EMPLACEMENT OF THE ADEN LAVA FLOWS, POTRILLO VOLCANIC FIELD, DOÑA ANA COUNTY, NEW MEXICO

Aden Crater and its flows are the youngest features in a diverse, monogenetic basalt field, known as the Potrillo Volcanic Field, which occupy the southernmost part of the Rio Grande Rift. The basalt flows encompass 75 km² and offer excellent examples of features of low-viscosity basalt associated with an Icelandic-type shield volcano. Five facies are recognized, including: (1) initial scabby flows; (2) inflated flows, (3) channeled flows, (4) lobate flows and (5) apical crater. The various facies developed as a function of the rheology and timing of flow emplacement.

Initial thin, fluid flows, 0.5-1.0 m thick, constitute a scabby facies near the vent. Early attempts to thicken failed as fluid lava broke through weak crust margins of early-formed tumuli. Lava escaping from small surface domes and larger flat-topped inflation plateaus allowed thickened portions of the flow to subside to form small, rootless shields and larger blocky-rimmed pits surrounded by lava channels radiating from the failed margins.

The distal flow field is subdivided into three subfaces facies based on flow physiography described in terms of inflation, in which a fluid interior acts on a thickening crustal layer. The thinnest flows, less than 1 m thick, spread across an uneven surface—filling small depressions and surrounding small rises in the surface. Partially inflated flows, 2-3 m thick, have steep rounded margins and a flat upper surface. Fully developed inflation plateaus, 4-5 m thick, have steep blocky margins, a large peripheral fracture around a relatively smooth, upper surface. Opposing flow-margins form deep intervening ravines throughout the field.

The low shield, 2.5 km basal diameter and 50 m in height, was formed as viscosity increased to the point that lavas began to accumulate over the vent. The upper slope consists of channeled flows that spilled from a lava lake in the broad crater atop the shield. The gradient downslope lessens, and lava spreads out of the channels to form an overlapping lobate flow pattern. The wide, shallow crater is bound by a three-meter-high spatter lava rampart. The interior contains remnants of the lava lake; an inner collapse pit; and a late stage spatter cone.