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EL PASO, TEXAS – North and west of El Paso is a giant hole in the ground in the middle of a vast stretch of Chihuahua Desert that covers northern Mexico, most of Southern New Mexico and all of West Texas. In fact, there are two giant holes right near each other in that great empty desert, both volcanic-rimmed craters amidst sand dunes, cactus, creosote bushes, ocotillo and yucca near the base of the East Potrillo Mountains: Kilbourne Hole and its smaller neighbor to the south, Hunt Hole.
Between 50,000 and 80,000 years ago the Aden Crater Lava Flow (part of the Afton Lava Flow event) covered the light brown sand dunes west of the Rio Grande Valley with a thick layer of dark brown lava from a series of low-intensity volcanic eruptions. That lava flow is reminiscent of the ‘a’a and pāhoehoe flows found in Hawaii today – examples of both being readily visible from your vehicle as you drive around the edges of the Aden Crater flow mass to the north of Kilbourne Hole itself.
Subsequent to that ancient Aden Crater Flow, some 20,000 years ago, long after that layer of lava had hardened into a 100+ square mile, several-yards-thick cap over those ancient desert sands, another volcanic eruption from deep below that desert floor drove a column of superheated water, gas, basalt rock and pyroclastic material through several seismic crustal cracks that run north-south in this region to the surface, bursting through the earlier volcanic cap and begining the formation process of these huge craters we now call Hunt and Kilbourne Hole. This geologic wonder that we have called a “hole” is technically called a “maar”, however.
This aerial view toward Ukinrek Maar, Alaska (above) illustrates what the explosive event might have looked like that formed Kilbourne Hole in New Mexico some 20,000 years ago. In this image a lava dome erupted in the 100-meter deep crater during a 10-day eruption cycle in 1977. This maar is about 300 meters in diameter, compared to the 1.7-mile wide Kilbourne Hole crater.
A maar, then, is a low-relief, broad volcanic crater formed by shallow explosive eruptions. The explosions are usually caused by the heating and boiling of groundwater when magma invades the groundwater table from mantle levels deep below the Earth’s crust. Maars often fill with water to form a lake, making them less likely to be discovered or studied.
In our locally-destructive event near El Paso, steam and pyroclastic eruptions along the Fitzgerald Fault broke through the Aden Lava Flow, belching destruction over a vast area of the Rio Grande Rift Basin to the south and east – downwind of the craters. Hunt’s Hole, ten miles south of Kilbourne Hole and the Potrillo Maar, even further south on the U.S.-Mexican Border, together threw millions of tons of ash and pyroclastic material into the atmosphere over several hundreds or maybe thousands of years of eruptions.
The Kilbourne Hole maar event itself is huge in comparison with the Ukinrek Maar event pictured above: a 1.7-mile diameter hole in New Mexico versus a 300-meter diameter crater in Alaska! The Kilbourne Hole maar eruption, alone, would have generated a gigantic column of ash and pyroclastic material some fifty times greater than the Ukinrek Maar. With the additional eruption events at nearby Hunt Hole and Portrillo Maar, the environment of the West Texas-Southern New Mexico region was dramatically changed. The landscape surrounding the Kilbourne maar and the area within a huge downwind ash swath to the East-Southeast, toward El Paso and Fabens, would have been severely impacted. Today, distinct layers of curious ash-like material are visible within sedimentary layers exposed in the escarpments near Cattlemen’s Steakhouse in Fabens, Texas (in a geologic feature called the San Felipe Arroyo) and along the Interstate 10 route east of El Paso…some forty to sixty miles away.
From what we know of the earliest Native Americans in this region, it is very possible that some of North America’s first inhabitants, the Ancient Ones – who were here long before the Anasazi and migrated from the Northwest into the Southwest Region, witnessed those cataclysmic eruptive events that shaped this volcanic feature and others in the area. One can only imagine the place these stories might have held in their mythology.
Throughout my life in this border city, the vast desert area surrounding El Paso has been one of our favorite playgrounds and weekend destinations. Only about an hour’s drive on washboard-rutted dirt roads west of the city limits, the sprawling geologic formations within the Potrillo Volcanic Field gave us dove, quail, snakes, coyote and rabbits to hunt by day, incredible sunsets in the evenings and inky dark skies decorated with millions of bright stars all night long. During a full moon, it is so bright you could almost read a book as you sit outside in that beautiful, clear, dry, high-desert air. After all, we’re at an elevation of over 4,000 feet MSL. Nighttime brings a chill to that desert air and as much as a thirty degree temperature drop from daytime.
Many a summer and fall weekend during childhood were spent with my cousins (Warren Brown and his sons) and other friends exploring the West Sandhills at the foot of Mount Riley – where Aden Crater, Hunt’s Hole and Kilbourne Hole reside. A campout among the sand, cactus, creosote, ocotillo and mesquite bushes, sleeping out under the dark desert sky, changes your perspective on life. The stars are almost as tangible as the mesquite wood crackling on the fire. Having a gigantic 1.7-mile wide geologic feature such as this on your doorstep when you arise at dawn puts a different point of view in play. When I first knew this crater, I had no concept of the ancient cataclysmic events that had formed it. Then, the conventional wisdom was that it was a meteor crater.
Occasionally, when walking through the dunes on the eastern edge of the crater, or just below the east escarpment, we stumble upon treasures thrown out of that volcanic eruption so many tens of thousands of years ago: olivine-filled basalt xenoliths. You have to look carefully to discern these volcanic “bombs” amidst the other volcanic ejecta surrounding the crater. The surface of the xenoliths containing the olivine treasure looks slightly browner and more ropey than normal basalt lava, and the stones are more round and very heavy…heavier than the rest of the rocks you might pick up and investigate.
You might also see the remnants of previously-discovered xenoliths glistening like emerald green crystal sand in the sun on the occasional flat rocky surfaces…where earlier treasure hunters used hammers to smash them open, hoping for those rare large olive crystals that can be cut to produce the exquisite Peridot semi-precious stone. Kilbourne Hole is one of only a few places on Earth where peridot of gem quality can be found absent heavy mining operations.
So, how again did this beautiful and fascinating geologic artifact form? Kilbourne and Hunt’s Holes are classic examples of a maar crater, as we described earlier, that formed as a result of the explosive interaction of hot basaltic magma carried up from deep underground with groundwater during a volcanic eruption. When a steam-saturated eruption column forms during an explosive event and then gravitationally collapses, a ring-shaped surge of ash and pyroclastic material (a super heated water-ash-rock slurry) travels radially outward from the central vent along the ground. The stratified, cross-bedded pyroclastic surge deposits surrounding the crater at Kilbourne Hole are spectacular and were formed as a consequence of such explosions from the center of the large crater, each explosive event forming beds of material in differing cross-layers full of small “volcanic hailstones” called lapilli. The area surrounding the rim was pelted by basalt “bombs” containing the mineral olivine thrown up from those deep eruptions, layer between ash and pyroclastic surge material.
Basalt lava flows, very similar to a’a lava in Hawaii, which erupted between 50,000 to 80,000 years ago through a set of vents called the Afton Cones (the Aden Crater and Black Mountain lava formations) located north-northeast of Kilbourne Hole, flowed southward and covered the present area around Kilbourne and Hunt’s Holes below the East Potrillo Mountains. The explosion that formed Kilbourne Hole erupted from deep below in what is known as the Fitzgerald Fault, and up through the Afton lava cap, indicating to us that the Kilbourne and Hunt craters are younger than the Afton Lava Flows themselves.
Pyroclastic surge beds and vent-associated breccia blown from those craters (including our precious xenolith “bombs”) overlie the Afton basalt flow. So does eons-worth of desert blow-sand. The crater we see today formed during the final stages of that huge maar eruption, partly from collapse of the edges inwards as the magma intrusion deep below subsided, and partially from expelling surface material during the explosive eruption events in the huge column ash and dust these maar generated.
To get a good glimpse of these strata, walk around the east rim of Kilbourne Hole, starting from a large cleared area just off that long ranch road you just rode for an hour and half of rutted washboard adventure out of El Paso.
See the lthick black-brown horizontal layer of basalt lava exposed along the edge of the crater rim, broken off and tumbled down the slope into the crater itself? That dark brown layer is the Afton Lava Flow which once covered the desert surface for miles around in several feet of lava, and pre-dated the explosions that created the crater. Eons ago, and deep below that lava covered surface, magma-heated steam, sand, rock and ash surged to the surface and broke through in a series of large cracks, spewing pyroclastic emissions into the air – spitting chunks of the Afton basalt lava surface all around.
One stunning gift today of that cataclysmic eruption: olivine-filled xenoliths thrown from that boiling caldron. Those layers of surge flows and their treasures are now exposed above the dark lava edge of Kilbourne Hole. Notice carefully the texture and makeup of the soil at your feet and the colors and textures of the layers in the exposed sediments as you wander around the rim trail to your right. They give clues about the makeup of each explosive eruption. That “cauldron” is now the present crater you are overlooking, almost 1.7 miles across – an artifact of a huge regional tectonic feature called the “Rio Grande Rift System”.
Look at the texture of the grains in those multi-colored bands, notice the occasional layer of “volcanic hail stones” – small light round spheres of ash material formed in the furious rush of updrafts and downdrafts created by the volcanic eruption and pyroclastic flows. Once in a while you might come across a large lava bomb embedded in those layers of surge flows. You might observe how its mass penetrated and deformed many layers of the surrounding sands, thereupon being buried itself by subsequent layered flows. You might even be fortunate enough to find a xenolith, untouched since landing in the ejected tuff and pyroclastic outflow around the crater, waiting eons for someone to open and find it’s glorious green olivine crystals inside.
Kilbourne Hole is unique because of the remarkable abundance of both crustal and mantle (peridotite/olivine-bearing) xenoliths found in basalt bombs ejected during the eruption and scattered throughout the area. Xenoliths are inclusions of rock made from pieces of once-molten mantle and crust, that were incorporated into the magma as it moved from a depth of about 40 miles (60 km) to the surface. Once ejected, the core of the xenolith cools, and the longer it takes to cool, the larger the crystal structure inside becomes. Of course, like Forrest Gump was so famously quoted as saying in the movie of the same name, the process of finding and opening a xenolith is a lot like exploring “a box of chocolates, you never know what you’re gonna get”. Small green crystals indicate quick cooling, large crystal structure mean that the xenolith cooled slower. Slower is better.
Mega-geologic features making up the landscape that cradles this huge volcanic crater are part of the Mesilla Basin. “Mesilla” (Spanish for “little table”) is a common name for many places around the Northwest El Paso-Las Cruces corridor. Another geologic basin adjacent to the east and separated from the Mesilla by the range of the Franklin Mountains, is called the Hueco Bolson (Basin). Both basins provide deep and abundant sources of drinking water for the communities nearby.
These basins are part of a series of linked basins between central Colorado and west Texas that began to form about 36 million years ago when the earth’s crust extended or stretched in an east-west direction here. Normal faulting associated with this kind of crust extension and thinning led to the development of these deep basins and prominent rift-flank uplifts, producing this continental-scale tectonic feature now known as the “Rio Grande Rift”. The Franklin and Organ mountains to the east and the East Portillo Mountains to the west of Kilbourne Hole are examples of rift-flank uplifts in our south-central region of New Mexico and West Texas.
The rate of rift-flank uplift and basin subsidence likely peaked between 4 and 10 million years ago. Crustal thinning during extension periods tends to trigger volcanic eruptions, which is exactly what happened in this region of Southern New Mexico. The crust thinning triggered a series of significant volcanic events up and down the Rio Grande Rift system. The basalt lavas at Kilbourne Hole are on the eastern edge of the extensive Potrillo volcanic field, which was active between 1.2 million and 20,000 years ago. Twenty thousand years is not that long ago, geologically speaking. This youthful volcanism suggests that the Rio Grande rift extension process is still active in this region. The more than 100 vents of the Potrillo volcanic field are aligned along older faults. Kilbourne Hole, Hunt’s Hole, and the Potrillo maar are aligned along the newer Fitzgerald-Robledo fault system.
Extension of the crust in this part of the Rio Grande Rift began about 36 million years ago. Rock debris that eroded from the developing rift-flank highlands, as well as wind-blown and playa-lake deposits, accumulated in the subsiding Mesilla Basin – the area between the north-south mountain ranges where the Rio Grande flows today. These basin-fill deposits of sand, clay and silts, known as the Santa Fe Group, are 1500 to 2000 feet thick beneath Kilbourne Hole today. The uppermost sand, silt, and clay of the Pliocene to early Pleistocene “Camp Rice Formation”, the youngest unit of the Santa Fe Group in this part of the basin, are exposed for us in the bottom of Kilbourne Hole. The Camp Rice Formation was deposited by a south-flowing braided river that emptied into a playa lake in the vicinity of El Paso. That huge ancient playa lake which extended for hundreds of square miles has generally been referred to by geologists at the University of Texas at El Paso as “Lake Cabeza de Vaca”.
The La Mesa surface, a flat surface that developed on top of the Camp Rice Formation, represents the maximum basin fill of the Mesilla Basin at the end of Santa Fe Group deposition about 700,000 years ago. This “La Mesa Surface” is about 300 ft above the modern Rio Grande floodplain, and is the “escarpment” or “West Mesa” that we see from El Paso or Las Cruces as we look across the Rio Grande toward Santa Teresa and Mount Riley to the west. This surface formed during a period of landscape stability, meaning during a time of relative volcanic quietness.
Basalt flows from the Portillo volcanic field are layered within the upper Camp Rice Formation and lie on top of the La Mesa surface. The material we know that makes up west El Paso’s alluvial fan formations is eroded and layered deposits from that same playa period. The Rio Grande started to cut down through those older Santa Fe Group deposits some 700,000 years ago in response to both climatic changes and connection of the upstream river system with the Gulf of Mexico as Lake Cabeza de Vaca was breached and drained southwards. This downcutting was not a continuous process; there were several episodes of downcutting, back-filling, and renewed incision. The episodic development of the Rio Grande system led to the formation of several terrace levels along the river between Las Cruces and El Paso that can be easily identified today.
Historically, the area we know so well around Kilbourne Hole could have been part of El Paso and Texas, but by the stroke of a pen between politicians far removed from the region. On the east side of the Rio Grande, the Franklin Mountains and the city of El Paso are now part of the State of Texas, whereas the Potrillo Mountains and Kilbourne Hole, on the west side of the Rio Grande, are in what is now New Mexico – a historic addition to our country that was part of the original Gadsden Purchase from Mexico, one of seven major territorial expansions which created the United States that we know today.
The Gadsden Purchase of 1853 is remarkable in that the United States purchased a strip of land from the Mexican government along the U.S. – Mexico Border for $10 million at the time (equivalent to $279 million in present day terms), today that land is part of New Mexico and Arizona and begins on the extreme west edge of Texas at El Paso, marked by International Boundary Marker No. 1 on the west banks of the Rio Grande near the former ASARCO plant at Executive Center Boulevard and Paisano Drive.
This territory grab was intended to assist the construction of a southern transcontinental railroad route (now the Union Pacific Railroad), taking the easier way westwards by traversing the Continental Divide south of the difficult passes of the Rocky Mountains to the north of El Paso, and just skirting the Afton lava beds we know so well at Kilbourne Hole and Aden Crater. The train whistles still echo across those vast Western sandhills today.
On May 21, 2014 President Barack Obama signed a Proclamation designating the area around Kilbourne Hole and other nearby regional historic and geologic features as “The Organ Mountain Desert Peaks National Monument“. Now, over one-half million acres of Southern New Mexico land are protected for posterity. Though the National Monument is made up of three distinct non-abutting regions, together they form a huge historical natural canvas that speaks to us of the remarkable time-line painted in this portion of the Chihuahua Desert.
Here is a link to the White House website and a copy of the actual Presidential Proclamation: