La Garita Caldera
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| La Garita Caldera | |
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Ash formations of La Garita Caldera, looking northeast (Wheeler Geologic Monument).
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 La Garita Caldera
Location of La Garita Caldera within Colorado
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| Location | Mineral County, Colorado, US, near Creede |
| Parent range | San Juan Mountains |
| Geology | |
| Mountain type | Caldera |
| Last eruption | 26.3 Ma (Fish Canyon Tuff 27.8 Ma) |
La Garita Caldera is a large volcanic caldera located in the San Juan volcanic field in the San Juan Mountains near the town of Creede in southwestern Colorado, United States.[1] It lies to the west of the town of La Garita, Colorado. The eruption that created the La Garita Caldera is among the largest known volcanic eruptions in Earth's history.[2][3]
Contents
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 40–18 million years ago, and was the site of massive eruptions about 28.01± 0.04 million years ago, 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 30,000 km.2 Its average thickness is 100 m. 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 deposit, known as the Fish Canyon Tuff, has a volume of approximately Lua error in Module:Convert at line 1851: attempt to index local 'en_value' (a nil value)., rating it an 8 on the Volcanic Explosivity Index. This is enough material to fill Lake Michigan. By comparison, the May 18, 1980 eruption of Mt. St. Helens was only 0.25 cubic miles (1.0 km3) in volume.[6]
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. However, because Tsar Bomba's reaction was complete within microseconds, while a volcanic explosion can take seconds or minutes, the power of the events are comparable if measured within the respective bounded timeframes. It is the most energetic event to have taken place on Earth since the Chicxulub impact, which was 400 times more powerful than La Garita.[citation needed]
Geology
Lua error in package.lua at line 80: module 'strict' not found. 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 quite large in scale. It is 35 by 75 kilometers (22 by 47 mi) and oblong in shape. Most supervolcano 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 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 Lua error in Module:Convert at line 1851: attempt to index local 'en_value' (a nil value).. The dacite lava-like unit call Pagosa Peak Dacite. These rocks had been identify as lava, because units has a high aspect ratio (1:50) and very high viscosity of the crystal-rich magma similar to those of flow-layered silicic lava. However, Pagosa Peak Dacite formed by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows.[7]
See also
References
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- ↑ Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ Mason, et al.
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Bibliography
- Lua error in package.lua at line 80: module 'strict' not found. (includes maps, photo collection, and links to on-line abstracts)
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- Lua error in package.lua at line 80: module 'strict' not found.[dead link]
- Largest explosive eruptions: New results for the 27.8 Ma Fish Canyon Tuff and the La Garita caldera, San Juan volcanic field, Colorado
- The Mid-Tertiary Ignimbrite Flare-Up
External links
- USGS Hawaiian Volcano Observatory: Supersized eruptions are all the rage!
- Maps:
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