The Peridot Mesa vent is a part of the San Carlos volcanic field

San Carlos volcanic field

San Carlos volcanic field is a monogenetic volcanic field in Arizona, USA. 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 lava flows. The basanite to hawaiite basaltic flows...

 located in San Carlos, Arizona

San Carlos, Arizona

San Carlos is a census-designated place in Gila County, Arizona, United States. The population was 3,716 at the 2000 census.San Carlos is the largest community in and the seat of government for the San Carlos Apache Indian Reservation....

, USA

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 on the San Carlos Apache Indian Reservation

San Carlos Apache Indian Reservation

The San Carlos Apache Indian Reservation, in southeastern Arizona, United States, was established in 1871 as a reservation for the Chiricahua Apache tribe. It was referred to by some as "Hell's Forty Acres," due to a myriad of dismal health and environmental conditions.-Formation:President U.S....

. The mesa

Mesa

A mesa or table mountain is an elevated area of land with a flat top and sides that are usually steep cliffs. It takes its name from its characteristic table-top shape....

 is about 3 km (1.9 mi) in diameter and is capped by a basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

 flow 3 to 6 meters thick that originated from a volcanic cone

Volcanic cone

Volcanic cones are among the simplest volcanic formations. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and size of the fragments ejected during the eruption...

 located in the Southwest corner of the mesa. The flow which surrounds this vent is of special interest to igneous petrologists

Petrology

Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....

 due to several unique qualities including the extremely high concentration of xenoliths.

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. 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

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

 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

Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...

 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

Diatreme

A diatreme is a breccia-filled volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled relatively shallow crater known as a maar, or other volcanic pipes.- Word origin :...

, 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

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

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

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 is directly correlated to the amount of partial melting and energy of the volcanic eruption. The flow followed topography down a depression trending to the NE carrying numerous nodules of peridotite

Peridotite

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...

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.