An 18-million-year-long eruption
Between 40 and 25 million years ago, a period of intense volcanic activity reshaped the western United States. This episode, the Mid-Tertiary Ignimbrite Flare-Up, was not a single event; it was a sustained series of colossal eruptions centered in three main areas: the San Juan volcanic field in Colorado, the Central Nevada field, and the Indian Peak field in Nevada and Utah. The geological trigger was the subducting Farallon Plate beneath North America. For millions of years, this plate had slid under the continent at a shallow angle, but as that process ceased, the mantle opened up, fueling widespread volcanism.
The flare-up produced enormous quantities of ignimbrite, a rock formed from the searingly hot, fast-moving clouds of ash, gas, and rock fragments known as pyroclastic flows. The total volume of erupted material included 500,000 cubic kilometers of ash flow tuff. The San Juan volcanic field, where the La Garita Caldera is located, was one of the most active zones, experiencing its peak activity between 35 and 30 million years ago. This period saw numerous volcanoes erupt and collapse, creating at least 15 distinct calderas in the region. The deposits from these eruptions are often found in thick layers, sometimes hundreds of meters deep where they were trapped by the caldera walls.
The Fish Canyon Tuff
The single largest known explosive eruption of the Ignimbrite Flare-Up—and one of the most powerful on Earth in the last half-billion years—created the La Garita Caldera about 28 million years ago. This eruption unleashed the Fish Canyon Tuff, a deposit of dacite rock with a volume of approximately 5,000 cubic kilometers. For comparison, the 1980 eruption of Mount St. Helens produced about one cubic kilometer of material. The energy released was thousands of times greater than the most powerful human-made explosive device.
The eruption was so vast that it created the oblong La Garita Caldera, measuring roughly 35 by 75 kilometers. The Fish Canyon Tuff covered at least 28,000 square kilometers of what is now Colorado with a deposit averaging 100 meters in thickness. This single layer of rock is remarkably uniform in its mineral composition, containing plagioclase, sanidine, quartz, and biotite. Following the main eruption, a resurgent block of this tuff was uplifted to form La Garita Mountain, which today stands over 4,179 meters tall. The hardened ash from the eruption is also visible at the Wheeler Geologic Area, where erosion has sculpted it into spires and towers.