Yellowstone's hydrothermals

I haven’t posted on this blog for a year (!) and I have a number of trips to get caught up on, starting with a fun week in Wyoming at Yellowstone and Grand Tetons during late summer of last year.

Hydrothermal feature called "The Fisher"
at the edge of Yellowstone Lake. 

Yellowstone is the oldest national park in the world and I’m sure visitors arrive in this northwest corner of Wyoming for many reasons. For me, the hydrothermal features were the most exciting part of my first visit to this iconic park last summer.

The park’s hydrothermal features owe their existence to a geologic hotspot that lies under Yellowstone. Like the hotspots under the Hawaiian Islands, Iceland, and other locations across the planet, geologic hotspots are places where hot magma rises closer to the Earth’s surface than elsewhere. The volcanic intrusions into the crust heat groundwater that then rises to the land surface in a variety of forms.

One geologic signature of hotspots that I find fascinating is the volcanic traces they leave over the surface of the Earth over geologic time. As the tectonic plates comprising the crust move over the mantle (in the case of Yellowstone it is movement of the North American Plate), the hotspot remains relatively fixed below the moving plates and a “trail” of volcanic activity develops at the land surface over millions of years. This phenomenon is very easily observed with the island chain of Hawaii that formed as the Pacific Plate has gradually moved to the northwest over the Hawaiian hotspot. The trail of evidence is the string of Hawaiian Islands and Emperor seamounts across the north Pacific. In the case of Yellowstone, the history of volcanic activity in that region over the last 15 million years or so can be seen as a series of surface volcanic features that stretches from northwest Wyomingto southeast Oregon.

The Yellowstone caldera and hydrothermal features inside the national park boundaries. Base map from NPS.

Yellowstone has experienced volcanic eruptions a few times over the last several million years, but today volcanic activity in the park is just manifest as earthquakes and numerous hydrothermal features. Four types of hydrothermal phenomena are present in the park. Each is fueled by heat from below the surface, but all involve water at different temperatures and in different quantities. Mud pots consist of little basins of heated mud of different consistencies at the ground surface. The mud inside the pot is formed when acids dissolve rocks. As steam rises through the mud, it gurgles or bubbles at the ground surface. In Yellowstone, I was able to view mud pots at the Artist’s mud pots area southeast of the Grand Canyon of the Yellowstone.

Four types of hydrothermal features at Yellowstone. By CNJ, after NPS display.

Mud plots.

Steam vents, or fumaroles, are a second type of hydrothermal feature. They release super heated water vapor through sub-surface vents. Fumaroles can be small, just quietly releasing a steady stream of steam.

A fumarole near the Artists' Paintpots.

Castle Geyser erupting.

Geysers have subsurface reservoirs that fill with heated water which is periodically ejected violently through an opening in the ground. Geysers can erupt with predictable periodicity or can have irregular timing. Yellowstone has the greatest concentration of geysers anywhere in the world. They are diverse in terms of eruption height and periodicity.

Old Faithful is among the class of regularly-erupting geysers, though it is not the tallest geyser in Yellowstone. It erupts approximately every 70 minutes, and I saw several eruptions during the few days we were in the park. For me, a more impressive geyser was Castle Geyser which only erupts about every 12 hours, but for an impressive 20-30 minutes at a time. I was fortunate to catch one of its episodes. Both Old Faithful and Castle Geyser are in the Upper Geyser Basin where there are a wide variety of interesting hydrothermal features.

Hot springs are the final type of hydrothermal feature. At hot springs, heated water forms pools at the ground surface. They can be rather quiescent or quite active like the Beach Spring which periodically alternates between calm conditions and a vigorous flush of bubbles that rise to the surface of the pool that lasts for a minute or two. 

Examples of hot springs in Biscuit Basin. Shell Spring (left) and Mustard Spring (right).

Hot springs are often lined with precipitated minerals and microbial assemblages, lending them a variety of colors. Blue hues are due to the reflection of other colors of light from the pool. Yellow colors are due to precipitated sulfur compounds. Photosynthetic cyanobacteria and other algae may lend green colors to the water. Bacterial mats at the edges of pools may be white, black, or reddish in color.

Many of the more attractive hot spring pools have two or three colors, but the chromatic display at Yellowstone is most brilliant at Grand Prismatic Spring. This spring is so large it looks like a small lake. Lying across a large flat muddy treeless expanse, steam billows from the superheated spring. When the steam clears, the Grand Prismatic has a beautiful spectrum of blue, green, yellow, and orange.

Grand Prismatic Spring as seen from a trail on the west side of the lake (left) and from up close (right).

Boardwalks facilitate an up-close view of the Grand Prismatic Spring from the east side at ground level, but a short trail that climbs a nearby hillside also leads to a view from above. Since we were backcountry camping for two nights in a forest just a short distance from Grand Prismatic Spring, we viewed it on several occasions and from different angles. In the cool mornings, the brilliant colors of the pool were generally obscured by a large cloud of steam perpetually rising from the spring. However, later in the day as air temperatures warmed, there was less steam to block the rainbow of colors.

Our last of several visits to Grand Prismatic also held an unusual surprise. While we were out on the farthest boardwalk near the edge of the spring, a bison crossed over the boardwalk behind us and into the mudflat to the east of the spring. It seemed unfazed by either the runoff from the spring (which I presume was rather warm), or the eager tourists eyeing the huge animal. In no rush, it seemed unsure of where to go next. We left before learning of the resolution of that event. Curiously I had earlier seen some animal prints in the soil close to the spring, and this surprise visit confirmed that bison do wander quite close to the hydrothermal features from time to time. 

Several examples of smaller springs in Upper Geyser Basin.

The Sapphire Pool in Biscuit Basin.