La Garita Caldera Explained

La Garita Caldera
Location:Mineral County, Colorado, US, around Creede
Range:San Juan Mountains
Map:USA Colorado
Coordinates:37.7564°N -106.9342°W
Type:Caldera, extinct supervolcano
Last Eruption:26.3 Ma (Fish Canyon Tuff 27.8 Ma)

La Garita Caldera is a large caldera and extinct supervolcano in the San Juan volcanic field in the San Juan Mountains around the town of Creede in southwestern Colorado, United States.[1] It is west of La Garita, Colorado. 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.[2] [3]

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 to 18 million years ago, and was the site of massive eruptions about, during the Oligocene Epoch.[4]

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, covered at least . Its average thickness is . The eruption might have formed a large-area ash-fall, but none has yet been identified.[5]

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 1200sigfig=2NaNsigfig=2, giving it a volcanic explosivity index rating of 8.[6] By comparison, the eruption of Mount St. Helens on May 18, 1980, was 0.25cumi in volume.[7] By contrast, the most powerful human-made explosive device ever detonated, the Tsar Bomba, had a yield of 50 megatons, whereas the eruption at La Garita was about 5,000 times more energetic.[8]

The Fish Canyon eruption was the second most energetic event to have occurred on Earth since the Cretaceous–Paleogene extinction event 66 million years ago. The asteroid impact responsible for that mass-extinction, equivalent to 100 teratons of TNT,[9] was approximately 420 times more powerful than the Fish Canyon eruption.

Geology

The Fish Canyon Tuff, made of dacite, 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, 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 large in scale. It is 35by 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 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 200-. 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 fountaining and lateral transport as dense, poorly-inflated pyroclastic flows.[10]

See also

References

Bibliography

External links

Notes and References

  1. Steven . Thomas A. . Peter W. . Lipman . Calderas of the San Juan Volcanic Field, Southwestern Colorado . U.S. Geological Survey Professional Papers . 958 . 1–35 . U.S. Government Printing Office . Washington, DC . 1976 . 2012-05-16.
  2. Web site: What's the Biggest Volcanic Eruption Ever? . livescience.com . November 10, 2010 . 2014-02-01.
  3. The 36–18 Ma Indian Peak–Caliente ignimbrite field and calderas, southeastern Great Basin, USA: Multicyclic super-eruptions . Best . MG . 2013 . Geosphere . 9 . 4 . 864–950 . 2013Geosp...9..864B . 10.1130/GES00902.1 . free.
  4. Ultra-high precision 40Ar/39Ar ages for Fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required? . Phillips . D . 2013 . Geochimica et Cosmochimica Acta . 121 . 229–239 . 2013GeCoA.121..229P . 10.1016/j.gca.2013.07.003.
  5. Central San Juan caldera cluster: regional volcanic framework . Lipman . Peter W . 2000 . Geological Society of America Special Papers . 346 . 9–69 . 0-8137-2346-9 . 10.1130/0-8137-2346-9.9.
  6. Book: Super Volcano: The Ticking Time Bomb Beneath Yellowstone National Park . 10 November 2007 . Voyageur Press .
  7. Mason, et al.
  8. Web site: 2021-08-25 . La Garita Mountains grew from volcanic explosions 35 million years ago . US Forest Service . en . 2022-04-23.
  9. Schulte . Peter . Alegret . Laia . Arenillas . Ignacio . Arz . José A. . Barton . Penny J. . Bown . Paul R. . Bralower . Timothy J. . Christeson . Gail L. . Claeys . Philippe . Cockell . Charles S. . Collins . Gareth S. . 2010-03-05 . The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary . Science . en . 327 . 5970 . 1214–1218 . 0036-8075 . 10.1126/science.1177265 .
  10. Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado . Bachmann . O. . Dungan . M.A. . Lipman . P.W. . May 2000 . Journal of Volcanology and Geothermal Research . 98 . 1–4 . 153–171 . 2000JVGR...98..153B . 10.1016/S0377-0273(99)00185-7.