Friday, August 11, 2017

Kilbournes Hole, Potrillo Volcanic Field

It has been decades since I visited Kilbournes Hole, in southern Doña Ana County, New Mexico.  It and its companion Hunts Hole (less-developed and to the East) are both defined as Quaternary "Maar" volcanoes.  These two craters are within the southern part of the Potrillo Volcanic Field.  Maar volcanoes form when rising magma encounters surface water or a shallow aquifer, resulting in a series of phreatic (steam explosions), which "blow a hole in the ground".

In this case, a southwest-extending "tongue" of the existing Afton Basalts overlay the site of the steam explosions.  Without the pre-existing basalt flows, the rim of the maar would likely have been more subtle, as is the rim of Hunts Hole.  The surface "host" consists of lightly-consolidated Quaternary basin and fluvial clastics, e.g, Camp Rice Fm. 

The basalt flows in the middle ground of the photo are the Quaternary Afton Basalts, slightly older than the previously discussed Aden Basalts.

Unusual for basaltic eruptions, the phreatic explosions pulverized the lavas and with the help of prevailing winds, deposited cross-bedded, Base Surge Ash beds, primarily on the east side of the maar.   

Other "Geologic Toys" scattered about the exterior of Kilbournes Hole are numerous "Volcanic Bombs", many containing Mantle Xenoliths, primarily of Olivine and Enstatite.  This particular Xenolith is about 5 - 6 inches across the largest dimension.

[I may add some additional notes, later.]

No comments:

Post a Comment