Monday, January 17, 2011

San Francisco Peaks: The Turbulent Geohistory of Tranquil Lockett Meadow

Who would ever suspect that this beautiful and quiet meadow, with its Aspen trees turning golden yellow in the October sun and surrounded by the San Francisco Peaks, was once the site of a cataclysmic volcanic explosion that dwarfed the size of Mount St. Helens?

Tranquil Lockett Meadow

The San Francisco volcanic field is scattered over an area of about 1800 square miles in the vicinity of Flagstaff, Arizona, about 50 miles south of the Grand Canyon. During the field's 6 million year old history, it produced more than 600 volcanoes, most of which are relatively small cinder cones that bled long rivers of basaltic lava many miles over the flat terrain. San Francisco Mountain was the only stratovolcano in the San Francisco Volcanic Field. It was built by eruptions between 1 and 0.4 million years ago. Stratovolcanoes have moderately steep slopes and form by the accumulation of layer upon layer of andesitic lava-flows with an intermediate viscosity. Also in the vicinity are several lava domes formed by dacite and rhyolite lavas, which form very steep-sided masses at the site of eruption.  

Many volcanologists believe that the San Francisco Peaks are a remnant of a catastrophic, lateral explosion of the parent San Francisco Volcano, while others insist that the stratovolcano violently exploded, and then collapsed into itself forming a crater or caldera within the six peaks we see today. The latter argument is bolstered by the observation that no assignable debris has been identified downfield of the cataclysm. Regardless, tranquil Lockett Meadow's serene nature, located within the Peak's Inner Basin, indeed belies its violent past. 

Interestingly, the San Francisco volcanic field both contradicts and confuses the tenets of modern plate tectonic theory. That theory states that volcanic activity generally occurs at the margins of the large plates that float about and collide on the Earth’s surface, where one plate interacts with another. The contemporary explanation for the San Francisco volcanic field’s intraplate location is that it occurs either from a “hot spot” or plume of magma rising up from the Earth’s mantle, or from the thinning of the Earth’s crust in association with an adjacent area called the Basin and Range.

Although there has been no eruption within the region for nearly 1,000 years, geologists consider the field to be dormant with a future eruption expected to be somewhere to the east, the implied direction of plate migration over the fixed plume.

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