The Peridot Mesa vent is a part of the

San Carlos volcanic field San Carlos volcanic field is a monogenetic volcanic field in Arizona. The field lies within the San Carlos Apache Reservation about east of Globe, Arizona. It is a small field covering approximately 50 km2 or less of volcanic cones and l ...

located in

San Carlos, Arizona San Carlos ( apw, Sengaah) is a census-designated place (CDP) in Gila County, Arizona, United States. The population was 4,038 at the 2010 census, up from 3,716 in 2000. San Carlos is the largest community in and the seat of government for the ...

USA The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

on the

San Carlos Apache Indian Reservation The San Carlos Apache Indian Reservation (Western Apache: Tsékʼáádn), in southeastern Arizona, United States, was established in 1872 as a reservation for the Chiricahua Apache tribe as well as surrounding Yavapai and Apache bands removed fro ...

. The mesa is about in diameter and is capped by a

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

flow 3 to 6 meters thick that originated from a volcanic cone located in the Southwest corner of the mesa. The flow which surrounds this vent is of special interest to igneous petrologists due to several unique qualities including the extremely high concentration of

xenoliths A xenolith ("foreign rock") is a rock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ign ...

Formation history

The formation of Peridot Mesa is still being studied and debated today, but several theories about the initial formation have come to be widely accepted among scientists. The initial eruption was an extremely violent, high-energy pyroclastic flow.Wohletz KH, 1978
''The eruptive mechanism of the Peridot Mesa Vent, San Carlos, Arizona.'' In: Guidebook to the Geology of Central Arizona (D. M. Burt and T. L. Péwé, Eds), State of Arizona Bureau of Geology and Mineral Technology, Special Paper 2: 167–176.
/ref> The presence of xenoliths which reach up to basketball size and beyond suggests that the magma had to have been traveling at extremely high speeds to have been able to carry such heavy pieces up with it. Current studies are being done at Arizona State University to determine a timeline history for these xenoliths based on analysis of the reaction rims around

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

crystals. The theory that a lava fountain created this unique flow is currently the most accepted theory. This is because the lava flow is "rootless" meaning that it begins outside of the

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

ring directly around the vent. This indicates that a lava fountain probably blasted lava away from its point of origin. The vent is best described as a diatreme, defined as a volcanic vent blasted through solid rock by exploding gases. The magma probably contained a large mass amount of water and

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

which are two volatile substances known to cause extremely high-energy and violent eruptions. The amount of volatiles in

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

is directly correlated to the amount of partial melting and energy of the volcanic eruption.Hirose and Kawamoto, Earth and Planetary Science Letters, Volume 133, Issues 3-4, July 1995, pp. 463–473 The flow followed topography down a depression trending to the NE carrying numerous nodules of

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...

xenoliths that had also been blasted out by vigorous fountaining. The volcanic neck of the vent is now collapsed. It is thought that the final stage of activity was marked by a cessation of fountaining, and the remaining volumes of magma hardened to create a plug in the vent.

References

{{coord, 33, 20, 02, N, 110, 29, 08, W, display=title Landforms of Gila County, Arizona Diatremes of Arizona Mesas of Arizona Volcanoes of Arizona