Volcanic geology of the Rio Puerco necks
— R. Bruce Hallett

The volcanic necks of the Rio Puerco Valley are among the most concentrated and best preserved examples in the world. They formed by erosion of poorly consolidated pyroclastic material surrounding a more resistant basaltic core. Throughout the valley, necks are preserved over a continuum of erosional stages. A partially exhumed scoria cone near Cerro de Nuestra Senora represents one of the least eroded volcanoes, whereas all that remains of Cabezon Peak is the solidified magma core. Mantle and/or crustal xenoliths occur in about 60% of the necks. Cerros Guadalupe and Negro, in particular, contain an exceptional variety and abundance. A new whole-rock ⁴⁰Ar/³⁹Ar plateau age of 2.658 ± 0.032 Ma for Cabezon Peak is consistent with volcanism on the adjacent Mesa Chivato and Mesa Prieta. The two principal types of volcanic features in the Rio Puerco Valley are (1) subaerial volcanic centers that produced surge and pyroclastic beds, lava flows and intrusive cores or crystallized lava lakes, and (2) wholly intrusive structures, such as dikes, which represent only a small part of the volcanic rocks in the valley. Based on field evidence, two models are given to explain the formation of the necks. In the first, a tuff ring or scoria cone is built around a central vent, then magma ponds in the vent and solidifies. Later, erosion of the outer cone exposes the plug. This model is more consistent with the larger, more symmetrical necks, such as Cabezon Peak and Cerro de Nuestra Senora. In the second model, magma supply rates are lower, the volcanic cone and conduit smaller, and thermal retention lower. The necks form when small pods of magma intrude into overlying volcanic tuffs and breccia near the waning stages of magmatic activity. Those that form by this process display irregular columnar jointing patterns. Most necks in the Rio Puerco Valley are consistent with this latter model.