Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from R. Greg Vaughan, research scientist with the U.S. Geological Survey.

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In Yellowstone, thermal areas are generally located along geologic structures, such as faults and fracture zones, the boundaries of intra-caldera rhyolite flows, and along the edges of the 2 resurgent domes: Sour Creek and Mallard Lake.

To clarify, a thermal area is a nearly continuous geologic unit that contains one or more thermal features (e.g., hot springs, geysers, or fumaroles), hydrothermally altered rocks or hydrothermal mineral deposits, heated ground and/or gas emissions, and is generally barren of vegetation or has stressed/dying vegetation.

Satellite-based thermal-infrared remote sensing with moderate spatial resolution (with a pixel size of ~90-meters, or ~300-feet) can be used to detect and characterize thermal areas that are large enough and/or warm enough to stand out above the background.  These data have been remarkably useful for assessing and updating maps of the park’s thermal areas.  But some thermal areas are too small in size or subtle in heat output to be clearly detected with these satellite thermal-infrared images.  Of course, high-spatial-resolution airborne thermal-infrared imagery could detect these subtle thermal areas, but due to their expense and logistical complications such data are not acquired regularly nor are they available for the entire park.

Visible signs of thermal areas include no vegetation or vegetation stress/mortality, bright hydrothermal mineral deposits, snow-free zones in winter, steaming, and bubbling/boiling water.  We do have high-spatial-resolution (with a pixel size of ~1 meter, or 3 feet) visible remote-sensing data covering the park that are regularly acquired by the National Agriculture Imagery Program (NAIP) and also from commercial satellite companies, like Maxar.  But even the visible signs of thermal areas are not always obvious.

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On the North side of the Mallard Lake resurgent dome, about 2.5 km (1.5 mi) southeast of Lower Geyser Basin’s Firehole Lake, there are some thermal areas that had not been detected and mapped until recently—and to our knowledge, these have not yet been visited in the field.  How did we miss these?

Well, first of all, this is a backcountry area not accessible by any trails.  Secondly, this is an area that was largely burned in the Yellowstone fires of 1988.  In the decades since the fires, the land has recovered and new trees have returned to this region, but there are still large areas of barren rock and soil that look bright but that are not thermal areas.  Many of the visible signs of these thermal areas are thus masked by the by the barren rocks of this fire-scarred landscape.  Even zooming in with high-spatial-resolution airborne images, hydrothermal mineral deposits are hard to notice.  In other words, the region does not look like a typically conspicuous thermal area. 

In addition, these thermal areas are scattered, with many small, isolated zones that are too subtle to stand out above the background in satellite thermal-infrared imagery.  So, how did we find them?  It’s all thanks to snow!

These thermal areas are warm enough to prevent snow from accumulating on them—and if you look at a high-spatial-resolution image of the area, acquired in the winter with snow on the ground, the warm zones are evident.

So, how do we know these are not newly emerging thermal areas?  Because archived Landsat satellite data going back to 1975 (before the 1988 fires) show this area to be a healthy lodgepole forest with numerous zones of no/stressed vegetation that align almost perfectly with the areas mapped based on recent high-resolution visible images of snow-free zones.  

Our future understanding and characterization of this newly recognized thermal area, as well as others in Yellowstone, will evolve as we continue to analyze and interpret both archived and recent remote-sensing data, combined with field work to accurately measure surface and subsurface temperatures, catalog individual thermal features, and sample emitted gases and thermal waters.  We hope to begin work on these subtle and interesting thermal areas on the Mallard Lake resurgent dome in the coming summer!

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