When you think of Colorado’s diverse geography, you might picture the iconic Rocky Mountains or the equally picturesque Eastern Plains—but likely not a tempestuous volcanic landscape.

Yet, one of the Centennial State’s most notable geological achievements is the world’s largest known volcanic eruption, which happened about 28 million years ago in the San Juan Mountains. The depression left behind, called La Garita Caldera, is about 22 miles wide and 62 miles long—large enough to be seen from space and close to the size of Mineral County, where much of it is located.

A caldera eruption isn’t like the volcanic events we know today. It occurs when pent-up pressure in a large magma chamber suddenly overpowers the strength of the surrounding rocks, causing the entire chamber to empty—an explosion—in a matter of hours or days. Once emptied, the remaining void can no longer support the mountain’s weight, so it collapses, leaving behind a huge, circular depression known as a caldera.

La Garita’s eruption was so impressive that it can’t even be ranked on the modern Volcanic Explosivity Index (VEI). This ranking system, created by the U.S. Geological Survey, is largely based on the volume of material ejected, which geologists estimate by mapping the extent of volcanic rock outcrops in the field. To put it in perspective, the infamous 1980 eruption of Washington’s Mount St. Helens, which ejected about 0.25 cubic miles of material, barely rated a five on the VEI. The eruption that caused La Garita to form discharged more than 1,200 cubic miles of material—so much that volcanologists have suggested that the explosivity index, which increases by a factor of 10 with each whole-number step, needs to be expanded to accommodate its estimated 9.2 ranking.

In the vast sweeps of time since La Garita was formed, ice, water, and wind have sculpted some of the ash layers into a fairyland of pinnacles and spires, which reach their most impressive form in the Wheeler Geologic Area about 10 miles northeast of the town of Creede (which, coincidently, also boasts the world’s largest fork). They’re stunning to behold, a simple reminder why the San Juans’ breathtaking scenery is truly off the charts.

Terri Cook
Terri Cook
Terri Cook is an award-winning freelance writer based in Boulder. More of her work can be found at down2earthscience.com.