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November 15, 2021

Many of Yellowstone’s hot springs, geysers, mud pots, and fumaroles look different depending on the season, year, or sometimes even the day one visits. Colloidal Pool, in Norris Geyser Basin, is an interesting example of a feature that changed over the course of summer 2021.


Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Lauren Harrison, postdoctoral researcher with the U.S. Geological Survey.

Map of Porcelain Basin and images of colloidal pool over time.
Comparison of (a) 1904 Historical map with (b) 1988 USGS map. Colloidal Pool is a large, labeled pool roughly located on a straight line between Hurricane vent and Whirligig Geyser on the 1988 map (b); this same transect on the 1904 map (a) shows no feature at that location (white circle). Note: maps are not at the same scale or orientation. Panels c and d show photos of Colloidal Pool’s variations in water appearance from the Yellowstone Flickr page. (c) Teal-blue water without foam (photo taken August 27, 2009, by Greg L. Jones). (d) Foamy conditions with opaque blue-brown water (photo taken August 4, 2019 by Joe Shlabotnik).

Colloidal Pool is a thermal feature in the Porcelain subbasin of Norris Geyser Basin, located just down the hill from Norris Museum. A 1904 historic map does not show a feature where Colloidal Pool now exists, so it must have appeared and substantially increased in size over the last ~120 years! Although Colloidal Pool is now a large shallow acid pool, usually with “opalescent bright blue” water, in the past the feature had spectacular geyser activity characterized by eruption of muddy water as high as 30 meters. The National Park Service, tasked with monitoring the >10,000 thermal features in Yellowstone by the Geothermal Seam Act of 1970, has measured the pH, temperature, and electrical conductivity (a measure of water salinity) of Colloidal Pool in 1998, 2018, and 2021. The water temperature and characteristics changed in 2021, including significant cooling and the appearance of foam on the surface of the pool. To investigate the source of these changes, YVO scientists took samples of Colloidal Pool water and solids during a field visit in June 2021.

The collected samples indicate that Colloidal Pool water had high concentrations of sulfate and lower concentrations of chloride, alkalis, silica, and trace metals compared to typical neutral-chloride thermal waters, like those from Old Faithful. The high sulfate concentrations come from the flux of H2S gas through the pool, which oxidizes to H2SO4 in contact with the pool’s water and causes the low pH. This tells us that in June 2021 Colloidal Pool water has no direct input of thermal water (which is indicated by the low chloride concentrations), but it does have a significant flux of geothermal gasses through the pool. The higher temperatures and electrical conductivities observed by measurements in 1998 and 2018 suggest that prior thermal water input did exist into Colloidal Pool but had ceased in 2021. This shift in water chemistry at a thermal feature is not uncommon and may be a result of the interaction between the level of the water table, amount of boiling of deep geothermal waters, subsurface fluid flow paths (controlled by precipitation of hydrothermal minerals and seismicity), and variable mixing with meteoric or other water types. For example, following the 1959 Hebgen Lake earthquakeOpal Spring changed from a discharging alkaline-chloride spring to acid-sulfate water.

Table 1: Measurements of Colloidal Pool  
Date Temp (°C) pH Conductivity (μS/cm)
26-Jun-1998 67.1 2.57 2200
22-Aug-2018 73.7 3.84 2520
16-Jun-2021 45 2.64 1810
High powered microscopic images showing clay particles and diatoms
Scanning electron microscopy (SEM) images of the Colloidal Pool colloids (images are a combination of backscatter and secondary electrons). The colloids are a mixture of clay particles, hydrated silica, alunite, and diatoms. (a) Image shows how hydrated silica (the whispy, lighter colored precipitate) uses larger particles, such as dust, clay, or things like pine needles that may fall into the pool, as substrates for precipitation. (b) Magnified image of hydrated silica precipitations on clay particles. (c) Highly magnified image of a diatom showing detailed shell structure. (d) Image that shows the platy habit of clay particles and several diatoms. Scale bars are on each image.

The colloids in Colloidal Pool were investigated using a high-powered scanning electron microscope (SEM) that can produce detailed images of very small particles and provide information on their chemical composition. The colloids sampled in 2021 fall into four main categories: 1) hydrated silica, 2) clay particles, 3) sulfur-rich particles, and 4) diatoms. Hydrated silica can precipitate at low pH, as observed in Colloidal Pool, although it typically does not form sinter deposits (like those that make up geyser cones) unless the pH of thermal waters is higher. Clays are typical of acid-sulfate conditions and form from the reaction of high-temperature acidic thermal waters with rhyolitic volcanic rocks (here the underlying Lava Creek Tuff). Alunite, a sulfur-rich mineral with the formula KAl3(SO4)2(OH)6, has been observed in many Yellowstone acid-sulfate thermal features and associated clay deposits, and it forms because of the high sulfate concentrations. Diatoms are microscopic aquatic organisms found in almost all surficial Earth waters from the ocean to glacial meltwater streams to nearly boiling geothermal pools. The diatoms found in Colloidal Pool belong to the genera Eunotia, which grow in acidic to near-neutral waters (pH 3-6) in shallow-water habitats. Diatom populations have also been documented in nearby Beowolf Spring (pH < 3 and at temperatures >50°C) in the 100 Spring Plain subbasin of Norris Geyser Basin.

As is common for many thermal features around Norris Geyser Basin, Colloidal Pool had rapid and noticeable changes in activity, pool color, and chemistry in 2021. The decrease in water temperature and electrical conductivity indicates a decrease in the flux of thermal water flowing into the pool. This could be due to below average precipitation, which can result in lowering of the water table. This, in turn, could lead to more boiling of subsurface geothermal water. Another possible explanation is that precipitation of hydrothermal minerals in the subsurface caused the closure or shrinking of gas and water pathways, which throttled down the thermal water flow to Colloidal Pool.

A hallmark of geothermal features in Yellowstone is how they change on timescales ranging from hours to years. Exploring why these changes occur, the timescales associated with different transformations, and the local and regional extent of variations are key data for scientists who study Yellowstone and other hydrothermal areas. This information is used to ensure visitor safety, assess Yellowstone hazards, and understand large-scale volcanic hydrothermal systems. And we all can do our part—next time you visit Norris Geyser Basin, stop by Colloidal Pool and observe whether the water is “opalescent bright blue” from thermal water input or if it is “brown and covered in foam” as it was in June 2021. Your observations can be valuable input to better understanding the timescales and causes of changes to Yellowstone features!

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