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.

We do not define the habitat of wolves just by observing where a wolf pack may be standing at any given moment; they move around.  Similarly, you can think of a thermal area as the “habitat” of individual thermal features, like geysers and hot springs.  Hot fluids also move around (underground), so just because a patch of ground in Yellowstone is either warm or cool doesn’t mean it will stay that way.

But here’s the rub: it can take as little as months to a few years for a new thermal area to appear and grow, and as little as days to weeks for a previously cool area to heat up to boiling temperatures.  But it takes years to decades for a previously active thermal area to cool off and be reclaimed by nature.  In other words, inactive thermal areas still look like active thermal areas for a long time. 

As a reminder, a thermal area is a portion of ground that has one or more thermal features that is mostly made up of hydrothermally altered ground and/or hydrothermal mineral deposits.  They also tend to be mostly barren of vegetation.  These are the visually obvious signs. 

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Active thermal areas also emit significant geothermal heat and magmatic gases, like CO₂, H₂S, and H₂O vapor.  Aside from steam that can sometimes be seen condensing above active thermal features, heat and gas emissions are generally not visually obvious signs - because heat and gases are not visible to our eyes.  But we can build specialized instruments to detect and measure heat and gasses being emitted.

Thermal areas are the surface expression of the geothermal heat that flows up from the deeper magmatic system.  So, how much of Yellowstone National Park’s total area (8900 km²) is actually a thermal area?  Only about 67 km² or 0.75%!  If you closed your eyes and threw a dart at a map of Yellowstone, it is very unlikely that you’d hit a thermal area. 

So, what portion of the total area of all the thermal areas (67 km²) is currently thermally active, that is, actively radiating significant geothermal heat into the atmosphere above?  That is a harder question to answer. 

First, Yellowstone’s thermal areas are spread out over a large and mostly inaccessible area.  The only way to routinely map, measure, and monitor all the Park’s thermal areas is with satellite-based thermal infrared remote sensing.  But the trade-off is that with thermal infrared sensors that can cover the entire Park, the moderate spatial resolution (90- to 100-m pixels) is not sufficient to detect and measure emitted radiance from thermal areas that are either too small or not hot enough.  In other words, there are some thermally active areas that are not fully accounted for in the analyses of satellite thermal infrared data.  

In addition, some areas may be thermally active, but their surface temperatures and thermal flux remain unknown.  This is because any above-background thermal emission is too subtle to be measured with currently available satellite sensors, and they haven’t yet been investigated with high-resolution airborne data or field observations.

But there are a few mapped thermal areas that we know are not thermally active.  Brimstone Basin, for example, is an acid-sulfate thermal area near the southeast arm of Yellowstone Lake.  It is characterized by bright, heavily altered ground and hydrothermal mineral deposits and is mostly barren of vegetation.  But it does not have any active thermal features and shows no discernible surface thermal activity.  It is an example of a place that was once warm but is no longer.  Interestingly, it still has substantial CO₂ and H₂S emissions that are comparable in magnitude to those in warm acid-sulfate areas. 

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To further complicate the picture, most thermally active areas contain some zones that are not warmer than the background.  Anyone who has explored Yellowstone’s geyser basins, has experienced this firsthand!  In general, about 10% to 20% of Yellowstone’s 67 km² of thermal area is not thermally emissive above background. 

Because active thermal features are spectacular and captivating, it often makes the news when significant changes are observed, like the new thermal area discovered near Tern Lake in 2018 or the increase in thermal activity at Norris Geyser Basin in 2003.  Understandably, it doesn’t make the news when they cool off.  This is because this type of change is harder to recognize and takes longer to happen, and also because it’s not necessarily a spectacular thing to witness.  But it happens and it’s important to recognize this, too.  Understanding the way thermal areas evolve in time and space helps to preserve these spectacular natural features and also to keep people safe around them.