La Garita Caldera

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. 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.

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 $28.01 million years ago$, during the Oligocene Epoch.

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 11000 sqmi. Its average thickness is 100 m. The eruption might have formed a large-area ash-fall, but none has yet been identified.

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 1200 cumi, giving it a volcanic explosivity index rating of 8. By comparison, the eruption of Mount St. Helens on May 18, 1980, was 0.25 cumi in volume. 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.

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, 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 35 by 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 - 300 km3. 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.