Wednesday, January 19, 2011

Diatremes of the Colorado Plateau: "Black Rocks Protruding Up"

Navajo Prayer
In harmony may I walk.
With harmony before me may I walk.
With harmony behind me may I walk.
With harmony above me may I walk.
With harmony underneath my feet, may I walk.
With harmony all around me may I walk.
It is done in harmony.

That’s what Navajo Indians or Dine' (di-neh) in their language call them, referring to the 80 or so volcanic structures that are found scattered within a large area referred to as the Navajo Volcanic Field. The field is contained within the Navajo Tribal reservation, which is about the size of West Virginia. The volcanic field is roughly centered at the Four Corners region of Utah, Colorado, New Mexico and Arizona.

Geologists call these volcanic structures diatremes. They form when superheated magma (lava) from the Earth’s mantle rises until it contacts a subterranean water source. That vaporizes the water into a gas. As the gas rises at a high velocity, it tears off portions of the Earth’s crust that eventually reaches the surface releasing an enormous amount of gaseous vapor. This is not a conventional volcano in the sense that we are accustomed. It’s more of a super-gaseous discharge, and it occurs very quickly. One famous geologist, Donald J. Baars, called it a “mega-geo-fart.” How descriptive!

A tectonic-mountain building event called the Laramide Orogeny occured roughly from the Late Cretaceous through Paleogene time (70-40 million years ago). It involved the shallow subduction of the Farallon Plate beneath the North American Plate at the western edge our forming continent. Previous to the Laramide Orogeny, the Farallon Plate had already begun its descent but at a steeper angle. This "first" phase of subduction initiated the Sevier Orogeny, which transitioned into the Laramide as plate mechanics and the angle of descent were later altered. It was the Laramide that was ultimately respnsible for the formation of the Rocky Mountains. In the process, the southwest region of the U.S. called the Colorado Plateau was uplifted, by the same tectonic forces deep within the Earth.

The Laramide finally ended around 30-40 million years ago. That’s about when the Navajo Volcanic Field had its birth, and the diatreme-activity began. The uplift of the plateau induced erosive forces to accelerate everywhere. Interestingly, diatremes such as seen in the photo were entirely positioned deep within the Earth when it formed. In time, the diatremes were exposed by the forces of erosion acting on the elevated landscape. Geologists say that they were exhumed.

Larger diatremes have romantic names like Shiprock, Alhambra, The Thumb and Agathla Peak. As far as I know, this particular diatreme was unnamed, having discovered it while exploring the backroads north of Monument Valley. Notice the black volcanic feeder-dike slicing through the strata off to the right of the diatreme. It was a conduit that conveyed molten magma belonging to the diatreme. It was thought that all diatremes are associated with dikes, until one within the field was found without one. Subsurface sonar there, however, has since substantiated the presence of a buried dike, yet to be exhumed.  

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