thumb|Kilbourne Hole (center) and Hunt's Hole (lower right), with [[White Sands National Park lit by the sun in the distance beyond the Organ Mountains]]
thumb|upright|[[Basalt cliffs of Kilbourne Hole, looking northwest from near the southwest corner. The cliffs come from the earlier Cenozoic Afton basalt flow; the magma that caused the maar explosion was also basalt.
In 1975, Kilbourne Hole was designated as a National Natural Landmark by the National Park Service. It is now part of Organ Mountains–Desert Peaks National Monument.
Geologic setting and formation
Kilbourne Hole and Hunt's Hole are found in the central part of the Potrillo volcanic field, which also contains the Afton-Aden basalt flows. The area is part of the Rio Grande rift, where the Earth's crust is being stretched and thinned. The rift is characterized by deep sedimentary basins, recent faulting and volcanic activity, and unusually high heat flow upwards from the Earth's mantle. Kilbourne Hole and Hunt's Hole are located on the same north-trending fault of the Fitzgerald-Robledo fault system.
A maar forms when rising magma encounters sediment beds saturated with groundwater. The magma heats the groundwater to the point where the vapor pressure overcomes the weight of the overlying beds (the overburden pressure) and the beds are catastrophically blown out. Country rock is fragmented and expelled into the atmosphere together with fragments of the magma, creating a deep crater, the bottom of which sits below the pre-eruptive ground surface. The eruption that is attributed with the formation of the maar depression was dated to around 20,000 years. As a result of the eruption, the maar also experienced a collapse similar to that of a caldera.
Kilbourne Hole erupted through alluvium (unconsolidated water-deposited sediments), the Camp Rice Formation and through the pre-existing Afton basalt flow. Like most maars, it has a shallow rim, composed of erupted material that was deposited as thin pyroclastic surge deposits.
The hole is over a mile wide, and over deep, with crumbling cliffs all around except at the southwest corner. The rim cliffs, measuring about in height, are composed of basalt and exhibit clear columnar jointing (a feature common to many basalt cliffs, including those of Devils Postpile National Monument near Yosemite National Park and Moses Coulee in the Channeled Scablands of Washington), with characteristic reddish-purple, polygonal (mostly hexagonal) columns. The base of the cliffs is obscured by scree composed of blocks of basalt that have been dislodged from the columns above by the work of erosion and mechanical weathering. The basalt flow that comprises these columns pinches out (thins) and eventually disappears as it approaches the southwestern rim of the maar. and a large number of xenoliths derived from the lower crust and mantle. These have been closely studied by geologists to learn more about geologic processes deep underground. These likely took less than three days to reach the surface from their place of origin, and show pristine composition and texture. Their characteristics show that they were little altered from their formation 1.6 to 1.8 billion years ago, other than some reheating during the opening of the Rio Grande rift.
In 2017, a NASA field team visited the hole to test various instruments that are planned to be used in future space missions. Jack Schmitt attended the tests, as well as astronaut Barry Wilmore who was there to assist in simulated moonwalks at the hole.
thumb|General location of Kilbourne Hole, Doña Ana County, New Mexico
Access
Kilbourne Hole is located within Organ Mountains–Desert Peaks National Monument and administered by the Bureau of Land Management. It is accessed via Doña Ana County Road A-011, driving west from the railroad. The hole is "on the right, past the big tan dirt bank."