The Cornudas Mountains of New Mexico and Texas:
Intrusives or Extrusives?
Laccoliths or Volcanos?
(Updated in 2016)
The source for this discussion is this publication Click on it to read it for yourself):
New Mexico Geological Society
Guidebook, 49th Field Conference, Las Cruces Country II,
1998, Geologic Framework of Tertiary Intrusions of the Cornudas
Mountains, Southern New Mexico, Constance
J. Nutt and J. Michael O'Neill, US Geological Survey, Denver,
Note these two mountains, viewed from the Guadalupe range, with some other mountains to their right and left. Several times on other pages on this website I called them volcanos, but now I have corrected that by referring the reader of those pages to this page. Where I do that referring to this page is here,
and finally here.
And here too, when we finally get up close to Wind Mountain in 2016:
Thanks to a tip from a geologist friend, Norbert Rempe, I looked into the geology of the Cornudas Mountains and learned that most are intrusive rock masses but few are volcanos. For example, Wind Mountain (above photo) is a laccolith, not a volcano.
Wikipedia defines and illustrates laccoliths: A laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.
So I was mostly wrong when I call them volcanos on the pages linked above. Most are laccoliths.
The two more prominent mountains in the above photos, with Wind Mountain as the larger of the two, are the result of a magma intrusion creating a dome under overlying rock. They are not an extrusive event (not volcanic). The way these mountains are formed is for a dike of magama to rise up and come onto a layer that resists its movement. It spreads out, slows down and begins to cool, allowing large crystals to form, and then it builds up sufficient pressure to move the overlying rock upward, and a dome forms under that upper layer of rock. The top of that dome may not extrude (become a volcano) but it now cools more rapidly nearer the surface and in contact with cooler rock, and it grows tiny cruystals that encapsulate the larger crystals --large enough to see with the naked eye--already formed in the sticky molten mass,. the new very fine crystals are too small to be seen with the unaided eye. So, it is not a volcano. It is a laccolith.
These intrusive mountains are the remnants of erosion-resistant intrusions of magma. In most cases, the magma stopped rising and cooled while still below the ground surface (remaining under pressure and cooling slowly). Magmas with similar chemistries will cool into rock with different mineralogies depending on the pressure and cooling rate. These intrusive mountains that cooled at some depth are largely nepheline syenite and quartz-bearing syenite.
Orthoclase is a potassium-aluminum-silicate, hence an alkali feldspar, The next most frequently encounterd alkali feldspar in syenite is albite, a sodium-aluminum-silicate. Nepheline is a mixed sodium-potassium aluminosilicate, and it also occurs in this setting, making the syenite whre it occurs a nepheline syenite. The extrusive (volcanic) equivalent of nepheline syenite is phonolite, which is abundant in the volcanoes among these mountains.
The ground the intrusive rocks were intruded into is no longer there, it has been eroded away. But three of the mountains and one valley are full of volcanic rock, the result of an extrusive event in which core-melt magma cooled more rapidly and under less pressure, and that explains why they are composed largely of the extrusive equivalent of the intrusive nepheline syenite called phonolite.
Wikipedia: Phonolite is a rare extrusive volcanic rock of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grain) to porphyritic (mixed fine- and coarse-grain). . . . The name phonolite comes from the Greek meaning (more or less) "sounding stone" because of the metallic sound it produces if an unfractured plate is hit; hence the English name clinkstone
Our New Mexico Geological Society Guidebook explains the way these mountains were formed in this diagram:
The names of the individual Cornudas mountains, some in New Mexico and some in Texas just across the border, are Wind, Black, San Antonio, Cornudas, Chattfield, Alamo, and Flat Top. Chess Draw is not a mountain but a phonolite-filled drainage into which this liquid rock flowed from the nearby volcanoes. Chattfield, Alamo and Flat Top are volcanic.
Chattfield is volcanic: meaning the magma flowed out onto the surface. San Antonio is formed by an intrusion of magma that did not break the surface. It is a laccolith. Both were formed by the same type of magma rising, but whether the magma cooled faster (volcano or laccolith top) or slower (laccolith deeper interior) determined the mineralogical composition of the dominant rock of each of these mountains.
Different pressure regimes and cooling rates made for different dominant minerals as shown in this table from our reference (pay no attention to the two-letter error, Corundas is Cornudas) [note that these mountain were formed a very long time ago! Ma means millions of years ago]:
Notice that the bulk of the Wind Mountain intrusion is nepheline syenite, coarse grained, indicating slow cooling. The upper part of the same rock mass is syenite porphyry, indicating more rapid cooling creating finer grains. At one time I though porphyrys had to have been extruded, but it turns out it isn't necessarily so, just being at the top of the intruding mass, and in contact with overlying rock, will also result in faster cooling. That was the case for this mountain since much of it is still covered by the rock it pushed up out of its way, with no basalt flows down the sides. A typical laccolith,
Wikipedia explains the creation of porphyry deposits this way: Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first, the magma is cooled slowly deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye.
OK, so I was less than half-right when I called these mountains volcanoes.
Was I not quite so right about anything else? Yes. I also declared [but no longer do] that these volcanos marked the edge of the Rio Grande Rift. In fact, as our source document indicates, that is not true. As shown on this map and as discussed in the source's text, the Rio Grande rift boundary is west of here.
The Cornudas Mountains sit on the dividing line between the Otera Platform and the Diablo Platform. The Diablo Platform, farther south, also has volcanoes. [See: Geology of the Sierra Diablo Region Texas By Philip B. King. Geological Survey Professional Paper 480, 1965.]
If there is any structure in the above figure that is meaningful in terms of the presence of intrusive and extrusive rock bodies, it is the Padernal Uplift, an anticlinal structure, meaning something pushed the land up in this elongated area, and this is the crest-line. The Rio Grande rift and its more recent volcanism lies directly to the west.
So, now I have it right, and if you are reading this, you have it right. That is a good thing.
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