The San Juan Mountains do not look like volcanoes. There are no cones, no craters, only a tangle of high peaks and deep valleys. But almost all of the range is made of volcanic rock, the debris of a volcanic field that erupted again and again between roughly 35 and 23 million years ago, and at least fifteen times it did so on a scale that left calderas, great collapsed basins, behind.
Calderas
A caldera forms when an eruption empties the magma chamber beneath a volcano so fast that the ground above it collapses into the void. In the San Juans the eruptions were explosive: clouds of ash and pumice so hot and heavy they flowed across the country and welded into rock where they settled, a kind of rock called tuff. Silverton and Lake City both sit inside old calderas, and the cliffs and basins around them are their walls and floors.
La Garita
The largest of them lies northeast of the range's center, around the town of Creede. About 27.8 million years ago the La Garita caldera erupted roughly 1,200 cubic miles of material, some 5,000 cubic kilometers, in what is counted among the largest explosive eruptions known anywhere on Earth. Mount St. Helens in 1980 put out about a quarter of a cubic mile. The ash-flow deposit left behind, the Fish Canyon Tuff, covered something like 11,000 square miles, in places hundreds of feet thick, and finer layers of its ash have been traced as far as Kansas and Nebraska. The collapsed basin it left is about 22 miles wide and some 60 miles long, much of it since worn down and refilled by later eruptions.
What the volcanoes made
Volcanism went on for millions of years after La Garita, building smaller calderas within and beside it, including the one around Creede. Hot water circulating through the fractured rock of the caldera rims carried silver, gold, lead and zinc and laid them down in veins, and those veins are why the San Juans became mining country: the Creede boom of the early 1890s, and the ores of Silverton, Ouray and the Red Mountain Mining District, all owe their existence to the volcanoes.
Where to see it
The Fish Canyon Tuff is easy to reach on the San Luis Valley side, where it forms the rounded rock outcrops at Penitente Canyon and Elephant Rocks near Del Norte. The best-known display is the Wheeler Geologic Area in the La Garita Wilderness north of Creede, where erosion has carved volcanic tuff into a maze of pale spires and hoodoos. It was made a national monument in 1908 and so few people managed to get there that Congress abolished the monument in 1950; reaching it still means a long hike or ride, or a rough four-wheel-drive road.