Friday, June 1, 2007

The Sound of Mylonites

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.

5 comments:

Anonymous said...

During my oral comps my professor asked me to identify a rock - it was a mylonite. Beautiful specimen.

I too am a structural geologist, although only at masters level. I studied duplexes in limestone. Very interesting.

Julian said...

(New commenter here, cruised on in through a long chain of geoblog links. Don't mind me!)

As someone with a music background with hopes/plans to start school in seismology this fall, your description of singing quartzite is downright enthralling to me. The way you describe the visuals of the mylonite gives a beautiful mental image in and of itself, and I'd also be excited to simply see the transition between brittle and ductile in terms of visuals alone. But to add the aspect of musical sound coming in quite unexpectedly...just reading about it makes me very excited, since it bridges my interests so very neatly.
I, for one, would love to hear a recording of that sound, if you decide to bring recording equipment along when you go back with your class.

(Also, the title of your blog cracks me up so hard. Unrelatedly.)

Kim said...

Thanks, Julian. I didn't bring any recording equipment along on the class trip (which was at the end of August). I should post some of my pictures, though.

A musical background might help you think about seismology in really interesting ways, since both music and seismology involve the physics of waves. Maybe you can figure out a way to let people hear a slow earthquake, or... well, I'm neither a seismologist nor a composer, but I wonder if patterns of reflecting and refracting waves could inspire compositions?

Julian said...

I'd be interested in seeing the pictures!

One of the first places my mind went when I started reading in depth about earthquakes was how I could portray them musically! There are a couple of things I've been already thinking about doing, involving different frequencies of wave within one quake and involving displacement of notes/themes as if across a fault (like thrusting the pitch of the first part of a theme up relative to the second part of the theme, or something - this is still an unrefined idea!), but I hadn't even thought of using reflection/refraction patterns as a basis of a piece. Definitely a cool idea!

I hope the music does, as you said, help me approach seismology in interesting ways. Other people have also pointed out the physics-of-waves connection to me, which makes me feel a whole lot less weird about putting, "I have no formal experience, but..." on those college applications.

kdinger said...

Have a look at Andy Michael's homepage (http://quake.usgs.gov/~michael/); he has an mp3 of a performance of a quartet he wrote for voice, trombone, cello, and seismograms, as well as some discussion and links to some other pages with earthquake sounds.