1 Distributed volcanic fields of the southwestern U.S.

This session invites submissions from various disciplines (e.g., petrology, tectonics, geophysics, geochronology) that explore distributed volcanism in the southwestern U.S. Topics of interest include spatio-temporal trends, magma compositions, and mantle and/or crustal controls on volcanism.

Monday, 18 May 2026
8:00 AM - 12:00 PM
Alvarado A

Mapping and Geochemistry of the Black Mountain-Santo Tomas Chain, Potrillo Volcanic Field, Southern New Mexico

Paper number: 1-1
Program: Technical Sessions
Time: 8:05 AM – 8:25 AM

Authors

Abstract

The Black Mountain-Santo Tomas volcanic chain, consists of monogenetic alkali basalts related to the intracontinental Rio Grande rift (RGR), is the eastern most periphery of the 1295 km2 Potrillo Volcanic Field (PVF) in southern New Mexico. This ~165 ka volcanic chain has at least five main eruptive centers along a south to north trend: Black Mountain, a cone north of Black Mountain, Little Black Mountain, San Miguel, and Santo Tomas, respectively. Mapping in the 1960s showed conflicting numbers of vents and flows at Black Mountain, which is the largest eastern chain feature, due to aeolian sands that prevented positive identification. Previous studies of magma assembly and monogenetic volcanism within the intra-continental rift system focused on the central PVF, and limited work on the eastern PVF has been done.

The purpose of this study is to refine the location of vents and flows at Black Mountain, then assess the relationship between this chain and the larger PVF using field mapping, melt composition, and isotope analyses. National Agriculture Imagery Program (NAIP) images and 1m digital elevation model (DEM) overlays were used to identify these vents and flows at Black Mountain, which creates a topographic profile because they are separate cooling units. Field mapping and geographic information system (GIS) analysis identified four vents and six flows associated with Black Mountain. Picked clean groundmass was used as an analogue for the melt composition for the Black Mountain-Santo Tomas chain. The melt composition of this chain has a higher silica content and is more evolved than previous whole rock composition data, possibly due to the effects of crystal accumulation. This updated map and future analyses will evaluate the extent of monogenetic volcanism related to the RGR within the eastern Black Mountain-Santo Tomas chain and the PVF.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410665

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