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.

Tuesday, August 8, 2017

A "Twofer"

An example of an image that serves two purposes...

Differential Weathering and the "universal melding of joy and grief" for field geologists.

This is a scanned 35mm slide from 40+ years ago, somewhere on the Georgia Piedmont (revisiting that past error of not labeling slides).

Prior to eons of Chemical Weathering, (based upon texture observations and a knowledge of prevailing local rock types), this was probably an Amphibolite, with some quartz-rich intervals (including the ledge upon which the Estwing prybar is perched).

"Differential Weathering" is due to the relative difference of susceptibility to Chemical Weathering between Mafic (Fe-rich silicate minerals) vs. Quartz.  Generally speaking, the minerals "higher" on the Bowen Reaction Series (including the Fe-Mg rock-forming minerals, e.g., Pyroxenes, Amphiboles, and Biotite) are more susceptible to chemical weathering at (or near) the surface.  In other words, at a quick glance, the ledges are probably Quartz.

As for the "universal melding of joy and grief", it is reference to finding (and then losing) tools in the field.  It was the subject of a past post, from 2011.  In other words, the post included the regrets of lost hammers, prybars, chisels, handlenses,...and other items.  We usually leave these things because we are tired.  Sometimes we are so obsessed with our newfound "toys" (that we may have carried several hundred yards to the vehicles), that we don't want to make one more trip back to the area to check for tools.

The Estwing prybar in the photo had such an "experience".  I found it at a now long-forgotten locality, kept and used it for a few years, then left it somewhere else.  (The "great circle of life"?)  I hope another Geologist found it and took it home.  So, if you do find a tool in the field, say a little "prayer of empathy" for the fellow scientist that lost it.

Monday, August 7, 2017

Thoughts on Traveling Alone

Over at "Itinerant Geologist" is posted some extensive thoughts about what that new blog is to be, a "travel geology blog" and the circuitous route of inspiration (40+ years).