North Cascades

Posted by Ellen

 

C-T scans have been in the news recently; evidently, they can sometimes be dangerous, zapping people with risky levels of radiation. My master's thesis involved a C-T scan, but fortunately a very safe one, of a rock instead of a human being. Rocks can sit there and take huge doses of X-rays without injury or complaint, making them ideal targets for this sort of procedure. Because there's no need for radiological restraint, scanning a rock can yield much clearer, more detailed results than scanning a live person. It's also a lot cheaper.

The rock I sent to Texas for a C-T scan was a metamorphic chunk of the North Cascades mountains in the state of Washington. It had garnets in it. Each garnet was surrounded by a shell of pure-white minerals: quartz and plagioclase. The rest of the rock--what we termed the matrix-- looked dark gray in color; it actually consisted of the same white minerals as in the shells, flecked with tiny black grains of a kind of mica called biotite.

We were trying to figure out why the garnets were set off from the rock matrix by the white shells, which we called coronas. Our hypothesis was that unusual conditions during the rock's metamorphism had permitted garnet growth but had simultaneously limited diffusion of elements that the garnets would consume during their growth. We wanted to know more about the geometry of the coronas, and about the separation between garnets and matrix. So we shipped a core of rock about an inch in diameter to a C-T lab at the University of Texas, where it was zapped with X-rays;  the results were reconstructed by a computer, rendered in 3-D, colorized, and made into a little animated movie.

I've posted the movie on YouTube. I recommend watching it--even if it's not your kind of movie, it only lasts 12 seconds. The garnets in the rock are rendered red, the corona shells green, and the matrix rendered as transparent, with a slight reddish speckling of biotite grains. When I first saw this movie, I'd spent months working with the rock sample, but I was surprised by how long and snake-like the coronas are, and how many clumps of garnets each corona engulfs. How did this happen? The short answer is that the rock got squished squished and heated and stretched and squished and heated again during its mountain-building experience, which coincided roughly with the era of the extinction of the dinosaurs. The long answer is published in a journal called Canadian Mineralogy.

And now, the rock is doing a star turn on the internet, in what I honestly believe to be the first true hard-rock video on YouTube.

Posted by Ellen

That's me with the chisel in my mouth, some summers ago, during geologic field work in the North Cascades, in the state of Washington. I'm climbing a hill called Lincoln Rock that rears up about twelve hundred feet above the apple orchards along the banks of the Columbia River. We'd been told there were some good garnet coronas up there--garnets with white rings around them---the metamorphic feature I was trying to interpret for my thesis project.

We'd also been told that Lincoln Rock was the one place in the North Cascades where a geologist named Bob Miller--a man who climbed cliffs for fun when he couldn't think of an excuse to climb them for research--fell badly and almost cracked his head open. This was my last day in the field that summer, and though I'd had wonderful fun, I was beginning to shift gears mentally, to look forward to getting back home so I could stop worrying about slipping and falling and leaving five children motherless.

Perhaps because of Bob's misadventure, but surely also because I was old and out of shape, I was by far the slowest climber. While I toiled upward inch by inch in the August sun, the rest of the gang was already lolling about in the shade of an overhang at the top of the hill, eating lunch and making fun of me. As I finally approached the scene of this snapshot, a Ph.D. student named Carlos Zuluaga asked if I wanted my picture taken. Then he suggested I put the chisel in my mouth. It seemed like a good idea at the time, it really did.

Anyway, there were indeed nice-looking garnet coronas all over the hilltop, and Carlos and the others kindly helped me smash them out of the outcroppings. We all made it down safely, with rocks in our backpacks. When I got a look at my Lincoln Rock samples under the microscope, however, I discovered that the garnets were rotten; they'd cooled too slowly after their metamorphic odyssey, and a mineral named chlorite had replaced much of the garnet. My thermodynamic models wouldn't work on rock with rotten garnet, so I put the Lincoln Rock samples in a drawer in the basement of the geology building, and maybe they are still there today.

Fortunately, I had plenty of other samples. And I'd love to be back up there again.....