Wednesday, May 7, 2014

Bizarro Rocks of the Beaver Dam Mountains

This was all Jerry’s idea. He wanted to go out to the Beaver Dam Mountains and look at the cool metamorphic rocks. There is a canyon on the other side of the mountains, he said. Why don’t we go check it out? Now, Jerry is a geologist but he is also a paleontologist, one of those peculiar people who studies dead things in mud. He claims metamorphic petrology sets his head to spinning. I only have an undergraduate degree in geology but I completed three years worth of field research in these rocks and did my senior project on them (you can read about some of that here!). He figured I could help him recognize a few things and so I said I would do my best to enlighten him. 

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Jerry becomes ONE with the metamorphic rocks

As Jerry, Tracy (his long–suffering wife), their friend Wendy (a chemistry prof), and I thrashed our way up the dry wash and stump–jumped across the rocky hillside, I tried my best to explain what I know. Soon, though, my head started spinning! I love bringing folks out here and showing them around, but trying to get a handle on these 1.8 billion year old rocks is a piece of work. My old geology prof Mark and I used to refer to my study area as Bizarro Land.
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View westward, into the Great Basin of eastern Nevada

It was not a big wash, and we did not go very far at all. The thing is, after practically every step we took, we saw something interesting that forced us to stop! You certainly do not break any land–speed records on these hillsides, unless that speed is glacial you want to win the prize for loitering. 

Current theory concerning the Beaver Dams is that this area was once at the edge of the early North American continent of Laurentia. Along with mountains to the south in Nevada, Arizona, and California, the Beaver Dams represent a section of an ancient suture zone of high–pressure and high–temperature conditions between Laurentia and the Mojave and Yavapai provinces. A major indication that this might be true is the presence of migmatites, one of the most unique types of rock you can find.
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Tracy as scale in the metamorphic rocks of the Beaver Dam Mountains

What are migmatites, anyway? Are they igneous? Are they metamorphic? This is a debatable question, one that most likely will not be answered to everyone’s satisfaction in my lifetime. Various sources give various definitions. I learned in school that migmatites are metamorphic rocks with igneous characteristics. However, since metamorphic rocks form by recrystallization of minerals that have not melted, and igneous rocks form by crystallization of minerals that have melted, it is still not easy explaining how and why these two types of material coexist in a single rock. What is apparent, though, it that they form at some threshold between high–pressure metamorphic conditions and complete igneous melting.

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I'm thinking migmatites with typical sinuous shape

There are at least two ways for migmatites to form. One is by the intrusion of a granitic melt into the pre–existing metamorphic rock. The other way is by partial melting, where minerals with higher melting points remain solid while the rest of the minerals become part of the melt. Most likely, though, these two explanations are not mutually exclusive. At the temperatures and depths where these rocks are found, both processes might ultimately occur together. 

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A sense of shearing and ductile movement jumps out from this rock!

What most impresses me about these Beaver Dam Mountains in general is the evidence for ductile shearing. You can observe these structures under a petrographic microscope (Check some images out here!) but in these particular rocks the shearing is also apparent to the naked eye. At lower crustal depths of possibly 25 miles and temperatures of perhaps 1300°F, these rocks moved plastically, twisting and contorting like silly putty squeezed into itself, over and over and over again. 

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What have we here?

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There is so much going on in this close-up of the previous image - pen for scale

Besides the migmatites there are tons of other intriguing rocks out here, strange rocks with stranger stuff in them. A lot of the mineralization is rusty red and looks like weathered garnet, common in these high–grade metamorphic rocks. Other minerals appear greenish (some but not all is certainly epidote, which often crystallizes in the fractures of metamorphosed basalts), while others have a darker brown hue. What are all these minerals? I have no idea but I would love to know. 

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I'm thinking migmatite - but with is that big reddish-greenish-brownish thing in the middle?

On the way back to the highway, my “low tire pressure” light came on so I pulled off to the side of the gravel road. There was not another soul in sight as we listened to the insidious hissing of a rear flat tire. Tracy and Jerry sprung into action and totally rocked that tire change while Wendy and I stood around and supervised. It took barely 15 minutes before we were on our way back to town, captivated by our latest metamorphic adventure into Bizarro Land. 

All I originally intended to do afterwards was to post some pictures of cool metamorphic rocks. As usual, though, the rocks and their explanations got in the way of my slothfulness. I wouldn’t have it any other way! 

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The newest addition to my patio


Some Helpful Explanations can be found here:
"Migmatite." World of Earth Science. 2003. 7 May. 2014 <>.
Vernon, R.H. and Clarke, G.L., 2008, Principles of Metamorphic Petrology

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