Repost from old blog: the sound of mylonites

I'm neck-deep in a five-week summer class, and spending my evenings reading for class prep and thinking about how to run discussions. So I'm on a blogging semi-hiatus, at least until I've got an hour or two free of other commitments. In the meantime, I'll occasionally post some of my old favorites. This one was my first blog post ever, and was included in the 2007 Open Lab.

NPR has had this series, off and on, in which listeners record interesting sounds and then explain them on the air. I didn't have a recording device with me last weekend, but I literally stumbled across some of the most musical rocks I have ever heard: the Snake Range décollement.

I had never been to the Snake Range before, although it loomed much larger than its true elevation over my grad school career. The Snake Range is in easternmost Nevada, nearly on the Utah border, and is home to Great Basin National Park. And it's crossed by US 50, the "loneliest road," the road that continues west across the Basin and Range after I-70 ends. And in the 80's and early 90's, it was a case study for controversies over how continental crust stretches. It's a metamorphic core complex, a range with a core of metamorphic rocks overlain by a veneer of faulted and thinned sedimentary rocks. The boundary between the metamorphic and sedimentary rocks has been called a thrust fault, a low-angle normal fault, and an exhumed ductile-brittle transition. The metamorphic rocks are only around one-fifth their original thickness - pebbles stretched to pencils, thick beds of quartzite turned into mylonitic flagstone that is quarried into lovely slabs, thin and strong, with streaks of silver muscovite shimmering on their surfaces.

I was scouting out the area for a class field trip. I'm teaching Advanced Structural Geology for the first time, and I wanted to take students to see some classic fault and shear zone rocks. The Snake Range décollement is an amazing example of a mylonite, a rock in which the minerals have been shrunk and strung out by deformation within the crystal lattices themselves. Hard to explain to students the first time, and darn hard to identify in the field, and important to be able to recognize and interpret. And very, very cool.

The rocks were everything I had hoped. Perfectly exposed, with beautiful structures. Easy to find, easy to look at. I climbed to the ridgeline, enjoyed the view, and then headed down.

And then I heard something I hadn't expected. Not the rattle of a snake -- they don't call it the Snake Range for nothing! -- but a musical sound from below my feet. The thin slabs of quartzite knocked against one another, and each differently sized rock rang with a different tone. It was too resonant to remind me of wind chimes. It sounded, for all the world, as if I were dancing across a xylophone.

I wish I could have recorded it. I was sliding down the ductile-brittle transition. And it was singing to me.

It was delightful.

Edit: If you want to visit this site, it is described in at least two field trip guides:

Miller, E.L., Gans, P.B., and Lee, J., 1987, The Snake Range decollement, eastern Nevada, in Hill, Mason L., ed., Cordilleran section of the Geological Society of America Centennial Field Guide, p. 77-82.

and in more detail here:

Gans, P.B., and Miller, E.L., 1983, Field trip 6; Style of mid-Tertiary extension in east-central Nevada, in Gurgel, Klaus D., Geologic excursions in the Overthrust Belt and metamorphic core complexes of the Intermountain region; Guidebook, Part I: Special Studies - Utah Geological and Mineral Survey, v.59, p.107-16.

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