La Garita Caldera

La Garita Caldera is a large volcanic caldera located in the San Juan volcanic field in the San Juan Mountains in southwestern Colorado, United States, to the west of the town of La Garita, Colorado. The eruption that created the La Garita Caldera was the largest known eruption since the Ordovician, with a VEI magnitude of 9.2. [cite journal |last=Mason |first=Ben G. |authorlink= |coauthors=Pyle, David M.; Oppenheimer, Clive |year=2004 |month= |title=The size and frequency of the largest explosive eruptions on Earth |journal=Bulletin of Volcanology |volume=66 |issue=8 |pages=735–748 |doi=10.1007/s00445-004-0355-9 |url= |accessdate= |quote= ]

Date

The La Garita Caldera is one of a number of calderas that formed during a massive ignimbrite flare-up in Colorado, Utah and Nevada from 40–25 million years ago, and was the site of truly enormous eruptions about 26–28 million years ago, during the Oligocene Epoch.

Provence

The area devastated by the La Garita eruption is thought to have covered a significant portion of what is now Colorado, and ash could have fallen as far as the east coast of North America and the Caribbean.Fact|date=April 2008

ize of Eruption

The scale of La Garita volcanism was far beyond anything known in human history. The resulting deposit, known as the Fish Canyon Tuff, has a volume of approximately convert|5000|km3|cumi|sigfig=2|sp=us, enough material to fill Lake Erie (in comparison, the May 18, 1980 eruption of Mt. St. Helens was only convert|1.2|km3|cumi|1|sp=us in volume).

By contrast, the most powerful human-made explosive device ever detonated, the Tsar Bomba or Emperor Bomb, had a yield of 50 megatons, whereas the eruption at La Garita was approximately 10⁵ times more powerful. It is possibly the most energetic event on Earth since the Chicxulub impact, which was 50 times more powerful.

Geology

The Fish Canyon Tuff, made of dacite, is known to be remarkably 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, a volcanic rock compositionally intermediate between basalt (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 quite large in scale. It is convert|35|by|75|km|mi|0|sp=us, an unusually oblong shape. Most calderas of explosive origin are roughly circular or slightly ovoid 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 can be considered a "supervolcano", albeit an extinct one.

La Garita is also the source of at least 7 major eruptions of welded tuff deposits over a time 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 made of dacite that is very similar to that of the Fish Canyon Tuff. This rock, which has characteristics of both lava and welded tuff, was erupted probably shortly before the Fish Canyon Tuff. The lava-like rock has been interpreted as having erupted as thick spatter during low-energy lava fountaining. The lava-like rock is also voluminous — up to convert|200|-|300|km3|cumi|sigfig=2|sp=us.

ee also

* Wheeler Geologic Area
* Yellowstone Caldera

References

* (includes maps, [http://pubs.usgs.gov/imap/i2799/photos.html photo collection] , and links to on-line abstracts)
*cite journal
author = Ben G. Mason
coauthors = David M. Pyle, and Clive Oppenheimer
year = 2004
title = The size and frequency of the largest explosive eruptions on Earth
journal = Bulletin of Volcanology
volume = 66
issue = 8
pages = 735–748
doi = 10.1007/s00445-004-0355-9
*cite journal
author = Daniel R. Askren
coauthors = Michael F. Rodden, and James A. Whitney
year = 1997
title = Petrogenesis of Tertiary Andesite Lava Flows Interlayered with Large-Volume FelsicAsh-Flow Tuffs of the Western USA
journal = Journal of Petrology
volume = 38
issue = 8
pages = 1021–1046
doi =
url = http://www.oxfordjournals.org/our_journals/petroj/online/Volume_38/Issue_08/pdf/ega047_gml.pdf
format = PDF
accessdate = 2007-05-02
* [http://staff.aist.go.jp/s-takarada/CEV/newsletter/lagarita.html Largest explosive eruptions: New results for the 27.8 Ma Fish Canyon Tuff and the La Garita caldera, San Juan volcanic field, Colorado]
* [http://www.colorado.edu/GeolSci/Resources/WUSTectonics/CzIgnimbrite/ignimbrite_intro.html The Mid-Tertiary Ignimbrite Flare-Up]

External links

* [http://hvo.wr.usgs.gov/volcanowatch/2005/05_04_28.html USGS Hawaiian Volcano Observatory: Supersized eruptions are all the rage!]