Jornada del Muerto Volcano
The Jornado del Muerto lava flows are not just another malpais, but a volcanological gem that is neither widely known nor appreciated. About 760,000 years ago and in a fanfare of lava fountaining, minor ash eruption, and, no doubt, noxious fumes, lava flows spread out over170 square miles (440 square kilometer) of the central Rio Grande rift valley. Before the eruption finished many years later about 3.2 cubic miles (13 cubic kilometers) of basaltic lava had accumulated, or about the same volume as the Laki fissure eruption that occurred in 1783 in Iceland. The total volume is only slightly greater than the much younger McCartys (El Malpais) and Carrizozo lava flows, so the Jornada accumulation ranks among the larger of the geologically young basaltic eruptions on Earth.
It has always been remote, but the Jornada del Muerto (the modern idiom for the Medieval Spanish would be "Dead Man's Route") was not always unvisited. For two hundred years after the first Spanish settlement in 1598, most of the movement along the Camino Real between Mexico City and the interior of Nuevo Mexico passed through the valley just to the east of this lava flow. Today it is about as remote as any place in New Mexico; it overlooked and was illuminated in 1945 by the world's first atomic explosion in the valley to the immediate east.
Those of us who study volcanic features have in the past generally thought of the Jornada del Muerto lava flows as a lava field like the McCartys (El Malpais) or Carrizozo lava flows, that is, a lot of lava flow and not much volcanic vent. However, technically-speaking the whole field is a volcano, or more specifically, it is a shield volcano (a type that is relatively flat or shield-shaped). The topographic relief in the accompanying map (Figure 1) shows that the surface is built up to a central highand lava flows slope radially away for 360 degrees around the central vent, the basic physiographic definition of a volcano.
Lava tubes similar to these frequently form in basaltic lava flows when the molten interior of a lava flow tends to be channelized in tube-shaped passages; once the lava drains characteristic open lava caves are left behind (Figure 3). The floors of the Jornada caves are covered with bat droppings and were once commercially mined. A few small wooden structures remain from that operation south of the vent region and at the end of the only road into the volcano's interior. Some of the lava tubes are gigantic. No doubt other lava tubes occur throughout the field that are uncollapsed and undetected.
The Jornada del Muerto flows are an outdoor musuem of structures developed from inflation or swelling of the flows from within during flow emplacement. These include mesa-like structures, domical swells, and circlular troughs. Based on our understanding of the relatively slow manner in which this particular type of lava flow and its lava tubes are formed, I estimate that the complete volcano formed in a time period less than 50 years.
Figure 4. The central vent region of the Jornada del Muerto volcano as seen from the south rim of the vent complex. This view shows that the vent region is essentially a shallow collapse approximately one half kilometer-wide. Click on the image for a larger view of this figure.
The central volcanic vent (Figure 4) from which the lavas erupted consists of an unusual raised elongated platform of basalt surmounted on the northern end by a small cinder cone. From a distance the profile of this platform resembles a squat cone, the flat-top of which is a little over one-quarter mile (0.5 km) in width. In fact, it resembles the profile of the raised rim of Meteor Crater. Instead of being a crater, however, the platform consists of relatively smooth lava surface that is broadly down-sagged toward the center and broken here and there by abrupt escarpments and cracks. The outer edge of the platform is surrounded by a wide moat-like crack and outward-sloping sheets of basalt that form a rampart sloping down to the surrounding lava surface. Similar volcanic vent structures occur on the seafloor and on the surface of Venus, but rarely on continents of Earth. To New Mexicans it may not seem like an easy place to visit, but compared to what one would have to do to see a similar feature anywhere else on Earth, the Jornada vent is an unusually accessible example of this type of volcanic vent structure.
Since the Jornada del Muerto has been sampled and dated, but not mapped, we cannot say with certainty how the unusual vent structure was created. But based on knowledge of some of the ways in which basaltic eruptions occur and my reconnaissance study of the Jornada del Muerto volcano, the vent complex may have started as a lava lake or as an unusual near-vent lava inflation structure. Cooling and draining of the interior of the lava lake or inflation structure during the final stages of the eruption caused the surface to founder and sag. The cinder cone at the north end of the vent complex was probably one of the last events. Ash and cinder eruptions often occur as a final phase, even in eruptions that are on the whole relatively non-explosive.
Not too far away to the north and west, nor very deep, there is another large body of magma, the Socorro magma body. It is continuing to accumulate, so perhaps another Jornada del Muerto-type eruption is in store for New Mexico in the near future.