La Garita Caldera

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Ash formations in of La Garita Caldera, looking northeast.

Location of La Garita Caldera within Colorado

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. It 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. The scale of volcanism was far beyond anything known in human history. The resulting deposit, known as the Fish Canyon Tuff, has a volume of approximately 5,000 cubic kilometers, enough material to fill Lake Erie (in comparison, the May 18, 1980 eruption of Mt. St. Helens was only 1.2 cubic kilometers in volume). 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.^{[citation needed]}

The eruption that created the La Garita Caldera was one of the largest known eruptions, if not the largest, to have occurred in Earth's history.^{[citation needed]} By contrast, the most powerful man-made explosive device ever detonated, the Tsar Bomba or Emperor Bomb, had a yield of only 50 megatons, whereas the eruption at La Garita released approximately 10 zettajoules. It is possibly only the second most energetic event to have ocurred on Earth besides the Chicxulub impact, which released an estimated 500 zettajoules (5×10²³ joules) of energy, equivalent to 100 teratons of TNT (10¹⁴ tons), making it 2 million times more powerful.^{[citation needed]}

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. La Garita Caldera also has erupted large intracaldera lavas composed of andesite, a volcanic rock compositionally intermediate between basalt (poor in silica content) and dacite (higher silica content).

The caldera, like the eruption of Fish Canyon Tuff, is also quite large in scale. It is 35 by 75 kilometers (approximately 22 by 47 miles), 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 200-300 cubic kilometers.