The Potrillo Volcanic Field is one of the youngest and mostly southerly fields in New Mexico. The young (~0-24 ka) eastern part of the field (Aden-Afton field ) (Hoffer et al, 1998) consists of several scoria cones, a shield volcano, and maars, as well as some much older basaltic masses such as Black Mountain. Three well-known maars, Hunt's Hole and Kilbourne Hole, and Potrillo, and a tuff cone, Riley occur in the eastern half of this field. The age of Kilbourne Hole is estimated at 80 ka, or approximately similar to Zuni salt Lake, another maar in northern and western New Mexico. Kilbourne Hole is widely known for its abundant mantle-lower crustal xenoliths (Padovani and Reid, 1989), but is better known in the volcanological community for its rim consisting of deposits emplaced through classic hydromagmatic base-surge eruptions, of which there are many excellent exposures.
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These calderas are part of the widespread volcanism that swept the Southwest during the Miocene. Like the Datil-Mogollon field to the immediate north, these have been broken up as part of the Basin and Range faulting and deeply dissected. Nonetheless, the centers of volcanism, and the fault-encircled extremely thick sections of outflow materials such as ash-flow tuffs and thick viscous lavas are evidence for the locations of the original calderas.
Sierra is an older volcano consisting of intrusive stocks and dikes and contemporaneous ash, breccia, and flows. It is not a modern volcano like Mount Taylor or the Valles Caldera, but is an old one that has been greatly eroded. Based on the attitude of existing volcanic units the bulk volcano may have been originally on the order of 3000 ft above the surrrounding terrain, 20 miles in diameter and contained about 185 cubic miles of volcanic material. Estimates of the age of the initial volcano is 38 to 26 million years. This means that it erupted at about the same time that the volcanoes of the Gila and Mogollon Ranges in western New Mexico were erupting.There was considerable volcanism in the Southwest during this interval of time, most of which just preceded the formation of the Basin and Range Province. Volcanism of this age is often referred to as "mid-Tertiary" volcanism, which in the lingo of southwestern volcanology means "old". It is a testament to the original great size of the volcano, together with later block-faulting that it remains as a significant mountain even today.
The Carrizozo flow field is 75 km long from the vent area to the distal margin in the Tularosa Valley. The entire flow covers ~330 km2 to an estimated thickness of 10 to 15 m, for a total estimated erupted volume of ~4.3 km3. The lava is intermediate in composition between alkalic and tholeiitic basalt. Its location and age is consistent with the regional volcanism elsewhere that is fundamentally associated with the Rio Grande rift. Various researchers have distinguished between upper and lower Carrizozo flow units, separated by a narrow “neck” in the medial reach. However, chemical analyses to date have revealed no evidence for distinct differences between the upper and lower lavas. Cosmogenic (isotopic changes induced by exposure to high energy particles) studies indicate exposure ages of 4800 yrs to 5200 yrs for the Carrizozo flow field. These results make the Carrizozo flow field the second youngest volcanism in New Mexico, after only the McCartys flow field in western New Mexico.
Relief map showing the lateral dimension of the low relief construct associated with the Jornada del Muerto lava flows.