Wednesday, April 9, 2008

Standing Underneath Volcanoes

Into the last two weeks of the semester, and things are getting hectic, so blogging has been intermittent. But field trips have been part of the fun for the last week, and last week's exploration of the innards of a subduction zone in the Coast Ranges now lead to the rest of the story....where are the volcanoes of the magmatic arc that must of have been part of a convergent boundary? When oceanic crust and sediment is subducted into the mantle, water released into hot mantle lowers the melting point of the rocks in the base of the crust, resulting in the formation of magma, and ultimately of volcanoes.

When tourists visit Yosemite National Park, if they think of geology at all, they will probably think about glaciers, and how they sculpted the valley. The story is more complex than that, of course, as Yosemite has been carved as much by river erosion and mass-wasting. But when I stand near the famous view of the valley at the Wawona Tunnel exit, I think of how I am standing three or four miles beneath the volcanoes that once existed there. The granitic rocks that make up much of the Sierra Nevada were the product of the subduction zone that lay to the west near the present site of the Coast Ranges, which I explored in last week's posts.

Although the volcanoes have long since eroded away at the latitude of Yosemite Valley, the cobbles and boulders of these vents can be found in Upper Cretaceous sediments in places like San Luis Reservoir (but that is a trip for a future post). Today's picture of a view of Yosemite Valley, taken from the same spot that tens of millions of other photos have been snapped, is really a plea to stop and think about how even a familiar scene can be newly appreciated. Can you imagine what it must of been like for those who discovered the valley for the first time (whether speaking of native Americans or of their European conquerors)? I hope in some future posts to get a few people to see Yosemite for the first time, despite having seen pictures a hundred times over.

For those who are not as familiar with this view of the valley, here is a short tour: the vast cliff on the left is El Capitan, with a 3,000 foot sheer cliff that is a magnet to rock climbers. The snowy peak in the upper end of the valley is Clouds Rest, which at 9,926 feet is the highest point visible from the valley floor (at 4,000 feet). Half Dome lies just to the right. In the center of the photo lies Sentinel Rock, which I have always thought was one of the less appreciated cliffs of Yosemite Valley; in any other setting, it would be renowned. The three triangular peaks on the right are the Cathedral Rocks, with Bridalveil Falls (620 ft) in their foreground. The last prominent spire on the right is the Leaning Tower, but a different angle is required to appreciate the origin of the name.

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