Was it the River or the Rift? The Geological Debate over the Rio Grande Gorge

by Eric Mordhorst

Last summer, I drove some visiting relatives out to see the Rio Grande Gorge, which lies just a few miles west of Taos. I parked the car at the rest area on the west side of the gorge, and my relatives cautiously followed me down the footpath until we reached a vantage point and could peer into this 600-foot chasm. My aunt took endless snapshots with her phone while the others stood slack-jawed and petrified. Gradually, our gazes panned down the walls of the gorge, following three different benches of basalt from the rim to the river. Each bench was separated by a vertical cliff wall that dropped close to 200 feet.

“It’s just beautiful,” my aunt exclaimed.

My uncle thumbed his way through a roadside geology booklet. “It says here,” he read. “The gorge was carved out by the river over the last half million years.” My aunt nodded without acknowledging what her husband was talking about. She didn’t care.

As my uncle read to us the geological history of the gorge, a young man in his late twenties arrived on the overlook. I recognized him as a local, although I didn’t know his name. He was also playing tour guide for a group of visiting relatives. After listening to my uncle, the local abruptly interjected with his own explanation.

“Actually, the gorge was formed by rifting.”  

“What?” my uncle replied.

“You know the earth’s crust sits on plates, right?”

“I guess.”

“Well, those plates cover the earth and fit together like giant puzzle pieces.”

“Okay.”

“When the plates rub together, faulting and earthquakes happen, and mountains like those are formed. I think it’s called convergence. It’s when plates collide.” He pointed to the Sangre de Cristo Mountains that stood ten miles to the east.

“Does your booklet mention the rift?” the local asked.

“I don’t know. What’s a rift?”

“It’s a strip in the earth’s crust that has been stretched like taffy.”

My uncle kept reading until he landed on the information about the rift. “Yes, here it is. It says, ‘the rift was caused by the Farallon plate moving away from the North American continent. Over the past 30 million years, the rift has torn a seam in the earth’s crust.’”

My uncle looked up from the booklet and gazed out over the gorge. “So, are we looking at a seam in the earth’s crust? Are you saying that this gorge was torn apart by earthquakes?”

“Little ones. Block faulting some call it.”

The local smiled and returned his attention to his relatives.

My uncle nodded. It seemed to make sense.

I was not surprised by this guy’s explanation because I had heard it before. Around Taos there is a long-running debate over whether the gorge was created by the rift (torn apart) or incised by the river (erosion). To settle this debate, we have to go back 70 million years to the mountain-building time of the Laramide orogeny and the formation of the Rocky Mountains. Back then an oceanic Farallon plate was sliding under the North American plate at a shallow angle, causing the plates to converge against one another.

Geologists Adam Read and Shari A. Kelley state in The Geology of Northern New Mexico’s Parks, Monuments, and Public Lands (2010) that over the next 40 million years, the convergence of the plates waned and the Farallon plate dropped away from the continental plate. As a result, the crust that ran parallel with the mountains began to stretch and extend toward the Colorado Plateau, and the Rio Grande Rift Valley opened. The rift is roughly 30 miles wide and sits between the eastern Sangre de Cristo Mountains and the western Colorado Plateau, and it extends from Leadville, Colorado, to northern Mexico.

In the 1970s, geologists Peter Lipman and Harald Mehnert helped establish a direct relationship between the rift and volcanic activity in Southern Colorado and Northern New Mexico. They claim that crustal stretching and thinning allowed the heat from the mantle to punch through the earth’s crust via a cluster of newly formed volcanoes that erupted in two bursts about 26 and 5 million years ago. The San Juan Volcanic Field developed simultaneously with the initiation of the rifting process around 26 million years ago. Most of the San Juan Volcanic Field today lies in Southern Colorado.

According to the Lipman and Mehnert 1979 study, between 2.8 and 5 million years ago, the crustal thinning and stretching created the Taos Plateau Volcanic Field: a network of shield volcanoes, lava domes, and cinder cones northwest of Taos. These volcanic vents produced a series of Servilleta lava flows that stretched 75 miles from Velarde at the southern end to the Colorado state line. Starting around 5 million years ago, the lava flows from the Taos Plateau Volcanic Field poured into the basins that the rift created through block faulting and filled in every low point like a jello mold. Eventually, the lava flows piled up and created a field of basalt rock that was up to 1,500 feet thick.

The Lipman and Mehnert study from 1975 found that most of the rifting took place between 7 and 16 million years ago, with the most intense rifting occurring around 10 million years ago. Read and Kelley (2010) reported that during the past 5 million years, the rifting has been relatively quiet. So, if most of the rifting took place between 7 and 16 million years ago, and the basalt flows in the gorge are from the past 5 million years, a period marked with little rifting, how can crustal stretching and block faulting be responsible for the Rio Grande Gorge?

Back at the overlook, my relatives were still enjoying the view when a man in his forties arrived with a small group of teenagers who looked kind of frightened. They were all quieted by the power of the place. The youth group leader hadn’t brought them to the gorge to scare them or to discuss the afterlife—he brought them to the gorge to talk about creation.

“This gorge was carved out by the Great Flood in 3,333 BC,” he told the group. 

A teenager named Marcy looked confused by the leader’s claim. “Are you referring to the flood in the Epic of Gilgamesh?” she asked.

“The what?” the leader replied.

“Gilgamesh. The oldest known work in world literature.”

“No, Marcy, this isn’t a story about literature, this is a story about truth.”

“Okay. Do you mean a series of floods that occurred at the end of the last ice age?”

“No, Marcy, we discussed this, there was no ice age, that’s a myth.”

“He’s talking about the flood in the book of Genesis,” another teenager informed her.

“That’s the one,” the group leader confirmed and slapped the teen with a high-five.

“You see, kids, when the flood started, it rained really hard. So hard that all of the streams from the mountains rushed down into the lakes, which quickly overflowed and all that water drowned the land as far as the eye could see.”

“Wow,” Marcy replied sarcastically. “How long did it rain?”

“The Bible says it rained for forty days and forty nights. Soon the rising oceans poured over the mountaintops and swallowed up all of North America and eventually the whole wide world. One day the rain finally stopped, and the oceans gradually began to sink again. As the land slowly reemerged, the receding waters carved out this gorge and left the river in its place.” 

“Where did all that water go?” Marcy asked.

“Back into the earth. See the cracks.” He pointed at the fissures in the volcanic basalt.

Marcy looked around unconvinced, but she kept quiet. Surprisingly, the youth leader’s creationist explanation may be closer to the truth than the local guy’s block faulting theory. There was a big flood that began around 430,000 years ago. A geological study of ancient Lake Alamosa, a giant lake that once filled the San Luis Valley, was conducted by Michael Machette and his team in 2013. The study suggests that the lake existed for almost 3 million years, “expanding and contracting and filling the valley with sediment until [430,000 years ago], when it overtopped a low sill and cut a deep gorge through Oligocene volcanic rocks in the San Luis Hills and drained to the south.” Machette et al. found that the sediment buildup in the lakebed was the likely cause of the overflow, and the draining of Lake Alamosa was responsible for the incision of the Rio Grande Gorge. They determined that the breaching began around 430,000 years ago and was finished by 200,000 years ago, taking Lake Alamosa around 230,000 years to completely drain. Lava flows from the Taos Plateau Volcanic Field had blocked the waters flowing south from Colorado, so the upper Rio Grande ended in a lake. The spill reconnected it, and the river began cutting down hard into the Taos Plateau.

In 2019, geologists T. Clow, W.M. Behr, and M.A. Helper established an incision history for the gorge. They focused on the five-mile stretch directly above the Rio Grande’s confluence with the Red River, a section of the gorge that is heavily terraced. The terraces descend like steps, and they all contain boulders that have been rounded, polished, and undercut by the river over time. Terraces are essentially abandoned streambeds that mark where the river once flowed, and geologists can measure a canyon’s incision history by studying them.

The Clow team found that the gorge cut down nearly twice as fast as it had on the other rivers. They concluded that the principal source of the incision came from the flood waters of Lake Alamosa, and they found little evidence that block faulting along the rift played a significant role in the incision: “Rates of dynamic surface uplift and/or slip along basin-bounding normal faults associated with the Rio Grande rift are over an order of magnitude too small to explain the fast incision.”

Geologists like to use words like “torn” or “ripped” when describing the rifting process. However, rifting is an event that cannot be readily seen at the surface; it takes place at a much deeper level in the earth’s crust and far beneath the basalt rock of the Taos Plateau Volcanic Field. Paul Bauer writes in his Rio Grande guidebook, “the river follows the topographically lowest point within the rift, carving several spectacular canyons along the way.” Bauer is stating that the rift was responsible for the landscape that the river follows, but the river carved the gorge. Greer Price, editor of the 2010 guide to Northern New Mexico’s public lands, agrees: “The growth of the rift has determined the course of the Rio Grande, which has carved into these thick alluvial sediments.” Knowing this, the rift theorists can find some comfort in being partly right. The rift was responsible for the topography of the landscape. The river, though, incised the gorge. After all, it’s called the Rio Grande Gorge, and not the Rift Valley Canyon.

As I stood at the overlook with my relatives, I withheld my commentary until I could be sure of the facts. Nobody was really listening to anyone’s explanation anyway. I decided it was better to keep quiet and allow them to have their own experience. If you batter people’s brains with geology (like I just have with you), they tend to stare off into the distance like despondent schoolchildren. I feel a little bit better having shed some light on the debate over the river and the rift. Now, if I could just figure out what the Taos Hum really is.

Sources

Bauer, Paul. 2011. The Rio Grande: A River Guide to the Geology and Landscapes of Northern New Mexico. Socorro: New Mexico Bureau of Geology and Mineral Resources. Quoted passage: p. 104.

Clow, T., W.M. Behr, and M.A. Helper. 2019. “Pleistocene to Recent Geomorphic and Incision History of the Northern Rio Grande Gorge, New Mexico.” Geosphere 15, no. 3: 820–838. Quote on p. 820. https://doi.org/10.1130/ges02017.1

Lipman, Peter W., and Harald H. Mehnert. 1975. “Late Cenozoic Basaltic Volcanism and Development of the Rio Grande Depression in the Southern Rocky Mountains.” Geological Society of America Memoir 144: 119–154.

Lipman, Peter W., and Harald H. Mehnert. 1979. “The Taos Plateau Volcanic Field.” U.S. Geological Survey. https://www.researchgate.net/publication/248030829.

Machette, Michael N., Ren A. Thompson, David W. Marchetti, and Roger S.U. Smith. 2013. “Evolution of Ancient Lake Alamosa and Integration of the Rio Grande During the Pliocene and Pleistocene.” In New Perspectives on Rio Grande Rift Basins: From Tectonics to Groundwater, Geological Society of America Special Paper 494: 1–20. Quoted passage: p. 1. https://doi.org/10.1130/2013.2494(01)

Price, Greer, ed. 2010. The Geology of Northern New Mexico’s Parks, Monuments, and Public Lands. Socorro: New Mexico Bureau of Geology and Mineral Resources. Quoted passage (introduction): p. 13.

Read, Adam S., and Shari A. Kelley. 2010. “The Rio Grande Rift.” In Price (2010), p. 183.