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. 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. In some spectacular events, molten rock came into contact with the water-saturated sands left by the ancestral Rio Grande. 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 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 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. It would change the face of the landscape. 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. Leading edge of a Potrillo Volcanic Field lava flow. ![]() It drained Cabeza de Vaca Lake, leaving behind water-saturated deposits of sand hundreds to thousands of feet thick. Eventually, the ancestral Rio Grande breached its natural land barriers, joined with Chihuahua’s Concho River, and rushed to the sea. Over time, the rift created the gorges and valleys that would become the home for the Rio Grande.įor 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. Crustal fractures opened avenues for molten rock, which sometimes issued forth or exploded violently onto the earth’s surface up and down the rift. ![]() Other parts subsided thousands of feet, yielding intervening basins. Some parts of the crust rose thousands of feet, producing mountain ranges. The rift, triggered by a collision of tectonic plates, formed as the earth’s crust bulged, cracked and warped. 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. ![]() NASA Earth Observatory images by Jesse Allen, using Landsat data from the U.S. 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. Geological SurveyĮxploring 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. Potrillo Volcanic Field Volcanoes in the Desert
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