The Potrillo volcanic field covers approximately of Doña Ana County. It is a monogenetic volcanic field that can be divided in three volcanic regions. The westernmost West Potrillo Field consists of more than 100 cinder cones, two maar volcanoes and associated flows that covers approximately . The central Aden–Afton field has a number of young flows, three cinder cones and three maar volcanoes, including Kilbourne Hole. Aden-Afton Field is approximately in extent. The easternmost Black Mountain-Sao Thomas alignment is a north-south belt of vents near the Rio Grande that includes Santo Tomas, San Miguel, Little Clack Mountain and Black Mountain. The field consists almost entirely of alkaline olivine basalt.