Potrillo Volcanic Field - Volcanology

Volcanology

The Rio Grande rift is a tectonically active structure that has evolved during two stages:

1. The first phase was from the late Oligocene to early Miocene and at that time thirty to fifty percent of the Rio Grande rift was made; volcanism at that time included basaltic andesites, andesites, and silicic ash flow tuffs.
2. The second stage began around the mid-Miocene to Quaternary and continues to the present. It is about ten percent of the Rio Grande extension; there is some evidence of the continuous rifting including young fault scarps, seismicity, high heat flow, and ongoing uplift as established by geodetic measurements. Mafic volcanism is related to this stage.

What is thought is that molten rock went into crustal cracks which sometimes exploded violently onto the earth’s surface up and down the rift. Several thousand years ago, here in the Potrillo volcanic field, white-hot magma rose from 50 miles (80 km) under the surface up through structural fractures toward the surface to change all the landscape appearance.

In some places molten rock intruded into depositional sand but it failed to erupt through the surface, so erosion took place to expose the igneous formations. In other places molten rock erupted explosively, producing clouds of solidified lava that later fell back to earth as ash, forming a cone around the vent. In some cases, it erupted several times as a succession of lava flows from a single vent forming widespread basalt layers taking on the form of a shield. At times, molten rock came into contact with water-saturated sand and it produced superheated and highly pressurized water vapor that exploded though the earth’s surface with a devastating force creating large craters known as maars or lakes.

In the Potrillo volcanic field, there are 10 to 15 foot thick layers of basalt spread across the volcanic field, from lava breaching the walls of the cinder cone and shield volcanoes, spilling down the flanks and spreading across the surrounding surface. The Potrillo area is generally classified as part of the southern Rio Grande rift and shows the Tertiary tectonic evolution of that structure. Extension took place in an intense 30–20 Ma phase, involving low-angle normal faults, and a less intense <10 Ma phase, involving high-angle normal. Mack & Seager (1995) argued that the Quaternary magmatism reached the surface via a transfer zone linking two adjacent N–S-trending, long-lived, extensional structures—the West Robledo and Camel Mountain faults.

The Potrillo volcanic field is separated into three areas; the eastern, central and western alignments. This division is based on the essentially north-south distribution of volcanic centers, which is delineated by units east of the east Robledo fault. It goes across from north to south, the eastern alignment includes Santo Tomas, San Miguel, Little Clack Mountain and Black Mountain. Most of the eastern alignment are scoria cones with and without breach flows with pahoehoe surface.

The central alignment includes Aden, Afton, the Gardner cones, and the two maars; the Kilburne hole and Hunt’s hole. The two maars erupted through portions of pre-existing Afton series. The Afton flows may be a fissure-emplaced upon which the Gardner cones were made. Aden is a shield site of many tube-fed lavas with explosive characteristics.

The Western alignment affects The Western Potrillo Mountains, a cavalcade of hundreds of coalescing cones and flows formed upon older, thick platform was the result from possibly fissure-fed stacked flows. Also within the western area are several maars: Riley, Malpais, and Potrillo. Potrillo maar is included with the western alignment due to its position west of the East Robledo fault.