The Portrillo Volcanic Field
The 500-square-mile Portrillo Volcanic Field bears vivid scars inflicted by the Rio Grande Rift’s violent geologic history—low and brooding igneous mountain ranges, sprawling lava flows, magmatic dikes and plugs, volcanic cones, and immense craters. It is covered by the veneer of the Chihuahuan Desert, with ragged blankets of sand, growths of stunted mesquites and creosote shrubs, clumps of cacti, and stands of yuccas.
In this desolate and unforgiving landscape, you sometimes discover traces of ephemeral prehistoric encampments, marked by thinly scattered shards of crudely fashioned brown ware pottery, fragments of stone tools, and burned rocks from centuries-old fire hearths. Some of the sites occur near playas—those shallow depressions that occasionally hold water and perhaps served as prehistoric corn fields. Others occur in the midst of desert scrublands, inexplicably distant from any water, food sources, or distinguishing geographic features. Surprisingly, in the mountainous areas, you can find occasional rock art sites, which recall the reverence that prehistoric people felt even for this grim land.
In a hard drive through the scrublands, the low-lying volcanoes and the lava flows appear almost frozen in time, and that impression was reinforced in the 1920s, when three young men, exploring the shield-shaped volcano called Aden Crater, in the northeastern part of Portrillo Volcanic Field, made a remarkable discovery.
In a 15-foot by 30-foot chamber at the bottom of a 100-foot-deep fumarole, or gas vent, near the eastern rim of the crater, they found the well-preserved skeletal remains of a late Ice Age creature known as a shasta ground sloth, an animal, now extinct, about the size of a black bear. Apparently by misstep or in flight, the sloth had stumbled into the fumarole. Somehow, it had survived the 100-foot fall. It made its way into the stony space that would become its tomb for 11,000 years. A covering of bat guano, said Time Magazine, January 7, 1929, “preserved the sloth-bones, teeth, tendons, hide and even a food ball in its stomach.” Located today at Yale’s Peabody Museum, the sloth’s remains provided biological scientists with an unequaled picture of the animal’s life in a late Ice Age world.
If the spirit moves you, you can still descend the 100 feet, by rope, into the fumarole and explore the room where the sloth met its end. Just bear in mind, as Professor Chester R. Longwell, Yale University, said after visiting the fumarole in 1928, “The descent into the pit is difficult… From the first landing the pipe continues down irregularly by a series of steep slopes, nearly horizontal stretches, and vertical drops, and the diameter varies greatly.” For the careless, it could be, as Longwell suggested, “A most appalling death trap...”
Roughly 10 miles south-southeast of Aden Crater, you will find several craters called “maar” volcanoes, the most famous and best preserved being Kilbourne Hole, which was awarded the status of National Natural Landmark in 1975. Typically, a conventional volcano consists of a cone or a shield of volcanic ash and cinders, lava flows and basaltic blocks. By contrast, a maar-type volcano features a hole blown through the surface of the earth by the pressure of superheated steam, which is created when magma rises to contact subterranean water.
Kilbourne Hole provides a dramatic illustration of the monumental force at work. In its sudden formation, tens of thousands of years ago, a steamy drumbeat of eruptions blew hundreds of millions of cubic yards of basalt, rocks and sand into the sky. It produced, according to Earl M. P. Lovejoy, El Paso’s Geologic Past, a funnel-shaped crater. It would span nearly two miles in length and a mile in breadth. Much of the ejecta fell back into the crater, partially re-filling it. Other material fell around part of the crater’s lip, creating a rim. Still other material fell in a scatter across the surrounding desert floor.
After the principal Kilbourne Hole eruptions, said authority Jerry M. Hoffer, lesser, “staccatolike eruptions,” produced steamy clouds of smaller ejecta that consolidated to form “a rampart of stratified tuffs [consolidated volcanic ash].” These have been described as “festooned dunes” because their stratified layers resemble festoons billowing in the wind. After the drama of the eruptions drew to a close, the walls of the crater slumped, widening the diameter. Winds deposited dunes of sand around the rim.
In the 1960s, during the Apollo manned flights to the moon, astronauts and scientists visited Kilbourne Hole to study its features, which bore similarities to possible maar-type volcanoes on the surfaces of other bodies, such as Mars, in our solar system.
Today, exploring Kilbourne Hole, still hundreds of feet deep, you will see graphic evidence of the monumental forces that lie beneath the surface of our planet. You can examine the strangely beautiful festooned dunes. You can climb the sand dunes at the crater’s northern end, where you might find traces of a prehistoric human occupation. You can see, in the very bottom, a cabin foundation, which an early settler constructed from Kilbourne Hole’s ejected stone. In a crater vent, at the north end, you can find “volcanic bombs,” or xenoliths, with green olivine crystals and black augite fragments embedded in a thin coating of lava. With good luck, you could discover a gem-quality olivine crystal called a peridot, the birthstone for August.
With exceptionally good luck, you may just discover the treasure of gold borne by a prospector’s pony abandoned and lost somewhere in the Portrillo Volcanic Field during an attack by Apaches during the late 19th century. According to legend, this pony – “portrillo” in local Spanish – gave the volcanic field its name.
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