I made regular trips between Colorado Springs and Dallas, TX for many years. It is a nice drive; not too long, and with great views. One of the areas you drive through is the Raton – Clayton Volcanic Field in the northeast corner of New Mexico, which is home to over 100 recognizable volcanic units covering over 7,500 square miles (20,000 square km), many of which you can easily see from the road.
The region is most accurately classified as high plains which get little precipitation so the volcanic vents and flows are relatively pristine.
Activity in the field took place over much of the last nine million years, with the most recent eruption being some 45,000 years ago. The eruptions have been grouped into three phases with distinctly different eruptions. The first two phases produced substantially more material than the final phase. Also of interest is the relatively low volume of erupted material across a substantial area. This low volume has led some researchers to speculate that the low volume of eruptible magma led to a substantial portion of it cooling and crystalizing in place with small portions left to find their way to the surface. This in turn led to multiple small vents that shut down after a short period of activity rather than several large ones active for a long time. The field erupted various flavors of basalt. There were a few ash-rich eruptions, but only seemed to take place when the basalt interacted with ground water causing phreatomagmatic blasts.
The Raton phase took place from 9 to 3.6 million years ago. It is the western most vents and was marked by basalt flows, ryhodacitic domes, and smaller flood basalts that covered a number of mesas in the central and western portion of the field. There is even some evidence of pyroclastic activity out of one of the eruptive centers. Note that the mesas were created as the area was uplifted.
The Clayton phase took place from 3.6 to 2 million years ago. It is marked by voluminous basalt flows, numerous shield and cinder cone volcanoes. Fissure eruption locations have also been identified, with one flow measuring over 12 miles (20 km) long. One of the volcanoes, Sierra Grande is unusual in that it most closely resembles an andesite shield volcano. It measures some 6 miles (10 km) in diameter and tops out at over 9,000 feet (2,650 meters) high. It is surrounded by cinder cones on its flanks and dates back some 3 – 4 million years.
The final phase is called the Capulin phase and dates from 2 million years ago to when the activity stopped some 45,000 years ago. It is dominated by basalts and andesitic basalts erupting as cinder cones. These vents also produced no small amount of basalt during their active times.
Among the most recognizable of these vents is the Capulin Volcano, which sits north of the highway. It is a scoria cone with at least three episodes of basalt lava flow around the base. The volcano dates back some 58,000 years and has been turned into a National Monument. There is a two mile road that winds around the cone and tops out at the top of the crater. Capulin stands some 1,000 feet (300 meters) high.
In addition to being scenic, Capulin was moderately complex, with a cone-building stage that followed an initial effusive stage. It looks like a fissure opened, which was followed by basalt eruptions covering some 16 square miles (41 square km). There are several vents identified during this phase of the eruption, which followed with the creation of lava tubes, lava lake, cascades, and a “boca” or mouth of the volcano. This phase was shortly followed by a cinder cone building phase, as cinders are found on top of the lava flows. Eventually the several vents coalesced into the main vent which built the cone.
After the cone was built, the first of three main lava flows issued from the “boca” at the side of the crater. It was during this phase that at least two active lava lakes and a tube system formed and were used by erupting lava. Most of the tubes have collapsed over time. Several spatter cones have also been identified. Total material erupted is on the order of 0.6 cubic kilometer, making this a medium sized cinder cone eruption.
Although the area is dry, there is plant and animal life present. Ranching is the primary activity in that part of the state. The lava beds are overgrown with a variety of grasses and shrubbery in places where it is not too thick. Trees also mantle the side of Capulin itself. Given that these are the high plains, trending into high desert, one needs to be a bit careful traveling across the lava, as the rough surface and lava tubes provide excellent shelter for all manner of creepy crawlers, including but not limited to rattlesnakes, scorpions and tarantulas.
Large Structures in New Mexico
The two main geologic features in New Mexico are the Rio Grande Rift, which lies roughly north – south, ending in central Colorado to the North and northern Mexico to the south and the Jemez lineament.
The Rio Grande River runs the length of the rift until the vicinity of El Paso, when it bends eastward and empties into the Gulf of Mexico. The river did not cut the rift. Rather the river takes advantage of the depressions at the bottom of the rift to flow.
The Rio Grande rift has been described by some as a failed rift of the North American continent. I believe the story is a bit more complex than that. The rift started pulling apart around the same period of time that the San Juan Volcanic Field was erupting remains of the Farallon Plate 25 – 40 million years ago. Those eruptions were primarily grey eruptions and it is thought that eruptible melt petered out some 19 million years ago.
The majority of volcanic activity along the rift was in Colorado and ended relatively long ago. Recent activity includes the Valles Caldera which erupted some 1.2 million years ago and outbreaks of basaltic volcanism. The newer volcanic centers are smaller and more effusive. There is currently a magma chamber identified under Socorro, New Mexico.
A big concern would be whether or not North America is actually rifting, as rifting more times than not leads to significant volcanic activity. From the papers, I reviewed, the rift is more related to the uplift and possible rotation of the Colorado Plateau to its west. One writer went so far as to speculate that a rotation of a degree or two of the Colorado Plateau, which he described as a microplate making up North America was enough to account for the formation of the rift. Regardless of the reason, the rift clearly does not extend to the sea.
Volcanic activity along the rift appears to be driven by a thinning in the plate itself which allowed melt to find its way to the surface and erupt. This may explain why the eruptive products are so different from what we saw from the San Juan eruptions (subducting plate dacites, andesites and rhyolites versus spreading center basalts).
The other large structure is the Jemez lineament, which lines up a number of volcanic fields from eastern Arizona to northwestern New Mexico. It runs roughly from central Arizona to the northeast corner of new Mexico. The Raton – Clayton volcanic field lies along it. The Valles Caldera lies at the intersection of the lineament with the rift. This line is almost parallel to that of the Yellowstone hot spot and led to some speculation that there may be a small or new hot spot involved. This speculation ended up being short lived, as the various volcanic fields do not line up in a time sequence from older to newer as you travel from west to east.
Instead, the lineament is now thought to be a suture point for cratons making up the North American continent some 1.2 billion years ago. As it is referred to as a “suture” this means that it is an area of weakness in the underlying bedrock. Given the flexing, uplift and possible rotation going on with the Colorado Plateau to the north and west, it is not beyond the realm of possibility that this suture has opened sufficiently to also allow eruptive melt to make its way to the surface from underneath the continent.
The lineament includes the Red Hill Volcanic Field in eastern Arizona and western New Mexico. It was active from two million years ago to near present. It is marked by cinder cones, maars and basaltic lava flows topping mesas.
The Zuni – Bandera Volcanic Field is found next to the east. It is also relatively new with activity measured back a half million years, although one vent is measured at over 7 million years old. Most recent lava flow is estimated at 3,000 years old. The field is marked by basaltic lava flows (aa), numerous cinder cones and a few shield volcanoes. Magma exhibits characteristics of typical intraplate volcanism, though some chemical signatures of enhanced and depleted mantle sources have been measured.
Close to the east of Zuni is the Mount Taylor Volcanic Field. Mount Taylor is a primarily basaltic stratovolcano that was active from 3.3 – 1.5 million years ago. It is surrounded by a number of smaller cones and vents. The mountain sits on the western edge of the Rio Grande Rift with appears to be its magma source. The volcano also had a climactic eruption during that time which destroyed the ancestral cone. Eruption is thought to be similar to what Mount Saint Helens has done in the past.
As the lineament crosses the Rio Grande Rift, we find the Valles Caldera and the larger Jemez Volcanic Field. The Valles Caldera was active from 14 million to 40,000 years ago with a pair of caldera-forming eruptions some 1.2 million years ago. The caldera was refilled with resurgent domes and newer and smaller domes which were active until around some 40,000 years ago. Eruptions produced significant tuff deposits which overlay the surrounding bedrock. Magma source appears to be a weak spot at the intersection of the lineament and the Rio Grande Rift. Valles Caldera measures some 14 miles (22 km) across.
North of the lineament is the Taos Plateau Volcanic Field. The majority of its activity took place between 4 – 1.8 million years ago, though several vents are measured at 22 million years old. The majority of the vents are shield volcanoes, cinder cones and fissure-fed sheet basalts. Like the last two volcanic fields, its magma source appears to be supplied by weak points at the intersection between the Jemez lineament and the Rio Grande Rift. There are at least 35 volcanoes identified in the field so far.
The last volcanic field between Taos and Raton is the Ocate Volcanic Field. It erupted between 8.4 million years to some 800,000 years ago. Like most of the other fields, this had multiple vents, at least 50 identified so far, producing basalt flows covering mesas, shield volcanoes and cinder cones. The interesting part about this field is that the entire region was uplifted during the period of the eruptions. So you have high mesas covered with basaltic lava flows, and the remaining activity filling up gullies that had been cut by erosion over the years.
With that, we are back at the Raton – Clayton Volcanic Field.
New Mexico is a lot more recently active than most people think. Most of the activity has been some form of basaltic eruptions with cinder cones, sheet basalts and cinder cones being most common. There were a few historic huge eruptions with pyroclastic flows, though these are relatively uncommon. Activity appears to be driven by magma moving through weak points in the North American plate, specifically tied to uplift and perhaps rotation associated with the Colorado Plateau.
The arid conditions of the high plains and the high desert preserve these features, making it difficult to date. Measured heat flow in a number of places is still quite high, and there is a suspected magma chamber near Socorro, NM, south and west of the Raton – Clayton Volcanic Field.
I don’t think activity in the state is over yet, though it may very be done in the Raton – Clayton Volcanic Field. The more we find out, the more we find we don’t know.