Expansion in this right an element of the Rio Grande rift started about 36 million years ago. Rock debris that eroded through the developing highlands that are rift-flank along with wind-blown and playa pond deposits, accumulated within the subsiding Mesilla Basin. These basin fill deposits, referred to as Santa Fe Group, are 1500 to 2000 legs dense beneath Kilbourne Hole (Hawley, 1984; Hawley and Lozinsky, 1993). The uppermost sand, silt, and clay associated with Pliocene to very very early Pleistocene Camp Rice development, the youngest product of this Santa Fe Group in this the main basin, are exposed into the bottom of Kilbourne Hole. The Camp Rice Formation ended up being deposited by a south-flowing braided river that emptied in to a playa pond into the vicinity of El Paso.
The La Mesa area, a surface that is flat developed in addition to the Camp Rice development, represents the utmost basin fill of this Mesilla Basin at the conclusion of Santa Fe Group deposition about 700,000 years ago (Mack et al., 1994). This area is mostly about 300 ft over the Rio Grande that is modern floodplain. The top created during a time period 
of landscape security. Basalt moves through the Portillo volcanic field are intercalated utilizing the top Camp Rice Formation and lie regarding the Los Angeles Mesa area.
The Rio Grande began to decrease through the older Santa Fe Group deposits after 700,000 years back as a result to both climatic modifications and integration associated with the river system with all the gulf. This downcutting had not been a constant procedure; there have been a few episodes of downcutting, back-filling, and renewed incision. This episodic growth of the river system generated the forming of several terrace amounts across the Rio Grande between Las Cruces and El Paso.
Basalt that erupted about 70,000 to 81,000 years back from a couple of ports called the Afton cones positioned north-northeast of Kilbourne Hole flowed southward. The explosion that created Kilbourne Hole erupted through the distal sides associated with Afton basalt flows, showing that the crater is more youthful than 70,000 to 81,000 yrs old. Pyroclastic rise beds and breccia that is vent through the crater overlie the Afton basalt movement. The crater formed druing the ultimate phases for the eruption (Seager, 1987).
Volcanic Features
Bombs and bomb sags
Volcanic bombs are blobs of molten lava ejected from the volcanic vent. Bombs have reached minimum 2.5 ins in diameter and so are frequently elongated, with spiral surface markings acquired due to the fact bomb cools since it flies although the fresh air(Figure 5).
Bomb sags are normal features within the pyroclastic beds that are suge. The sags form whenever ejected volcanic bombs impact in to the finely stratified rise beds (Figure 6).
Figure 5 – Volcanic bomb from Kilbourne Hole. Figure 6 – Hydromagmatic deposits exposed in cliffs of Kilbourne Hole. The arrow features a bomb that is volcanic has deformed the root deposits. Photograph by Richard Kelley.
Xenoliths
Most of the volcanic bombs at Kilbourne Hole have xenoliths. Granulite, charnokite, and anorthosite are typical xenoliths in bombs at Kilbourne Hole; these xenoliths are interpreted to express bits of the low to crust that is middleFigure 7; Hamblock et al., 2007). The granulite may include garnet and sillimantite, indicative of the metasedimentary origin, or the granulite may include pyroxene, suggestive of an igneous beginning (Padovani and Reid, 1989; Hamblock et al., 2007). Other upper crustal xenoliths include intermediate and silicic-composition volcanic stones, clastic sedimentary stones, basalt and andesite that is basaltic and limestone (Padovani and Reid, 1989; French and McMillan, 1996).
Mantle xenoliths (Figure 8) include spinel lherzolite, harzburgite, dunite, and clinopyroxenite. Research of these xenoliths has supplied data that are important the structure and heat for the mantle at depths of 40 kilometers under the planet’s area ( e.g., Parovani and Reid, 1989; Hamblock et al., 2007). Some olivine into the mantle xenoliths is of enough size and quality to be looked at gem-quality peridot, the August birthstone.
Figure 7 – Crustal xenoliths from Kilbourne Hole. Figure 8 – Mantle xenolith from Kilbourne Hole.
Surge beds
A pyroclastic rise is hot cloud which contains more fuel or vapor than ash or stone fragments. The turbulent cloud moves close into the ground area, usually leaving a delicately layered and cross-stratified deposit (Figures 3 and 6). The layering types by unsteady and pulsating turbulence in the cloud.
Hunt’s Hole and Potrillo Maar
Most of the features described above will also be current at Hunt’s Hole and Potrillo maar (Figure 9), that are found towards the south of Kilbourne Hole. Xenoliths are uncommon to absent at Hunt’s Hole (Padovani and Reid, 1989), but otherwise the maars are comparable. Contrary to Kilbourne Hole, Potrillo maar isn’t rimmed by way of a basalt movement, and cinder cones and a more youthful basalt movement occupy a floor of Potrillo maar (Hoffer, 1976b).
Figure 9 – View to your western from Potrillo maar looking toward Mt. Riley and Mt. Cox, two middle Cenocoic dacite domes . Photograph by Richard Kelley.