La Garita Caldera
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La Garita Caldera is a large volcanic caldera located in the San Juan Mountains in southwestern 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. La Garita was the site of truly enormous eruptions about 26–28 million years ago, during the Oligocene Epoch. This was the Fish Canyon Tuff, which has a volume of approximately 5,000 cubic kilometers. The scale is unimaginable — for comparison, the May 18, 1980 eruption of Mt. St. Helens (doing an estimated 2 to 3 billion USD in damages) was only 1.2 cubic kilometers in volume. The area devastated by the eruption must 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.
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 Caldera 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 very similar to that of the Fish Canyon Tuff. This rock has characteristics of both lava and welded tuff and 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 rather voluminous -- up to 200-300 cubic kilometers.
