The southwestern region of the United States is geologically fascinating. The Colorado Plateau, with a smorgasbord of awe-inspiring rock formations and geological features, rises dramatically from the Rio Grande rift. The rift is a large, sunken region in the middle of the North American continental plate.
Map: USGS
Description: Scott Nash – File:Tectonic plates.png, Public Domain, Link
Plate Tectonics
Globally, there are seven major plates, ten minor plates, and a bunch of micro plates. Collectively they form the earth’s surface, like the planet’s scales or armor. The plates move on the softer layer underneath; pushing, pulling, colliding, and tearing, provoking earthquakes and volcanoes on the fault lines.
New Mexico is within the North American plate. However, this region has been shaped dramatically by the movement of plates on the west coast of North America in the Cascadia subduction zone.
Cascadia Subduction Zone
The Cascadia subduction is a convergent plate boundary, an area where one plate collides with another, submerging under the larger mass. In this case, the subduction zone stretches from northern Vancouver Island in Canada to Northern California.
It is a long, sloping subduction zone, where the Explorer, Juan de Fuca, and Gorda plates have been moving east for millions of years. They are sliding under the much larger North American Plate. The Explorer, Juan de Fuca, and Gorda plates are small remnants of the massive, ancient Farallon Plate, which is now mostly subducted under the North American Plate. The process of one plate sliding under another creates cracks, provoking seismic and volcanic activity. For example, the Cascadia subduction is responsible for the volcanoes in the Pacific Northwest, like Mount St. Helens, as well as the seismically active San Andreas fault in California.
Ripple Effect
Additionally, further inland, the movement of the plates along the Cascadian subduction zone sculpted the western side of the continent. The pressure caused the land east of the subduction zone to ripple, like pushing a rug from the edge and shoving something underneath it. The Rocky Mountains, extending from Canada to northern New Mexico are part of that “ripple” effect.
Additionally, basaltic magma surged upward from the earth’s mantle, seeping through cracks and crevices in weaker sections of the earth’s crust. Major earthquakes, intense volcanic eruptions, and prolific lava flows ensued. This period of mountain building and faulting formed the Colorado Plateau and the Rio Grande rift.
Age of Volcanoes
An intense period of volcanic activity commenced 40-60 million years ago. A lot of familiar mountains in the southwestern United States formed during this volcanic melee. The Superstition Mountains in Arizona, the San Juan Mountains in Colorado, the Mogollon–Black Range–Magdalena–Socorro–Organ Mountain province in New Mexico, Sierra Blanca, the Cerrillos Hills, and the Ortiz Mountains in New Mexico, as well as the iconic butte and wings of Shiprock were part of the fireworks. Though New Mexico’s volcanoes are not currently active, the “Age of Volcanoes” continues today. Most of New Mexico’s volcanoes are not dead. For example, a seething pool of magma lurks a few miles beneath the surface of the idyllic, pastoral beauty of the Valles Caldera, which is the third largest super volcano in the United States.
Colorado Plateau
The Tertiary period was intense in the Four Corners area (northwestern New Mexico, northeastern Arizona, southeastern Utah and southwestern Colorado). There are reasons the dinosaurs were panicing and trying to migrate out of the area. This region, now known as the Colorado Plateau, was a shallow circular valley during the Mesozoic Era (Triassic, Jurassic, and Cretaceous Periods). Layers of rock and sediment were deposited in flat layers for millions of years. However, the pressure of one plate sliding under another off the west coast created chaos that reverberated inland, creating cracks and fissures in the earth’s surface.
As the continent began to tear asunder approximately 20 million years ago, the Colorado Plateau remained solid and stable. The solid mass was like the cylinder in a washing machine, with the land surrounding it churning. Basins formed to the south and west of the plateau, with a particularly large, long series of basins east of it. That sequence of basins is known as the Rio Grande Rift.
Rio Grande Rift
There are only five known active, or recently active, continental rifts in the world. The Rio Grande rift is one of them. It extends from southern Colorado down the center of New Mexico into northern Mexico. The rift, and the mountains on the margins, continue to be geologically active today. Specifically, the rift is growing, gradually getting wider.
The Rio Grande Rift began forming between 29-35 million years ago, inducing a lot of geological activity on the margins. For example, the earthquakes along the eastern margin resulted in the uplift of New Mexico’s central mountains, including the Sandia, Manzano, Manzanita, and Sierras de los Pinos Mountains.
A Series of Basins
Continental rifts form basins (topographic depressions). Over millions of years the basins fill with sediments. The notable escarpment known as La Bajada, between Santa Fe and Albuquerque, forms a margin between two basins. Santa Fe was built in the basin north of La Bajada and Albuquerque was built on top of the eroded debris filling the basin south of La Bajada. The basin sediments are three miles thick beneath Albuquerque. Furthermore, there are geologists that believe the rift will eventually spread and become an ocean (millions of years from now). Any remaining residents of Taos will have beachfront property and mountain views.
Volcanic Landscape
Volcanism has sculpted the western half of North America since the onset of rifting millions of years ago. As the continental plate was crunched, stretched and thinned in the basins surrounding the Colorado Plateau, fissures and faults formed. These created paths for magma to reach the surface. As a result, volcanoes erupted in the center of the rift from Taos to Carrizozo. There were several alignments known as “fissure vents,”including a prominent sequence west of Albuquerque. Fissure vents occur where several small volcanoes erupt in a line. The fissure on the west side of Albuquerque extends from the Sandia Mountains to the Rio Puerco. It produced the “Three Sisters” that dominate Albuquerque’s western horizon.
Dormant. Not Dead.
Geologists consider the volcanic features in the Rio Grande rift to be dormant rather than dead. New Mexico wouldn’t have the abundance of hot springs if there wasn’t magma heating the water beneath the surface. For example, Valles Caldera near modern-day Los Alamos, New Mexico, is one of the world’s largest and youngest calderas.
1.2 million years ago a magma chamber collapsed during a large “supervolcano” eruption. In the process, the Valles Caldera collapsed on top of an older caldera, the Toledo Caldera. Furthermore, there may be an older caldera under there. Geologists don’t know. However, measurements of ground surface motion detect magma intrusion a few miles beneath the surface. Similarly, the heat from ongoing geologic activity is apparent in the hot springs along the Rio Grande, including Ojo Caliente, or the ‘Hot Eye’ southwest of Taos.
Evolution of the Rio Grande
Most geologists believe that the Rio Grande formed as a major river 1–2 million years ago. As snow began to fall at high elevation in the newly formed Rocky Mountains, the snowmelt flowed downhill from Colorado into New Mexico.
Whereas most river valleys are created by the rivers that flow through them, the ancestral Rio Grande took advantage of the basins that form the Rio Grande rift. The series of rift basins formed a perfect low area for water to flow. Initially, the river flowed through the Upper Arkansas, San Luis, Española, and Albuquerque Basins, flowing 340-miles from the headwaters in the San Juan mountains to around Socorro, New Mexico. The rift is not simply a canyon, or gorge, that contains the river. It encompasses more than 160,000 square miles, stretching from central Colorado almost to Big Bend National Park in Texas.
Optimal Rift Overlooks
- The view from the Overlook on N.M. Highway 68 is fantastic. The mountains of Colorado are readily visible to the north, the snow-topped Sangre de Cristo Range are to the east, and the vast and serene Taos Plateau surrounds you. The rift narrows and widens within a short distance. It is about 8 miles south of the Ranchos de Taos post office.
- The Rio Grande Gorge Bridge on U.S. Highway 64 is about 10 miles northwest of Taos. There are sidewalks with observation platforms at mid-span, which allow people to gaze down at the narrow ribbon of water cascading through the canyon 650 feet below. Additionally, for an exceptional vantage point, visitors can access the West Rim trailhead from the parking lot on the southwest side of the highway.
- The Wild Rivers Recreation Area is approximately thirty-five miles north of Taos, three miles north of Questa, off Highway 378. The Red River flows into the Rio Grande at La Junta Point. In addition to numerous hiking trails, there are developed campgrounds. Several of the trails descend into the gorge to the banks of the Rio Grande.
Call the U.S. Bureau of Land Management’s Taos Field Office at (575) 758-8851 for more information. Read more about the Rio Grande del Norte National Monument here.
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