Kyanite, which has the same chemical composition as andalusite, but which grows at higher pressures, at greater depth.

That square in the middle – here, I’ll color it so you can see it –



- that square looks for all the world like andalusite. The outcrop is filled with those crystals, pulled apart, but still, in many places, andalusite. But not here.

That one crystal told me the story that I had gradually come to suspect in the years I worked in that field area. The rocks were heated, and deformed, and then buried a little more. (The deformation is evident in the curved lines of biotite around the andalusite. In non-technical terms, that rock’s been squashed after the square thing grew.)



I argued, once, that it had all happened while the granitic magma was intruding. I don’t think anyone has been back to argue otherwise since I stopped working there. But those textures – and other, more mysterious ones on the other side of the granite – those tiny textures are the best evidence I’ve got for a few kilometers of slip on a tricky, poorly exposed fault.

Tiny grains. Tiny textures. They would be easy to miss in any kind of random sampling scheme. But they’re there, and they tell a story.

(Edit: and the tiny thin section photos tell a story about how computer storage has gotten a lot cheaper over the past seven years. Those pictures are little because I took them in the early days of digital photomicrography. And I filled the &#$% hard drive of the computer attached to the camera, and had trouble printing the darn things, too.

Better take new pictures now that we've got a new scope.)