La Garita Caldera

DateEdit

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 7001280100000000000♠28.01±0.04 million years ago, during the Oligocene Epoch.

Area devastatedEdit

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 km² (11,000 miles²). Its average thickness is 100 m. The eruption might have formed a large-area ash-fall, but none has yet been identified.

Size of eruptionEdit

The scale of La Garita volcanism was the second greatest of the Cenozoic Era. The resulting 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 0.25 cubic miles (1.0 km³) 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. 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, at 240,000 gigatons, was approximately one thousand times more powerful than La Garita.

GeologyEdit

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 is considered an extinct volcano.

La Garita is also the source of at least 7 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 erupted probably shortly before the Fish Canyon Tuff. The Pagosa Peak Dacite has been interpreted as having erupted as thick spatter during low-energy lava fountaining and has a volume of about 200–300 cubic kilometers (48–72 cu mi). These rocks had been identified as lava because the unit 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, the Pagosa Peak Dacite formed by low-column pyroclastic fountaining and lateral transport as dense, poorly-inflated pyroclastic flows.