What is tufa?


Good morning. We're standing immediately in front of a display spring or what we call the cascade in Hot Springs National Park. My name is Steve Rudd. I'm the Natural Resources Program Manager here. One of the interesting rock formations that you'll find in geothermal parks is similar or the same as what you see behind me and we call it tufa. Now what is tufa and what sets it apart from say sandstone or limestone? Tufa is an evaporite, or at least that's what geologists call it. The reason is simple: it wouldn't be here if it weren't for the thermal water. As the water passes through the parent rock thousands of feet below, because it's hot, it is able to pick up a lot more in the way of minerals and other chemical compounds as it passes through. Those then become part of a solution, or in some cases a suspension, of carbonates. As the water makes it to the surface, and it does right here in the midst of the geothermal spring field, and is exposed to the atmosphere and air, then the water component simply begins to evaporate into the atmosphere. It leaves the other elements behind in the form of the other deposits you're seeing here. As I said, we call it tufa. If you visit Yellowstone National Park and go look at the terraced hot springs close to Mammoth at the northern end of the park, you'll see a slightly different kind of evaporite, but it's still all very chemically similar. Those are called travertines and they derive all their various color banding from the ores that are oxidizing in the open air and sunlight. Here the color tends to be a more drab gray just because of the nature of the rock that it's coming from at great depth.

Last updated: April 10, 2015

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