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La Garita Caldera is a large volcanic caldera located in the San Juan volcanic field in the San Juan Mountains in southwestern Coloradomarker, United Statesmarker, to the west of the town of La Garita, Colorado. The eruption that created the La Garita Caldera was, perhaps, the largest known explosive eruption in all of Earth's history (the Siberian Traps may have been larger but the cause is still being debated).


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


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 Caribbeanmarker.

Size 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 , enough material to fill Lake Michiganmarker (in comparison, the May 18, 1980 eruption of Mt.marker St. Helensmarker was only in volume).

By contrast, the most powerful human-made explosive device ever detonated, the Tsar Bomba or Emperor Bombmarker, 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 impactmarker, which was 50 times more powerful.


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 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 .

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