Potrillo Volcanic Field
Volcanoes in the Desert
by Jay W. Sharp
Exploring the Potrillo Volcanic Field a 500-square-mile Chihuahuan Desert landscape of lava flows, cinder and shield cones, immense craters, structural fracturing, low and sullen mountains, depositional and wind-blown sands, playa lakes, cacti and scrub brush you feel that you can touch the pulse of our restless planet. The field, primordial and otherworldly, lies about 30 to 40 miles west of El Paso and the Rio Grande, immediately north of the Mexican border, in south-central New Mexico.
Origins and Evolution
The Potrillo Volcanic Field is a progeny of the titanic forces that have produced the 29-million-year-old Rio Grande Rift, a still-active structural fault that begins in Colorado’s Rocky Mountains and extends southward for more than 600 miles, through the heart of New Mexico, across the western tip of Texas and into Mexico’s state of Chihuahua. The rift, triggered by a collision of tectonic plates, formed as the earth’s crust bulged, cracked and warped. Some parts of the crust rose thousands of feet, producing mountain ranges. Other parts subsided thousands of feet, yielding intervening basins. Crustal fractures opened avenues for molten rock, which sometimes issued forth or exploded violently onto the earth’s surface up and down the rift. Over time, the rift created the gorges and valleys that would become the home for the Rio Grande.
For millions of years, the river, carrying waters and sediments derived from the rising mountains, emptied into an ephemeral land-locked body of water called Cabeza de Vaca Lake, which covered parts of western Texas, southern New Mexico and northern Chihuahua. Eventually, the ancestral Rio Grande breached its natural land barriers, joined with Chihuahua’s Concho River, and rushed to the sea. It drained Cabeza de Vaca Lake, leaving behind water-saturated deposits of sand hundreds to thousands of feet thick.
Several hundred thousand years ago, in the area that would become known as the Potrillo Volcanic Field, white-hot molten rock or magma, extruded by the internal pressures of our planet, rose from depths as deep as 50 miles up through structural fractures toward the surface. It would change the face of the landscape.
In some places, molten rock intruded into depositional sand but failed to erupt through the surface, leaving it to the forces of erosion to strip away the overburden, exposing igneous formations. In other places, molten rock erupted violently, explosively, ejecting clouds of congealed lava that fell back to earth as cinder, forming a cone around the vent. In some instances, it erupted over time as a succession of lava flows from a single vent or clustered vents, forming wide-spread basalt layers that took on the form of a Roman warrior’s shield lying flat on the sand, its battle side up. In some spectacular events, molten rock came into contact with the water-saturated sands left by the ancestral Rio Grande. It produced superheated and highly pressurized steam that exploded through the earth’s surface with monumental force, creating large craters that geologists call “maars” or lakes.
Often, lava breached the walls of the cinder cone and shield volcanoes, spilling down the flanks and spreading across the surrounding sandy surface, leaving 10- to 20-foot thick blankets of basalt spread across miles of sand. In some places, the lava flows look like ocean waves that froze in place as they ran up onto a beach. In some areas, molten rock withdrew from subterranean spaces, leaving voids into which the surface collapsed, leaving linear or circular craters. Some 20,000 to 40,000 years ago, the molten rock of the Potrillo Volcanic Field at last grew still, leaving behind a landscape littered with volcanic landmarks and debris that speak to the violence of the past. As the earth quieted, winds gathered exposed sand and re-deposited it like a shroud over many the volcanic features.
The Potrillo Volcanic Field Today
Today, in the Potrillo Volcanic Field, you can find intrusive volcanic features (of uncertain age) in some of the low mountains. These include, for example, dikes, which are linear masses of igneous rock that cut through overlying strata, and plugs, which are more vertical masses of igneous rock that filled, or “plugged,” volcanic vents. You can find more than 150 volcanic cinder cones, although most of them lie in the western and more inaccessible part of the field. Some of the cones, perhaps half a mile in diameter, rise more than 500 feet above the desert floor. You will discover various accessible shield volcanoes, five maar volcanoes and numerous lava flows. You can easily explore the particularly interesting Aden Crater, one of the shield volcanoes, and Kilbourne Hole, one of the several maar volcanoes within the field.
According to authority Jerry M. Hoffer, Aden Crater, in the northern part of the field, originated with a succession of lava flows that issued from a single vent, building up a shield cone over time. Volcanic “spatter” droplets of molten rock erupted next, forming a rim around the crater. Molten rock flows returned, issuing from the vent to become trapped by the spatter rim as a “lake” of lava. Small explosive eruptions became Aden Crater’s dying gasp, and as the last of the lava drained away down the vent, the immediately surrounding surface collapsed, leaving a large pit within the crater.
Strangely, back in the 1960s, investigators exploring a fumarole a vent through which gasses escaped discovered the desiccated remains of a giant ground sloth. It had lived in the region near the end of the last Ice Age, roughly 11,000 years ago, during the same time that bands of nomads hunted big game of the area. Somehow, it had fallen into the vent to its death. [You can read more about that sloth here.]
Kilbourne Hole, a world-famous National Natural Landmark located in the eastern edge of the field, has long attracted scientists who want to compare the maar with similar features on other bodies in the solar system. Lunar scientists and Apollo astronauts studied Kilbourne Hole back in the 1960’s, during our manned flights to the moon. More recently, scientists have compared Kilbourne Hole with what may be maar-type volcanoes they have seen during unmanned flights to Mars. (Maars on Mars?)
One of the largest, best-preserved maar volcanoes in the world, Kilbourne Hole, which formed between 24,000 and 100,000 years ago, extends for about two miles in length and about a mile in width. Its bottom lies several hundred feet below the surrounding desert floor.
Early on, its unusual appearance led some people to speculate that it had been caused by a meteorite that crashed into earth and rebounded, like a tennis ball, back into space. Compared with volcanoes like Aden Crater, which developed over time, Kilbourne Hole appeared with great suddenness when the superheated steam produced by magma and subterranean water blasted some 500 million cubic yards of sand, rock fragments and shattered basalt into the sky. The explosion, according to Earl M. P. Lovejoy, El Paso’s Geologic Past, produced a funnel-shaped crater. Much of the blast material fell back into the crater, refilling much of the void. Other material fell around the crater lip, forming a rim. Subsequently, in what was probably a series of “staccatolike eruptions,” according to Hoffer, clouds of steam, ejected material and debris combined to construct “a rampart of stratified tuffs [consolidated volcanic ash].” The rampart is sometimes described as a “festooned dune” because its stratified layers look something like festoons billowing in the wind. After the explosions ended, the walls slumped into the crater, widening its diameter, and winds deposited sand dunes around the rim.
Among rockhounds, Kilbourne Hole is renowned for its “volcanic bombs,” or xenoliths, which encase olivine crystals and, with extremely good luck, gem-quality peridot. A volcanic bomb begins as a “glob” of liquid lava blown from the crater vent into the air. It cools and begins solidifying while airborne. If still sufficiently pliable when it falls back to earth, it may take on a roughly spherical shape as it tumbles over the surface. Within Kilbourne Hole’s volcanic bombs, the mineral olivine forms greenish or yellowish transparent or translucent crystals, sometimes of high enough quality to be considered a peridot.
Olivine comprises some 50 percent of the earth’s mantle, the 1800-mile thick and dense boundary between our planet’s crust and the molten interior regions. Among the most abundant of the minerals on earth, it has also been found on the moon and on Mars as well as in meteorites. It appears to be common in our solar system.
The peridot, the birthstone for August, has been held in high value for thousands of years. Peridot, mined by slaves, served as settings in jewelry for the ancient Egyptians. It is found in Pakistan, Brazil, South Africa, Kenya, Mexico and other countries. Some ancient cultures believed that peridot brought peace and good luck. Mystics still believe that peridot can slow aging, heal various ailments, and renew vitality. Others believe that it will increase one’s self-confidence and patience.
While volcanic bombs and olivine occur in one of the other maar volcanoes in the Potrillo Volcanic Field, you will find that they are most abundant in Kilbourne Hole, primarily in the vents at the north and south ends of the crater.
Prehistoric Man in the Potrillo Volcanic Field
Surprisingly, in this landscape of fiery violence, restless sands and desert shrubs, man has had a presence for centuries. You will find the evidence (which you must, by law, leave in place) around playa lakes, which have thin scatters of potsherds, flint flakes and grinding tools. You will find still more evidence on top of the wind-deposited sand dunes at the north end of Kilbourne Hole, where occasional widely scattered fire hearths, potsherds, projectile points, flint flakes and grinding tools still turn up on occasion.
You may hear tales about how an early miner had to kill and abandon a pack pony, which was carrying a treasure in coins, out in the volcano field while fleeing Apache Indians. The miner made good his escape, but upon returning, he could never find the pony’s carcass or the money. Presumably it remains lost somewhere on the desert to this day. He did, however, give the region its name because the Spanish word for pony is “potrillo.”
From historic times, you can still see, in the very bottom of Kilbourne Hole, the rock foundation of an rudimentary structure. Today, ranchers still run livestock across the Potrillo Volcanic Field, providing water with windmill driven wells and supplementing meager grazing with commercial feed.
Exploring the Potrillo Volcanic Field
If you set out to explore the Potrillo Volcanic Field, which comprises a mix of federal, state and private land, you will not if you use caution need a four-wheel drive vehicle to negotiate the region’s dirt roads, but you will certainly be well served by a high clearance vehicle. Since the area is remote, out of reach of some cell phone services, you should let an acquaintance know where you plan to go and when you expect to return.
Ideally, you should convoy with other vehicles. You will need sturdy walking shoes and preferably a good walking stick for hiking over the rough-surfaced lava and sand. You should take water and food in the event you get stranded. Keep a close watch for rattlesnakes, both diamondbacks and westerns, which especially favor the rodent burrows and the numerous recesses around the craters and lava flows. Give a wide berth to individuals walking across the desert, because illegal immigrants and even drug couriers sometimes manage to cross the border and elude the Border Patrol and then pass through the area.
There are various and usually poorly marked ways to reach the Potrillo Volcanic Field. Good recent maps such as the Bureau of Land Management’s Deming, Las Cruces and El Paso topographic maps, are essential for navigating the area.
One option, which will give you the opportunity to explore Aden Crater, a large lava flow and Kilbourne Hole, starts near the hamlet of La Union, New Mexico, 10 or 12 miles north of El Paso. Beginning at the intersection of State High 28 and Mercantile Avenue, take the avenue west. This will feed you into a dirt road that crosses the river, ascends the valley escarpment and traverses the desert, bringing you to a railroad track, about 8.8 miles from your starting point. After you cross the railroad track, turn right, following the road that parallels the track for 20.7 miles. From here you can see Aden Crater to the south-southeast. You should will find a small, washboard road, which you follow for two miles to a short turnoff to the crater.
There is a barren area that will serve for parking. Here you will find a dim footpath that will take you up the crater wall and into the crater. You will find the fumarole that entombed the giant ground sloth just under the rim, on the east side of the crater. Returning to the road that parallels the railroad track, turn right, retrace your route for 3.7 miles, where you will find another turnoff and a rough road that within less than a mile will conduct you into the heart of a massive lava flow. (A dilapidated windmill serves as a landmark.) Returning again to the road that parallels the train track, turn right, retrace your original route for 17.6 miles. Turn right, at a cattle guard. Follow the road westerly for 11.5 miles, which will bring you to a turnoff, to another rough road that will take you to Kilbourne Hole. You will come quickly to Hunts Hole, another large maar volcano, which you will circle, keeping it on your right.
Continue on the dirt road for about three miles, and you will arrive at the southeastern edge of Kilbourne Hole. You will find the festooned dune at the edge of the crater, off to your right. You will discover the volcanic bombs, with the olivine crystals and, with good luck, peridot, at vents at the north end and, to a lesser extent, the south end of the crater. Return to the road that parallels the railroad track, turn right, and within half a mile you will find the road that crosses the track and takes you back across the desert, down the valley escarpment, over the river to La Union and your starting point.
While exploring the Potrillo Volcanic Field is a powerful experience, I urge that you be especially vigilant and cautious, especially if you’re not familiar with the desert. Should you take a serious fall, get snake bitten, or get stranded in this isolated area, a long way from medical care, you could have a serious problem.
If you want to learn more about the Potrillo Volcanic Field, which comprises a mix of federal, state and private land, refer to Jerry M. Hoffer’s 1976 “Geology of Potrillo Basalt Field, South-Central New Mexico,” Circular 149, New Mexico Bureau of Mines & Mineral Resources and Earl M. P. Lovejoy’s El Paso’s Geologic Past, Science Series, Number 7, Texas Western Press. Those have been my primary resources for this article.
For additional information, about maps, directions and road conditions, contact the Bureau of Land Management, the federal agency that has jurisdiction of the federal lands in the field, at the following address:
Las Cruces District Office
1800 Marquess Street
Las Cruces, New Mexico 88005-3370
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