A landscape of spectacular rock formations around Colorado hint at the region’s turbulent volcanic past. While only one volcano in the state is considered active today, visitors of the San Juan mountains can see evidence of ancient eruptions.

The La Garita Caldera is an enormous depression in the topography of the Gunnison and Rio Grande National Forests. A caldera is formed when the contents of a volcano have been ejected and the earth collapses into the empty magma chamber.

The La Garita caldera is an astonishing 22 miles wide and 62 miles long. By some accounts, this was formed by the largest known volcanic eruption on the planet to date.

The Centennial State explosion happened approximately 27 million years ago–so large that it exceeded the highest rating on the Volcano Explosivity Index (VEI). Colorado College geology professor Emily Pope says this can be explained by the underlying chemistry of the lava.

“The volcanoes that we see erupting in Hawaii–they are formed from a lava that is very rich in iron and magnesium and very poor in silicon,” explains Pope. “And that causes the lava to have a really low viscosity.”

That kind of thin, easy-flowing lava is described as “mafic.” When it erupts out of a volcano, it looks more like an overflowing carbonated drink than an explosion. This kind of volcano is still quite dangerous, but their eruptions are far smaller than the kind that produced Colorado’s enormous caldera.

“If you have a really silicon-rich volcano [like the one that created the La Garita caldera], the lava behaves more like yogurt,” explains Pope. Thick, slow-flowing lava like this is described as “felsic”–and it makes for an entirely different level of explosion.

“It doesn’t come out easily,” says Pope. “So you build up a lot of pressure in the volcano until you reach the point where the built-up gases can’t be held anymore.”

The breaking point brings a catastrophically large eruption. Relatively felsic lava is what caused Mount Vesuvius to destroy Pompeii in 79 C.E. and Mount St. Helens to be the most destructive volcano in U.S. History.

“The 1980 eruption of Mount St. Helens produced a big ash cloud, it coated the whole state,” says Pope. “The La Garita volcano eruption was around 5,000 times bigger than that.”

Yellowstone’s “supervolcano” is also quite felsic, according to Pope. Its last eruption, around 600,000 years ago, was rated an 8 on the VEI–the highest official value on the scale. However, nobody is expecting super-volcanic activity in the region in the foreseeable future. The chances of Yellowstone erupting again soon are very low, and the volcanic activity behind the La Garita Caldera is now considered extinct.

Experience the site:

Visit the La Garita Wilderness to see fascinating geological formations left over from the region’s volcanic history.

Driving along CO 149 between South Fork and Creede, the caldera is evident. High walls rise up on either side with a dome-like shape forming in the middle.

Explore the La Garita Wilderness’s Penitente Canyon trail system and the Fish Canyon Tuff.

There’s also the Wheeler Geologic Area, which is home to a sprawl of otherworldly rock spires. The area is fairly remote: access via either a 7-mile hike or a rugged 14-mile road recommended for high-clearance 4WD or ATVs only.

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