The Albuquerque Volcanoes

1. How old are they?

The Albuquerque Volcanoes are as young as140,000 years. This is very young by the standards of geologic time. Volcanoes younger than 200,000 years in the Southwest are all very well preserved and look much like they did immediately after they erupted.

Rather than erupting individually like many small fields of volcanoes in the Southwest, the Albuquerque Volcanoes all formed simultaneously. This is because they all erupted from the same fissure eruption. The actual eruption probably took place over a period of months to years. To the south and west of Belen and Los Lunas there is another series of cinder cones called the Cat Hills volcanoes. These are slightly younger and consist of more typical cinders and agglomerate.

Small volcanoes do not reactivate like big volcanoes, but the area around Albuquerque remains potentially active, mainly because of its location in the Rio Grande Rift. A new volcano could erupt, if not along the Albuquerque Volcanoes, at least somewhere within the rift.

One of the larger earthquake swarms in New Mexico occurred in 1972 beneath the Volcanoes. There is no particular evidence that this was new magma moving up a crack. Instead, the earthquake swarm may have been re-adjustments of dikes and faults at great depth as the deeper portions of the dike continue to cool and contract.

Many people visiting the Volcanoes in the winter months have noticed "warm" air in the caves. This is probably not "volcanic heat", but instead is ambient air trapped in subsurface that feels warm compared with the cold outside air. If one were to visit the same caves during the summer, one would find them cool compared with the outside air. The caves simply record the temperature of Albuquerque "averaged" over a year, which is around 55 degrees F, a temperature that feels warm in the winter and cool in the summer.

Figure 1

Figure 2

Figure 3

If you were to look down on the Albuquerque Volcanoes (Figure 1) you would see that they are arranged in a nearly straight line. This also is apparent if you stand on top of one volcano and look across to the next one (Figure 2). This is simply because they erupted along a fracture in the crust. Fractures of this type along which eruptions take place are referred to as "fissure eruptions". Many of the eruptions in Hawaii are fissure eruptions in which the fissures frequently extend outward from summit craters or calderas. The fractures in Hawaii are thought to result from the gradual sliding of parts of the sides of the large volcanoes that make up the Hawaiian islands.

Fissures also occur in Iceland where the crust is being pulled apart along the Mid Atlantic ridge. In New Mexico the fractures have an origin similar to those in Iceland. Here the fractures are part of the faults that formed where the crust has been pulled apart by forces similar to those in Iceland. The eruption that resulted in the Albuquerque Volcanoes probably looked similar to eruptions in Iceland (Figure 3). Although the Albuquerque Volcanoes appear to occur in a straight line, they are actually arranged in slightly offset segments like the fissure in this example from Iceland. This is probably a good analogy of what the Albuquerque Volcanoes looked like when they started erupting and before the eruptions centralized on several spots. Once the eruptions centralized on these spots, the cones that we see today began to build up.

The stretching that helped create the Albuquerque Volcanoes has resulted in the development of the Rio Grande Rift. The rift valleys of eastern Africa are another example of similar stretching and pulling apart of Earth's crust. The East African Rift and the Rio Grande Rift are two of the youngest and larger continental rifts currently active on Earth. (This is probably one reason why East African and New Mexico are among the most geologically, and ecologically, interesting and exotic places on Earth.)

Fissure eruptions occur when conditions are right in the mantle for the existence of partially melted rock (magma) and faults act as conduits for the rise of magma to the surface. When a vertical crack filled with molten magma (known as a "dike") intersects the surface, a line of eruption takes place. If the dike is particularly large and deep, the line of eruption or fissure is long, like the Albuquerque Volcanoes. Other dikes are smaller and the associated fissure eruptions at the surface are short.

Most of the scoria cones and volcanoes in New Mexico and throughout the Southwestern U.S. are the result of smaller fissure eruptions. This is one reason why the Albuquerque Volcanoes are so unusual and geologically interesting. They are an excellent example of fissure eruption. In addition, few large cities in the United States have such young basaltic volcanoes nearby (within a few minutes drive of downtown) that are also an important part of the metropolitan landscape.

The Albuquerque Volcanoes are also an unusual type of scoria cone. Many scoria cones in the Southwest are piles of cinder. Examples are Capulin Volcano, Bandera Crater, and Sunset Crater. Some of the Albuquerque Volcanoes like Vulcan, the largest cone, are cones formed from spatter and thin lava flows coating an interior of cinder.

The Volcanoes are basalt, a type of lava that consists largely of the minerals plagioclase, olivine, pyroxene, and magnetite. There are two principal types of basaltic lava in Southwest: alkali basalt and tholeiite, defined on the basis of the relative abundances of sodium and potassium. The Albuquerque Volcanoes are made of tholeiite that is low in sodium and especially potassium. These different types of lava reflect the differences in the depth and composition of the mantle rocks that were melted and erupted. By comparison, the youngest lava of El Malpais (McCartys lava flow) is tholeiite. Many of the volcanic fields, like Mount Taylor's collection of small volcanoes, also on the western horizon of Albuquerque, are alkali basalt.