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|La Garita Caldera|
Ash formations of La Garita Caldera, looking northeast. (Wheeler Geologic Monument)
|Location||Mineral County, Colorado, USA, near Creede|
|Range||San Juan Mountains|
|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. It lies to the west of the town of La Garita, Colorado. The eruption that created the La Garita Caldera is the largest known explosive eruption in Earth's history.
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 massive eruptions about 28–26 million years ago, during the Oligocene Epoch.
The area devastated by the La Garita eruption is thought[by whom?] to have covered a significant portion of what is now Colorado, and some research suggests the aerial extent may be as far as the east coast of North America and the Caribbean.
The scale of La Garita volcanism was one of greatest in recent geologic history. The resulting deposit, known as the Fish Canyon Tuff, has a volume of approximately 1,200 cubic miles (5,000 km3), 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. The eruption was energetically equivalent to 240,000 megatons of TNT.
By contrast, the most powerful human-made explosive device ever detonated, the Tsar Bomba or King of the Bombs, 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. It is one of the most energetic events on Earth since the Chicxulub impact, which was 400 times more powerful than La Garita.
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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) oblong 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 200–300 cubic kilometers (48–72 cu mi).
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