Migrating Volcanism
Changing Buoyancy of the Farallon Plate

Devils Tower is a thousand-foot-high rocky sentinel that dominates the landscape in northeastern Wyoming. Together with the nearby Missouri Buttes, it represents the easternmost migration of volcanism across the western half of North America during the Mesozoic and Cenozoic eras.

The volcanism began when eruptions were fed magma from the great volumes of molten material that have solidified as batholiths in the western United States. This was the origin of Half Dome in Yosemite National Park. The volcanism then migrated eastward are reached as far as Devils Tower fifty million years ago. As the volcanism migrated, the Rocky Mountains rose. Then there was a return of volcanism to the west as the growth of the Rocky Mountains ended. This was all due to the changing position of the Farallon Plate beneath North America.

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Devils Tower is an erosional remnant of a volcanic neck, a pathway of molten rock to the surface. The region has been deeply eroded, leaving a tower of highly resistant rock displaying spectacular columnar jointing, evidence of the once-molten origin of the rock.

The migration of the volcanism and an end to the initial growth of the Rocky Mountains happened when the Farallon Plate deep beneath North America tore apart. That left a void—and hot mantle material moved in to fill it. That, in turn, set off a new round of volcanism to the west.

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The tearing of the Farallon Plate caused an ignimbrite flare-up across Idaho, in Utah and Arizona, and in Colorado and New Mexico. An ignimbrite is a huge volume of volcanic ash that is poured out during highly explosive volcanic eruptions. The Mogollon-Datil volcanic field in southwestern New Mexico formed at this time. So did the San Juan volcanic field in southwestern Colorado. That was thirty million years ago.

Volcanic ash is easily weathered and eroded. And many large caves are produced. Because thick deposits of volcanic ash are easy to dig into with stone tools, for centuries people enlarged and reshaped the caves to make them into dwellings. Such is the origin of the Gila Cliff Dwellings in northwestern New Mexico, which were constructed and occupied at least as early as the sixth century C.E.

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Devils Tower and a Full Moon.