La Garita Caldera is a large

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

in the

San Juan volcanic field The San Juan volcanic field is part of the San Juan Mountains in southwestern Colorado. It consists mainly of volcanic rocks that form the largest remnant of a major composite volcanic field that covered most of the southern Rocky Mountains in t ...

in the San Juan Mountains near the town of Creede in southwestern

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

United States 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 primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. It is west of

La Garita, Colorado La Garita is an unincorporated community in Saguache County, in the U.S. state of Colorado. History A post office called La Garita operated intermittently from 1874 until 1972. The community takes its name from the nearby La Garita Mountains ...

. The eruption that created the La Garita Caldera is among the largest known volcanic eruptions in Earth's history, as well as being one of the most powerful known supervolcanic events.

Date

The La Garita Caldera is one of a number of calderas that formed during a massive ignimbrite flare-up in

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

and

Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...

from 40–18 million years ago, and was the site of massive eruptions about , during the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

Epoch.

Area devastated

The area devastated by the La Garita eruption is thought to have covered a significant portion of what is now Colorado. The deposit, known as the

Fish Canyon Tuff The Fish Canyon Tuff is the large volcanic ash flow or ignimbrite deposit resulting from one of the largest known Explosive eruption, explosive eruptions on Earth, estimated at . (see List of largest volcanic eruptions). The Fish Canyon Tuff erupti ...

, covered at least . Its average thickness is . The eruption might have formed a large-area ash-fall, but none has yet been identified.

Size of eruption

The scale of La Garita volcanism was the second greatest of the Cenozoic Era. The resulting Fish Canyon Tuff has a volume of approximately , giving it a Volcanic Explosivity Index rating of 8. By comparison, the eruption of

Mount St. Helens Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...

on 18 May 1980 was in volume. By contrast, the most powerful human-made explosive device ever detonated, the

Tsar Bomba The Tsar Bomba () ( code name: ''Ivan'' or ''Vanya''), also known by the alphanumerical designation "AN602", was a thermonuclear aerial bomb, and the most powerful nuclear weapon ever created and tested. Overall, the Soviet physicist Andrei Sa ...

, had a yield of 50 megatons, whereas the eruption at La Garita was about 5,000 times more energetic. However, because Tsar Bomba's reaction was complete within nanoseconds, while a volcanic explosion can take seconds or minutes, the power of the events is comparable if measured within the respective bounded timeframes. The Fish Canyon eruption was the second most energetic event to have occurred on Earth since the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event, also known as the Cretaceous–Tertiary extinction, was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. The eve ...

66 million years ago. The asteroid impact responsible for that mass-extinction, equivalent to 100 teratons of TNT, was approximately 420 times more powerful than the Fish Canyon eruption.

Geology

The Fish Canyon Tuff, made of

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

, is uniform in its petrological composition and forms a single cooling unit despite the huge volume. Dacite is a silicic volcanic rock common in explosive eruptions, lava domes and short thick lava flows. There are also large intracaldera lavas composed of

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...

, a volcanic rock compositionally intermediate between

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 ...

(poor in silica content) and dacite (higher silica content) in the La Garita Caldera. The caldera itself, like the eruption of Fish Canyon Tuff, is large in scale. It is and oblong in shape. Many calderas of explosive origin are slightly ovoid or oblong in shape. Because of the vast scale and erosion, it took scientists over 30 years to fully determine the size of the caldera. La Garita is considered an extinct volcano. La Garita is also the source of at least seven major eruptions of welded

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 ...

deposits over a span of 1.5 million years since the Fish Canyon Tuff eruption. The caldera is also known to have extensive outcrops of a very unusual lava-like rock unit, called the Pagosa Peak Dacite, made of dacite that is very similar to that of the Fish Canyon Tuff. The Pagosa Peak Dacite, which has characteristics of both lava and welded tuff, was likely erupted shortly before the Fish Canyon Tuff. The Pagosa Peak Dacite has been interpreted as having erupted during low-energy pyroclastic fountaining and has a volume of about . These rocks were identified as lava because the unit has a highly elongated shape (1:50) and very high viscosity of the crystal-rich magma similar to those of flow-layered silicic lava. The Pagosa Peak Dacite formed by low-column

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

fountaining and lateral transport as dense, poorly-inflated

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...

References

Bibliography

* (includes maps
photo collection
and links to on-line abstracts) * *

* ttps://web.archive.org/web/20120716192901/http://www.colorado.edu/GeolSci/Resources/WUSTectonics/CzIgnimbrite/ignimbrite_intro.html The Mid-Tertiary Ignimbrite Flare-Up

External links

USGS Hawaiian Volcano Observatory: Supersized eruptions are all the rage!
* Maps: ** ** ** {{DEFAULTSORT:Garita Caldera, La Volcanoes of Colorado

La Garita Caldera La Garita Caldera is a large caldera in the San Juan volcanic field in the San Juan Mountains near the town of Creede in southwestern Colorado, United States. It is west of La Garita, Colorado. The eruption that created the La Garita Caldera ...

Extinct volcanoes Geology of the Rocky Mountains

Calderas of Colorado Supervolcanoes VEI-8 volcanoes Oligocene calderas Oligocene North America