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Geologic History. Expansion in this right the main Rio Grande rift began about 36 million years back.

Expansion in this the main Rio Grande rift started about 36 million years ago. Rock debris that eroded through the developing highlands that are rift-flank in addition to wind-blown and playa pond deposits, accumulated when you look at the subsiding Mesilla Basin. These fill that is basin, known as the Santa Fe Group, are 1500 to 2000 legs dense beneath Kilbourne Hole (Hawley, 1984; Hawley and Lozinsky, 1993). The uppermost sand, silt, and clay of this Pliocene to very very early Pleistocene Camp Rice development, the unit that is youngest associated with the Santa Fe Group in this the main basin, are exposed into the base of Kilbourne Hole. The Camp Rice Formation ended up being deposited by way of a south-flowing braided river that emptied in to a playa lake within the vicinity of El Paso.

The La Mesa area, a flat work surface that developed along with 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 back (Mack et al., 1994). This area is all about 300 ft over the contemporary Rio Grande floodplain. The top created during a time period of landscape security. Basalt moves through the Portillo field that is volcanic intercalated with all the top Camp Rice development and lie in the Los Angeles Mesa area.

The Rio Grande started initially to decrease through the older Santa Fe Group deposits after 700,000 years ago as a result to both climatic modifications and integration of this river system using the gulf coast of florida. This downcutting had not been a process that is continuous there have been several episodes of downcutting, back-filling, and renewed incision. This episodic growth of the river system generated the forming of a few terrace amounts along the Rio Grande between Las Cruces and El Paso.

Basalt that erupted about 70,000 to 81,000 years back from a set of vents called the Afton cones positioned north-northeast of Kilbourne Hole flowed southward. The explosion that created Kilbourne Hole erupted through the distal edges of this Afton basalt moves, showing that the crater is more youthful than 70,000 to 81,000 yrs old. Pyroclastic rise beds and vent breccia blown through the crater overlie the Afton basalt movement. The crater formed druing the last phases associated with eruption (Seager, 1987).

Volcanic Features

Bombs and bomb sags

Volcanic bombs are blobs of molten lava ejected from a volcanic vent. Bombs have reached minimum 2.5 ins in diameter and are also frequently elongated, with spiral surface markings acquired since the bomb cools since it flies although the fresh air(Figure 5).

Bomb sags are typical features into the pyroclastic suge beds. The sags form whenever ejected volcanic bombs effect to the finely surge that is stratified (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.


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 center crust (Figure 7; Hamblock et al., 2007). The granulite may include garnet and sillimantite, indicative of the metasedimentary origin, or the granulite may contain 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 basaltic andesite, and limestone (Padovani and Reid, 1989; French and McMillan, 1996).

Mantle xenoliths (Figure 8) consist of 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 underneath the planet’s area ( ag e.g., Parovani and Reid, 1989; Hamblock et al., 2007). Some olivine within the mantle xenoliths is of adequate size and quality to be viewed gem-quality peridot, the August birthstone.

Figure 7 – Crustal xenoliths from Kilbourne Hole. Figure 8 – Mantle xenolith from Kilbourne Hole.

Surge beds

A pyroclastic surge is hot cloud which contains more gasoline or vapor than ash or stone fragments. The turbulent cloud moves close towards the ground area, frequently leaving a delicately layered and cross-stratified deposit (Figures 3 and 6). The layering kinds by unsteady and turbulence that is pulsating the cloud.

Hunt’s Hole and Potrillo Maar

Most of the features described above may 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. in comparison to Kilbourne Hole, Potrillo maar is certainly not rimmed by a basalt movement, and cinder cones and a more youthful basalt movement occupy a floor of Potrillo maar (Hoffer, 1976b).

Figure 9 – View into the western from Potrillo maar looking toward Mt. Riley and Mt. Cox, two middle Cenocoic dacite domes . Photograph by Richard Kelley.

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