The Rio Grande Rift in New Mexico is a failed rift in North America, a rift that did not complete the separation of North America and the growth of a new sea between the two sides. Any time you have an intracontinental rift, what typically happens is the plate thins out along the rift allowing some amount of hot, young basalt to reach the surface.
This has happened in New Mexico, as it is littered with small volcanic fields along the Rio Grande Rift and along a generally southwest to northeast line of crustal weakness referred to as the Jemez Lineament. The location that the rift and the lineament cross is the site of the Valles Caldera, a 22 km wide caldera north of Albuquerque. The eruptions forming the caldera took place some 1.47 and 1.14 Ma. The second one produced over 300 km3 of ignimbrite flows. Oddly enough, the Bandelier Tuff eruption is also dated at 1.25 Ma. Last activity was some 40,000 years ago, with dome building and rhyolite eruptions. There is still an active hydrothermal system in the area.
Granyia wrote about Valles Caldera in December 2015. https://volcanohotspot.wordpress.com/2015/12/07/valles-caldera-jemez-mountains/
I wrote about the Jemez Lineament and the Raton – Clayton Volcanic field in Jan 2014. https://volcanocafe.wordpress.com/2014/01/21/the-raton-clayton-volcanic-field/
Any time you have relatively recent intraplate volcanic activity, you pay particular attention to anything odd along lines of geologic weakness. One such location is the Socorro magma body south of Albuquerque.
Central New Mexico is a dry and arid region with the Rio Grande river flowing through it from Colorado to the Gulf of Mexico near Brownsville, Texas. It bisects the state and forms the southern border between Texas and Mexico.
Major cities in the Albuquerque Basin include Albuquerque to the north and Socorro to the south. Los Alamos is just to the east of the Valles Caldera. Las Cruces are near the river well to the south while Santa Fe is near the river to the north.
The area supports a significant military presence with the White Sands Missile Range and Holloman Air Base near Alamogordo. The Sandia National Laboratories are located in Albuquerque. The Trinity site is located between Socorro and the Carrizozo lava field to the south and east of the basin.
There is a major tourist attraction with White Sands National Monument to the south. The Mescalero Reservation is to the east of Alamogordo and includes the Sierra Blanco Volcanic Complex. Roswell is well to the east of Sierra Blanco.
It is harsh, desert land but with stunning beauty.
The low velocity zone (there’s that phrase again) that is one of the pieces that define the Socorro Magma Body (SMB) is centered underneath the town of Socorro, NM, about 120 km south of Albuquerque. It is relatively thin at perhaps 0.1 – 0.5 km at a depth of 19km. There is an area of roughly 5,000 km2 called the Socorro seismic anomaly (SSA). It is also centered on the 3,400 km2 called the Socorro Magma Body (SMB) that has been defined by reflection data.
The seismic anomaly is home to 45% of total quakes larger than 2.5 Richter in New Mexico. The operating theory is that the earthquake activity is due to an injection of magma into sill(s) at the 19 km depth and in turn into vertical dikes created by injection of magma into cracked brittle crust overlying the region.
Volume of the melt is not well constrained. Either is its actual shape. Competing explanations are multiple thin sheets or the entire layer is a single large volume of melt. Total volume is in the range of 100 – 1,000 km3. Some other researchers put that volume in the several thousand km3 range.
There are observed occasional dikes moving upward above the magma body to depths within a few km of the surface. This is based on what is believed to be shallow bodies in the vicinity of Socorro. Given known volcanic activity in the region over the last half million years, this should not be a surprise.
A variety of surveys over the region for a century measure broad crustal uplift. The obvious question is how long that uplift has been going on. Speculation is that uplift has been going on for tens of thousands of years. Current rate is few mm / year in the center of the anomaly. Further investigation is demonstrating that long term inflation has not been going on and perhaps the movement has been fairly recent which would mean the magma is very young and very fresh and very hot. Verification for this view comes from observation of surface stream channels in the uplift area which have not changed direction as they should have during what would be a possible extended period of uplift.
A 1980 paper in Geology suggests there was 10 cm of uplift between 1934 – 1978. The uplift area in this paper measures 7,000 km2. Interestingly leveling data also suggests relative uplift within the Albuquerque – Belen Basin well north of the suggested northern boundary of the Socorro body. Either it is much larger than expected, or a second new magma body lies beneath the basin closer to Albuquerque.
A 2001 paper by Fialko and Simons took issue with the magma injection rate into the location. They made the point that the injection rate was very slow and based on the uplift rate and expected thermal properties of a thin magma chamber, it would freeze (solidify) quicker than new magmas could be added. The paper ends with the authors discussing various elastic crustal deformations that would allow magma slowly injected into a known chamber remaining liquid. Assumptions made in this paper get weaker the slower magma is injected and the longer the chamber has existed. If it is relatively new, the numbers would tend to work better.
A more recent 2011 NSF funded paper better constrains the uplift to two episodes. One prior to 122 ka and one starting after 60 ka and is still ongoing. Most of the earthquakes have been between 3 – 12 km in depth.
But so far, no volcano or lava flow at the surface. Yet.
Albuquerque – Basin Volcanoes
I find it interesting to set things like the Socorro Magma Body (SMB) in perspective and take a look at the surrounding region. This look will be constrained in the region between Socorro and Albuquerque, the Albuquerque – Belen Basin portion of the Rio Grande Rift / Rio Grande Valley.
The Lucero Volcanic Field is about midway between the much larger Zuni – Bandera and Mt. Taylor Volcanic Fields which lie across the Jemez Lineament to the west of the Socorro – Albuquerque axis. The field is not particularly large and has suffered no small amount of erosion. Age of the field is 8 Ma with the last eruption being some 1.1 Ma. The majority of activity was basaltic with small scoria cones, lava flows and small shield volcanoes built.
North of the SMB is the Tome – Black Butte – Los Pinos volcanoes, which lies just to the south of Albuquerque.
Just to the west of the Albuquerque lies the Albuquerque – Cat Hills Volcanic Field. This also called the Petroglyph National Monument. The volcanoes are a line of lava flows, spatter and scoria cones along a long rift that opened up some 150,000 years ago. This field is unusually close to the main city.
To the north of Albuquerque as we approach the Jemez Lineament and the Valles Caldera volcanic field we find the San Felipe Volcanic Field. This is a 2.5 Ma field similar to the Albuquerque volcanoes. It has a low shield volcano, multiple scoria / cinder cones and voluminous lava flows that issued from north – south fissures. There are multiple faults cutting the lava flows over time. The Canjilon Hill is a complex volcanic center that includes Maars and volcanic intrusions. It is south of the main field and represents a volcano eroded by the Rio Grande.
South of the SMB we find the Jornada del Muerto which erupted lava covering over 440 km2 some 760,000 years ago. The entire field is technically a shield volcano. Total output was 13 km3, or about the same size as the Laki eruption in Iceland in 1783.
Finally we have the Carrizozo lava flow, one of the youngest lava flows in New Mexico, dated at around 5,000 years ago. Like the Albuquerque Volcanoes, this was also a fissure eruption. Total coverage was 330 km2 with an estimated volume of 4.3 km3. The flow is primarily basalt and is 73 km long. There are older volcanic structures nearby including the Sierra Blanca Volcanic Complex to the east. Carrizozo, Sierrra Blanca and Jornada all lie generally to the north of White Sands National Monument, on either side of Los Alamos and the site of early atomic testing.
Note that these last volcanic fields are all associated with the Rio Grande rift and the Rio Grande Valley.
The entire area surrounding the SMB is littered with recent volcanic activity. The majority appears to be basaltic with fissure eruptions, scoria / cinder cones, and small shield volcanoes quite common. Given the widespread recent volcanic activity in the region, there is no reason to believe the SMB does not represent a continuation of this activity.
The Rio Grande Rift is the surface expression of widespread extension of the western US. It has three major basins and several smaller basins. The Albuquerque Basin is the middle basin in location. It is also the largest at 160 km long by 86 km at its widest point. The rift is generally wider as you travel farther south in New Mexico. The southernmost portion is more complex and eventually indistinguishable from the Basin and Range province to its west. Some researchers connect it to the Basin and Range province, but there are as usual alternative views.
Normal rifts are bound on either side by faults which typically drop the middle valley some distance below the common surface. The Rio Grande Rift is unique in that it only has a fault on one side of the valley for most of its length. This is called a half-graben with a flexible hinge on the opposite side. Motion along the faults are vertical with total displacement as much as 9 km from bottom of the fault to top of the uplifted mountains on the eastern side.
Formation of the rift proceeded in three phases. The first was crustal extension with normal faulting between 20 – 30 Ma. Major materials associated with extension was basaltic and andesitic volcanic activity. Second phase took place between 10 – 20 Ma. This was rifting and created the horst – half-graben system. This phase was marked by basaltic lava flows and eruptions and significant erosion down the walls of the growing rift. Final phase was between 5 – 10 Ma with uplift mainly on the east side via block faulting. This was marked by further erosion and basaltic (mostly) volcanism.
Volcanism along the rift is varied, with the oldest and most violent being the farthest north in Colorado with the San Juan Volcanic Field and its multiple calderas, pyroclastic flows and rhyolite. The farther south you move along the rift, the more the volcanism changes to intraplate basaltic activity. The initial activity was some 30 – 35 Ma. Recent activity is as young as 5,000 years ago.
There is a continuing disagreement over precisely what the Rio Grande Rift actually represents. The consensus appears to be that it is simply an extension of the neighboring Basin and Range Province that is west and north of New Mexico. An alternate explanation is the clockwise rotation of the Colorado Plateau by a few degrees explains the rift almost completely.
As with all arid and semi-arid regions, water is a significant importance and its availability and use occupies the full attention of governments and inhabitants at all levels. Given the shape of the underlying sedimentary rock in the rift valley, it has attracted interest from oil and natural gas exploration companies. The state government using the availability of water as an excuse has done its level best to obstruct that exploration.
The Socorro Magma Body represents the latest example of an intrusive magma source in the region. Given the recent volcanic activity, it does not appear that volcanic activity will be stopping any time soon. Should it occur, my guess (and it is only a guess) is that some sort of basaltic eruption is most likely – fissure, spatter cone, scoria cone or small shield volcano(s). When or where that takes place in the vicinity of the SMB is anyone’s guess.