Sunday, August 19, 2012

The Abandoned Lands...A Journey Through the Colorado Plateau: Calderas and Inside-Out Caverns

Our journey through the "Abandoned Lands" of the Colorado continued over the flank of one of the larger geological features of New Mexico, the Valles (or Jemez) Caldera, a huge volcanic edifice whose summit collapsed during a catastrophic rhyolite ash eruption about 1.15 million years ago. The volcanic center produced the incredible scenery that we observed at Kasha-Katuwe Tent Rocks National Monument, and the setting for the strange ruins we saw at Bandelier National Monument. It was a highly unusual sight for us after a week on the road in the southwest in that it was also lush and green. I guess it helps that we were at 9,000 feet, so that even in a drought year there had been a bit more precipitation.

The highway skirts the outer edge of the twelve-mile wide caldera so there is no one place that one can see the full scale of the feature. The meadows in the picture above are only 2 miles across.We'll have to depend on radar imagery to get a full sense of the shape of things (below).
Image of the Valles Caldera courtesy of the New Mexico Bureau of Geology and Mineral Resources.
The eruptive activity of the caldera coated thousands of square miles with ash, and being an eruption that was hundreds of times larger than St. Helens, destroyed all life over a vast region. At first the  caldera basin contained a lake. Eventually the edge of the caldera was breached, the lake drained, and Jemez Creek eroded the deep gorge of San Diego Canyon. Numerous other smaller canyons have been carved into the margins of the volcano as well, including Frijoles Creek which we explored in Bandelier.

It has been around 60,000 years since the last volcanic activity at the caldera, but magma still simmers in the heart of the volcano. Heat flow under the caldera is high, and hot springs can be found around the margins. We found one of them while driving down San Diego Canyon, something so out of place that I hit the brakes and came to an unexpected and unannounced stop (to the chagrin of the five vans following me...). It has to do with the "inside-out" cavern of today's post title. We had happened upon Soda Dam, an outstanding example of a travertine bridge.
Caves usually form because limestone layers are dissolved as acidic groundwater moves through fractures and crevices. The familiar speleothems (cavern features) like stalactites, stalagmites and flowstone result later on when carbonate-rich water drips into the cavern and a bit of evaporation causes the calcium carbonate (called travertine) to precipitate.  In this case, hot acidic water has certainly dissolved limestone, but faulting forced the hot carbonate-rich water to the surface at springs. Instead of filling caves with features, the features form on the surface of the Earth instead. An inside-out cave.
The travertine built up around the springs and the deposits ultimately built up a dam-like feature that crossed the canyon floor, forming a natural bridge. The travertine bridge has numerous cave-like features, including flowstone and rimstone pools. I suspect it once had stalactites and stalagmites in the small cave openings, but the site is too accessible from the highway for such features to survive the inevitable vandalism.

The hot springs have been active for nearly a million years, as evidenced by travertine deposits high on the canyon walls. Other deposits nearly the valley floor are around 60,000 years old, giving an idea of how rapidly San Diego Canyon was carved. The Soda Dam itself is about 7,000 years old.
The springs flow at a small fraction of their original volume. The dam was breached during highway construction so that the main flow of the springs no longer reaches the travertine bridge, other than a small trickle. The travertine is disintegrating, both naturally and from vandalism.
Jemez Creek forms a nice waterfall at the Soda Dam, and a sumptuous-looking swimming hole can be found at the base.

A nice source on Soda Dam and the many other geologic features of northern New Mexico can be found in The Geology of Northern New Mexico's Parks, Monuments, and Public Lands from the New Mexico Bureau of Geology and Mineral Resources.
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