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The Trans-Pecos Volcanic Field

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Big Bend National Park, close to the area of frequent volcanic explosions 30 million years ago. (Flickr / Tom Haymes)

For scientists and laypeople alike, the geology of Trans-Pecos Texas is dauntingly complex. The landscape has been shaped by disparate forces and events, over hundred of millions of years. But for sheer drama, no chapter in that story is more gripping than the period of volcanic activity.

The Trans-Pecos was roiled by volcanism over 30 million years. Magma pressed upward against the surface, lava flowed, and eruptions buried thousands of square miles in ash. Evidence of that violent activity can be seen from El Paso to the Chisos Basin.

The Trans-Pecos Volcanic Field, or Trans-Pecos Magmatic Province, began about 48 million years ago.

The dinosaurs had come and gone, and a vast inland sea that had submerged most of Texas had receded.

Off the coast of Mexico, an oceanic plate was grinding itself beneath the continent. In this collision, water migrated deep beneath the earth's surface. Water lowers the melting point of rock, and, in the earth's mantle, magma began to accumulate and rise.

Something had to give, and eventually the liquid rock burst to the surface. Some of the earliest results can be seen along Highway 118, near Study Butte.

Dr. Kevin Urbanczyk is a geology professor at Sul Ross State University, and one of the leading researchers on West Texas volcanism.

“In the mid-40s, of millions of years, there were multiple intrusions and explosions in the north of Study Butte area. Most of that really bumpy topography down there is mid-40, roughly, magmatism. That's older stuff, so what we see down there is eroded down to sort of the plumping systems.”

Trans-Pecos volcanism featured explosions and lava flows. There was also “intrusive” activity, in which magma never reached the surface. In the ensuing ages, overlying rock was eroded away to expose the igneous material. The exposed forms are often dome-shaped or conical. Brewster County's Hen Egg Mountain and Mitre Peak, south of Fort Davis, are examples of such intrusions.

From Study Butte, the center of volcanic activity shifted to the north and west. Between 39 and 35 millions years ago, a series of eruptions and lava flows laid the rocks of the Davis Mountains. During the same period, volcanism to the southwest formed the Sierra Vieja and Van Horn mountains.

Explosive volcanic activity formed the Eagle and Quitman ranges, in southern Hudspeth County. Magma swelled beneath the surface between Alpine and Marfa, creating a shield-shaped mountain, 2,000 feet high. Eventually, the mountain erupted. The collapsed remains of that volcano form the landscape of Paisano Pass.

The Chisos and Chinati mountains were laid in lava and ash between 35 and 32 million years ago. The Chinatis were the site of the largest eruption in West Texas – the Mitchell Mesa Tuff. It buried thousands of square miles in fiery ash and left a 240-square-mile hole in the ground.

“It's up there. It was big. And if you'd have been in Alpine at that time, you probably wouldn't have survived.”

About 30 million years ago, the forces of plate tectonics changed. Subduction on the Pacific Coast ceased. With the stress removed, the continental plate began to spread and extend. Explosive volcanism continued – in present-day Big Bend Ranch State Park, and the adjacent Sierra Rica mountains in Mexico. But as the crust thinned in West Texas, volcanic activity was more often characterized by lava flows. Those flows continued until about 17 million years ago.

Trans-Pecos volcanism remains relatively understudied. New research suggests it may have begun earlier than previously thought. And Urbanczyk said that the processes were so diverse that referring to a single “volcanic province” is misleading.

At Sul Ross, Urbanczyk has recently acquired a new X-ray device that can assess the chemical makeup of volcanic rocks. He said the analysis could provide new insights into the region's volcanic history.

“The Trans-Pecos has been looked at for decades, with various instruments. But now I have one that is state-of-the-art, and I might just go out there and just piece it all together again, just make sure everything is right.”

Underfoot and on the horizon, evidence of volcanism is inescapable in Far West Texas. Its geology remains an open question.

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