Thursday, March 20, 2014

Out of the Valley of Death: Mountains Hidden Within Mountains

I don't think there's anything nicer for a teacher of the earth sciences than to have a classroom in the outdoors. The planet is always around us and to step outside a classroom is to step outside of book learning and theoretical constructs, and into the actual chemical and physical reactions that affect every aspect of our lives. Yes, there is always gravity, and we are always respirating whether we are indoors or out, but there is nothing quite like being right on top of the volcanoes or the faults that can cause geological mayhem. We can tell the story of how rocks and mountains and continents came into being, but it is something else entirely to stand on the rocks that actually tell us the story step by step. There is just no substitute for field experience in a geology class.

So here is our class sitting in the midst of one of the greatest geological parks in the world, Death Valley. Rocks from nearly all the periods and eras of the geological time scale can be found within the boundaries of the park, and the park includes some of the oldest rocks in western North America (one has to get to Montana and Wyoming to find rocks that are older). They are quite literally sitting on the trace of a major fault line, and the darker slope on the left is a small volcanic cone that erupted along the fault. It's all the geological mayhem in one spot than anyone could ask for!
We had spent the day working our way through geological time, with a stop at a roadcut containing Miocene tuff and examples of faulting, a stop on the alluvial fan below a mountain of Paleozoic-aged fossil bearing limestone, and an exploration of a former rift valley containing late Proterozoic sediments. We had now reached the base of the Black Mountains of Death Valley, the deepest crustal rocks exposed anywhere in the park. They are exposed here because extreme extensional forces have  ripped the crust apart, and the deep trough of Death Valley gives us a peek into the deeper parts of the continent. These are rocks from the early Proterozoic around 1.7 billion years ago. The radiometric age date of 1.7 billion years records the time that these rocks were metamorphosed, so the actual age of the protoliths (the original rock before metamorphism) is millions of years older still. There are hints of rocks and detrital grains (the Mojave Block) that may be as old 2.3 billion years, more than half the age of the Earth.
A look at the rocks where they spilled out onto the valley floor offers a hint of the massive changes these rocks have experienced. They are composed of gneiss and schist, rocks that normally develop in regions of extreme temperature and pressure deep in the crust. About the only geological circumstance in which such conditions occur are in places where continents or exotic terranes are colliding with each other, in the way that India is colliding with Asia today to form the Himalaya Mountains. Such a mountain range once existed right here. And it eroded away, almost completely. Then, the continent split, and a portion of the ancient mountain range rifted away to become part of Australia and/or Antarctica. The remainder of the rocks were buried under tens of thousands of feet of younger sediments during the Paleozoic era. And there they remained for several hundred million years.

It wasn't until intense extensional forces ripped the crust apart, causing the graben of Death Valley to sink and form the imposing western face of the Black Mountains. As the mass of the overlying rocks slid off the deeper crustal rocks, the deep rocks domed upwards to form the curving footwall of the strange "turtlebacks" of Death Valley. In other words, the roots of an ancient mountain range rose to form the core of a modern day range, the Black Mountains: mountains hidden with mountains.
And the drama was visible within the rocks we were sitting on. It was a most amazing classroom!
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