Tucked away in the San Juan Mountains, Rainbow Hot Springs is a geologist’s dream

The La Garita caldera near Creede, Colorado formed about 27 million years ago from an eruption believed to be one of the earth’s largest volcanic events. The oblong caldera perimeter measures an impressive 22 by 47 miles. Some of the erupted ash, lava and volcanic tuff went south toward Wolf Creek Pass and Pagosa Springs. An unusual rock formation having the characteristics of both lava and welded tuff formed, called Pagosa Peak dacite. This rock permeates the landscape around Wolf Creek Pass and Sheep Mountain by the west fork of the San Juan River.

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The Rainbow hot spring.Robert Stump

The West Fork trail heads north along the west fork of the San Juan River into the Weminuche Wilderness where evidence of the volcano’s activity abounds. The river slices through the layers of volcanic rock to reveal distinctly different characteristics corresponding to various stages of the eruption cycle. It also creates great opportunities for waterfalls that can be spotted all along the river valley.

The southernmost extent of the La Garita caldera lies a few miles north of the trailhead where the landscape reveals more interesting rock units in the caldera’s interior like Fish Canyon tuff and calc‑alkaline lavas. Around five miles north of the trailhead, another special feature exists thanks to the volcano: Rainbow Hot Springs.

Though the volcano is long dormant, magma that fueled the explosive eruption remains hot near the earth’s crust. In turn, heat conducts through rocks near the surface that heats the water emerging as hot springs. Surfacing hot springs are not common in the La Garita caldera despite the plentitude of hot rock near the surface. Geologic and hydrologic conditions must line up perfectly to create a hot spring as enjoyable as Rainbow Hot Springs.

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Fireweed (Chamerion angustifolium) along the west fork of the San Juan River.Robert Stump

The overlying, colder rock must erode enough for water to come into contact with the hotter rock underneath. We can thank the river for that. The overlying rock must contain sufficient interconnected faults for water to permeate to the warmer depths, then heat and rise back to the surface. We can thank the unique, extended eruption cycle for that, which jostled the normally impermeable rock to create fractures. 

Finally, there must be water. A caldera forms as the ground collapses into itself after the volcano explosively ejects its contents. There may have been a lake in the southern part of the caldera at one time that finally started cutting its escape to the south which became the west fork of the San Juan River. Glacial activity likely helped this along. Seasonal precipitation and melting snow in the higher mountains surrounding the caldera feed water into the river, following millennia-old paths cut over and through rock. Some of that water finds the exact route that brings it perfectly in contact with the hot rock and back to the surface.

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A plunge pool from a waterfall on Rainbow Creek.Robert Stump

A stroll in the area reveals several small hot springs emerging from the rock with cold surface springs only a few feet away. But the most notable of the springs is the Rainbow hot spring that emerges from a large crack in the rock and cascades down the surface. The micro-climate of the hot spring created by the constant flow of warm water over the rock allows algae to grow with a myriad of colors — hence the name of the hot spring.

A massive wildfire caused by lightning burned about 20,000 acres of the west fork in June and July of 2013. It was part of the West Fork fire complex comprising three wildfires that burned more than 100,000 acres.Yet, the hot spring remained relatively unscathed despite the massive loss of vegetation in the area. Surprisingly, sediment transported by floods due to the lack of vegetation never plugged or buried the hot spring, and massive amounts of trees carried into the valley bottom never made the spring inaccessible. We can thank sheer luck for that.

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(June 2013) Smoke from the West Fork fire complex at sunset.Robert Stump

So, the hot springs are still accessible today and remain a popular destination outside of the winter months. If you visit, be sure to also check out the many beautiful waterfalls in the area, some of which are as elusive as the hot springs. Keep your eyes open for wildlife as well.  Several herds of bighorn sheep roam the cliffs along the canyon. You will find plenty of mule deer up there as well. The deer and other critters seem to be attracted to the mineral-rich rocks exposed in certain layers, as you can see them licking and chomping at the rock.

Where exactly are the hot springs, you ask? The thing about hot springs like these, being so special as they are, is that they are meant to be discovered. Discovery is part of the adventure!

ROBERT STUMP is no geologist but finds geology fascinating. It’s just another item in the ol’ adventure bag!