When you think of Texas the image of volcanoes does not come to mind. But one of the most
beautiful landscapes of west Texas is the result of volcanism some 30 million years
ago. Some of our crew went out to Alpine Texas with an expert who has been working in the
area for over 50 years, Dr. Don Parker Hi, Iím Don Parker. Iím professor emeritus
of geology at Baylor University and currently professor of geology at Wayland Baptist University.
Iím a member of the American Geophysical Union and proud to be so and have been studying
the volcanic rocks of West Texas for a long time. Many people are surprised to learn that
Texas actually has volcanoes. These are really quite old and largely concentrated
in far southwestern Texas in the Davis Mountains and Big Bend country.
This volcanism occurred between 47 and 17 million years ago and was related to the subduction
of a large oceanic plate along the west coast of Mexico.
Volcanoes erupt in two basic styles: liquid lava flows like those of Hawaiíi and explosive
eruptions like those of Mt. St. Helens, which geologists call ìpyroclastic.î
Big pyroclastic eruptions result from the violent release of volatiles, which are largely
water, from magma. Many pyroclastic volcanoes have central regions that collapsed to form
calderas because so much magma was ejected, emptying the underlying magma chamber.
Crater Lake in Oregon is a spectacular example of a caldera associated with a pyroclastic
volcano. The rapid expansion of superhot steam that is released from the magma violently
breaks up the magma and produces fine particles (termed ìashî) and pumice. These eruptions
can also throw out huge blocks of rocks. Such eruptions produce towering columns of escaping
gas and particles, which can be dispersed over wide areas in the form of air fall deposits,
or in dense avalanches of hot gas and suspended particles termed pyroclastic flows.
Itís not too hard to tell lava flows from air fall ash deposits from pyroclastic flows
once you know what youíre looking for. Lava flows are hard and dense, pyroclastic flows
have chunks of rocks that are all different shapes and sizes, and air fall tuffs contain
nothing but fine ash. Most West Texas volcanoes erupted violently
and produced huge calderas, pyroclastic flows and associated lava. These eruptions were
more like those of Crater Lake volcano in Oregon than those of Hawaiíi.
One of the best examples of West Texas volcanism is the Paisano Volcano in the Davis Mountains
just to the West of Alpine, Texas. The caldera that formed here was about 8 kilometers or
5 miles across. The amorphous shape we see on a geologic map is due to millions of years
of the caldera eroding away leaving behind only remnants of its original structure. Itís
in these rocky remains that we see the products of this once majestic volcano.
For example, we can see the evidence on the sides of the mountains of different lava flows
that the volcano produced. These lavas spread out 15 to 20 kilometers from the volcano and
in some places are as high as. We can also find Pyroclastic rocks in the area. Here we
have the fine grained ash and the large rocks that were caught up in the hellacious avalanche
that once rushed down the side of the erupting volcano.
The next time you are in this area try and spend some time looking at the rocks. Whether
you are exploring the nearby town of Alpine or checking out the arts that Marfa has to
offer try thinking of Texas a little differently, not just as a rugged landscape to drive past,
but as a once massive volcanic field which took millions of years to develop and erode
until finally settling into the beautiful landscape we see today.
Thanks to Don Parker and AGU for making this video possible and thank you for watching.
If you liked this please check out our channel for more videos that we made with AGU for
their 100th birthday and stay tuned for more from UTDís GeoScience Studios