Friday, January 9, 2015

What a Geologist Sees - Part 13

One of the many interesting geologic features of the greater El Paso area is Kilbourne's Hole, located approximately 30 miles WNW of El Paso, on the La Mesa Surface (a plateau), in southern Doña Ana County, NM. This area lies within the Rio Grande Rift and is included in the Potrillo Volcanic Field, which covers approximately 500 square miles. Some of the northernmost Potrillo cinder cones are visible on the southside of I-10, west of Las Cruces, NM.


Kilbourne's Hole is a low-relief, asymmetrical volcanic crater referred to as a "maar" volcano. It measures approximately 1.7 miles by 1 mile (see the first link for an aerial photo). A short distance south lies a smaller maar, Hunt's Hole. The two maar volcanoes lie within the southern end of a small, triangular-shaped graben basin, bounded on the west by the Robledo Fault and on the east by the Fitzgerald Fault (the two faults converge near the Mexican border). To the north-northeast of Kilbourne's Hole lie the Gardner Cones and the Afton Basalts. The distal ends of some of the Afton flows are visible in the background of the upper photo and covered at least a portion of what is now Kilbourne's Hole, before Kilbourne's Hole was formed. Due north of Kilbourne's Hole lie the Aden Basalts (where I did my Master's Thesis) and Aden Crater, a small shield volcano at the NW "corner" of the Aden Basalts. [Most of the Aden Basalts were erupted from "fissure eruptions" and are termed "flood basalts.]

For years, the origins of Kilbourne's Hole and Hunt's Hole were the subject of much speculation, including their being meteor-impact craters. The formation of a maar in the Philippines in the 1960s provided the needed answers.

Evidence suggests that Kilbourne's Hole formed as the result of repeated, large steam explosions, probably over the course of a few weeks to a few months.

The Aden and Afton Basalts were erupted onto the surface of the Camp Rice Formation, which includes river sediments from the prior meandering of the Rio Grande River during the Pliocene and Pleistocene Epochs of the late Tertiary Period/early Quaternary Period. When buried, river gravels and sand may serve as aquifers and it is believed that as a rising magma body contacted a shallow sand aquifer and the resulting steam explosions produced the crater. [These explosions are termed "phreato-magmatic explosions".]

Normally, basaltic eruptions like those in Hawaii and Iceland, do not produce much ash, unless there are phreatomagmatic explosions. When the explosions take place, the steam explosions pulverize the erupting lava before it can reach the vent, producing ash that is composed of minute particles of volcanic glass, rock fragments, crystals, and dust (each ash eruption will have different percentages of these components).

With some maar eruptions, a pyroclastic "tuff ring" of ash forms around the margins of the maar. The deposition of the tuff ring is probably at least partially controlled by prevailing winds during the explosive events. The ash itself is deposited as a pyroclastic "base surge" from; 1) The direct lateral components of explosive events; and 2) As lateral deposits around the maar after the collapse of the vertical ash column, i.e., after the explosions, some of the ash remains in the troposphere as "pyrocumulus" clouds and the rest collapses and spreads as it reachs the ground.

The undulating "cross-bedding" of the ash, seen in the photos, is produced by the formation and migration of ash dunes and ripples during and shortly after the eruptive events. The tuff ring is deposited on top of the Afton Basalt flows and exposures of the Camp Rice Formation.

Also produced by the explosions are ejecta, aka "volcanic bombs". In the case of Kilbourne's Hole, the volcanic bombs are sometimes cored by "mantle xenoliths", i.e., pieces of the Earth's mantle (carried in the magma/lava) that were already solidified before they reached the surface. When broken open, these mantle xenoliths reveal their composition of olivine and enstatite. When the olivine grains are big enough and of the proper color and clarity, they are termed "peridot" and some of the Kilbourne's Hole xenoliths have produced good-quality peridot.

[I will include a photo of one of these xenoliths in an upcoming "What a Geologist Sees" post. I wish I had collected more of these volcanic bombs when I lived in the El Paso area and I wish I had taken more slides of Kilbourne's Hole while I was working to the north in the Aden Basalts. I did collect some different types of volcanic bombs from within Aden Crater and from another maar to the west, Riley Maar.]

[I am not sure if there has been a Master's Thesis done on an analysis of the tuff rings and the distribution of the volcanic bombs at Kilbourne's Hole.]

All of the features within the Aden-Afton graben are geologically young, less than 2 million years in age, erupted during the Pleistocene Epoch of the Quaternary Period. These volcanic features and the others of the Potrillo Volcanic Field are related to the crustal thinning of the Rio Grande Rift, a crustal feature which is thought to extend from (approximately) the Big Bend area of Texas/Mexico northward into south-central Colorado (some interpretations may vary).

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