Tuesday, March 6, 2018


Aside from being a nifty word for Scrabble, "Xenolith" is an important component of the concepts of "Inclusions" and "Relative Age Dating".

For a quick review, a Xenolith is a piece of pre-existing rock that "falls" into a body of magma in which the temperatures are not hugh enough to assimilate the "foreign rock".  As the xenolith is an already-solidified piece of rock, it is older that the "host magma" (or lava) into which it falls.  Thus, within the xenolith/host rock relationship, the inclusion (xenolith) is always older than the host rock.

My best luck at collecting Xenoliths has been in Kilbournes Hole, New Mexico and the Eagle Mts., Texas (decades ago) and the back lot of a granite monument cutting business, west of Elberton, GA (below and next post).

In this particular Franklin Mts. outcrop (below), there are at least two sets of Xenoliths, the black basalt/disbase blocks and the greenish-gray contact-metamorphosed limestone, both of which "fell" into the Proterozoic Red Bluff Granite magma. 
This second photo, immediately west and uphill of the above, shows remnants of a larger Castner Marble (limestone) xenolith.

This photo is of a xenolith in the East Quarry area of Stone Mt., Dekalb County, Georgia.
 This particular xenolith is from the Elberton Granite.  [This image was "squared-off" - using Photoshop - for education purposes.]
In the case of the Mantle Xenoliths erupted in "volcanic bombs" at Kilbournes Hole, New Mexico, the ultramafic fragments were carried upward by the basaltic magma and erupted in a "Maar Volcano" setting.

Previous bloggings about the subject:

Kilbournes Hole, NM 

What a Geologist Sees - Part 3

What a Geologist Sees - Part 13

What a Geologist Sees - Part 14

Boulder xenolith, Eagle Mts., TX  

Saturday, December 30, 2017

Life Changes

As life continues to evolve,...

Got married a few days ago.  Moving about 25 miles to my bride's existing home, including books, rocks, and such.  Job changes.

2018 New Year's Resolutions will give a new impetus to resume blogging.

Wednesday, September 20, 2017

Will be back soon.

I am not doing a mental "walkabout", just busy with daily tasks, including cleaning up remaining debris from Irma's visit last week.  Nothing major, though we were without electricity for almost three days.

Saturday, August 12, 2017

Between Posts, a Brief Hiatus

When I resumed blogging on a regular basis, my intent was to blog everyday.  But life intervenes.  Nothing bad, just a social life and class preparation.

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.]

Thursday, August 10, 2017

Aden Volcanic Field

Aden Volcanic Field, it was like home for several years, while I worked on my Master's Thesis.  (This aerial photo, taken by Rollag and Associates, shows the northwestern part of the volcanic field.)  Aden Crater, a small shield volcano (approx. 2000 ft. across) is in the center left of the photo.  (The photo is oriented with North at the top.)

The Aden Basalts and associated Afton Basalts are part of the approximate 500 square miles of the Potrillo Volcanic Field, a regional feature related to the Rio Grande Rift.  The area is located in the South-central part of Doña Ana County, New Mexico, about 20 miles Southwest of Las Cruces.
The two irregular craters in the center bottom of the photo were part of my thesis project.  There were three other craters to the Northeast that were also given cursory coverage in the report.  The enigmatic craters, composed of boulder ramparts surrounding a collapsed floor, have been a source of curiosity among UT El Paso geologists since the middle 1960s.  More recent work on these types of craters in Hawaii by Tim R. Orr (USGS) yielded an explanation for their formation and gave them a name "Lava Tube Shatter Rings" (I called them "Explosion-Collapse Craters".)
Above is a photo of Quaternary-age Aden Crater, showing the typical low-profile of a basaltic Shield Volcano.  This photo was taken from one of the Lava Tube Shatter Rings within my field area.  The base of Aden Crater is about 4 miles across.  [The remainder of the photos in this post are mine, except for the aerial photo of the Lava Cone, also by Rollag and Associates.]  

The entirety of the Potrillo Volcanic Field offers a wide range of relatively-young basaltic volcanic features, e.g., Cinder Cones, Maar Volcanoes, Shield Volcanoes, Spatter Vents and Spatter Mounds, "Volcanic Bombs", "aa" and "pahoehoe" textures, Lava Cones, Lava Tubes, Columnar Jointing, Fumeroles, Ash Deposits,...(a few of which are shown here in this post.)

Outside of Aden Crater (a late-stage feature), most of the basalt flows are thought to be the result of "Fissure Eruptions".
Above and below are images of an intact Lava Cone (a miniature Shield Volcano).  The surface vent is about 6 feet across and the empty, roughly circular chamber below is about 30 feet in diameter and perhaps 20 feet deep.  This Lava Cone is unusual in that usually, once eruptions have ceased and remaining lava within have withdrawn and subsided, the cone usually collapses in upon itself. 
Another interesting feature, near the primary Lava Tube Shatter Ring (highlighted by my Thesis), was this tiny Spatter Vent, i.e., a tiny, sputtering volcano.
Within Aden Crater itself, near a subsided vent, was this Spatter Mound, shown below.  [A 5 foot long "Jacob's Staff" shows the scale.]  Usually, spatter activity is within the dying stages of a volcano's life span.

(Below) A few hundred yards East of Aden Crater are these crude Columnar-Jointed, Fissure-erupted Basalts.
[All of these photos were taken during a span of 32 - 28 years ago.  The area was and remains a Federal Wilderness Study Area, so I don't know what the current status is, vis-à-vis visits to the area.  As it is a few miles North of the Mexico Border, there might be some hazards in the area, due to smuggling, etc..  It goes without saying that when traveling in the desert, one should always carry a first aid kit, several gallons of water, local maps, etc..]

Wednesday, August 9, 2017

Class-Prep Tuesday

A short one-day sabbatical from blogging (on both blogs), to prepare for Physical Geology Lecture and Lab classes beginning next week.

Be back tomorrow.