Tuesday, July 7, 2009

Fire Down Below - a Geological History of the Colorado Plateau


I've been seeing a fair number of geobloggers giving apologies for not blogging recently, and then I notice I haven't posted anything in nearly two weeks. It's obviously field season! I've actually been home for a couple of weeks, but other projects took precedence, not the least of which was a lot of weeding. I spent most of the last month discussing the active volcanoes of the Hawaiian Islands, and I will return to that project soon. For the moment I am taking the blogger's perogative to change the subject. I have come close to completing a year-long project on the geology of the Colorado Plateau, and there is a matter of some serious volcanic activity in Cenozoic time across the plateau country.

The Plateau is above all about horizontal rocks. They may form cliffs, but it a simple truth that the landscape is dominated by sedimentary layers of sand, silt, clay and lime. Throughout Paleozoic and Mesozoic time, thousands of feet of rock were laid down, but in Cenozoic time things were changing. The land was rising and buckling so that the last sediments formed on the plateau were in freshwater lake basins of somewhat limited extent that form the colorful rocks of Bryce Canyon and Cedar Breaks today. The Laramide Orogeny, which had lifted the Rocky Mountains, was also responsible for the vast changes in this region as well. The tectonic conditions responsible for the deformation was to have one other effect on the land: volcanism.

Driving across this strange and scenic landscape, one is struck every so often by the sight of rocks just sticking up into the sky. The ancients were pretty sure that these were the remains of monsters who had been frozen in stone. Although geologists have a wonderful time debating the reasons, the fact is that from about 30 million years to around 20 million years ago, volcanism swept across the region, leaving behind all manner of volcanoes, calderas, volcanic necks, and strange laccolithic mountains (see the next post).

Today's pictures include a couple of volcanic necks, including Shiprock in New Mexico. These are the cores of volcanoes that have been exposed by intense erosion.


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