Science Spotlight: Geothermal Colors

by Leandra Xochitl Marshall, published 27 June 2022
Grand Prismatic Spring. Photo Credit: United States Geological Survey.
Grand Prismatic Spring. Photo Credit: United States Geological Survey.
Volcanic processes, where vapors and water heated by magma rise to the surface, can create striking geothermic landforms such as fumarolic fields, hydrothermal or geothermal springs, acidic brines, salt chimneys, pillars, terraces, and pools.

These geothermal features, which are studied by scientists seeking to understand the mechanics behind volcanic processes, exhibit remarkable spectrums of color. What causes the various colors present in geothermal landscapes?

The light properties of geothermal pools

Morning Glory Pool. Photo Credit: Leandra Xochitl Marshall.
Morning Glory Pool. Photo Credit: Leandra Xochitl Marshall.
The colors of geothermal pools are most affected by the optics, or light properties, of water and suspended particles. The colors reflected by the pools are dependent on the wavelengths of light absorption and scattering, which are called wavelength-selective absorption and wavelength-dependent scattering. Light scattering potential increases as water depth increases. Optical scattering by suspended particulates is a dominant color-producing mechanism.

Turbidity, or the amount of suspended particulates in the water, increases the optical scattering and absorption of thermal waters. The grain size of these particulates also affects light scattering and absorption. Water composition contributes to turbidity and existing optical properties, such as pH, salinity, sulfur content, minerals, and heavy metals.

Clear water pools, with lower turbidity, have reduced scattering and absorption that allow observed colors to be reflected by the bottom material of the pool. The color in these pools is explained by a combination of optical properties of the water and the spectral reflectance of the bottom material. These pools are generally a clear blue, and of neutral-chloride or acid sulfate-chloride composition. Any present particles, such as aqueous silica, are generally dissolved rather than suspended.

High turbidity pools, with larger amounts of light scattering particles, can come in a wide variety of colors, such as cloudy grays, yellows, greens, blues and more. For example, Japanese scientists determined that cloudy blue water is caused by neutral-chloride or acid sulfate-chloride composition water with larger suspended white particulate amorphous silica or aqueous silica. In Lake Onneto, Japan, the blue, green, and greenish-yellow colors are created by color-producing agents in the water, the optical properties of the water, and yellowish bottom sediments.

In Africa, scientists determined that Dallol volcano's Black Lake is colored by magnesium chloride particles, the Yellow Lake by calcium chloride and magnesium/potassium chloride particles, and the White and Pink Lakes by sodium chloride particles from halite dissolution.

The composition of geothermal deposits

Hydrothermal pools and terraces in Dallol, Ethiopia. Photo Credit: Electra Kotopoulou, Wikipedia.
Hydrothermal pools and terraces in Dallol, Ethiopia. Photo Credit: Electra Kotopoulou, Wikipedia.
Geothermal features are also colored by mineral deposits, which are formed from the solids present in volcanic gas and fluid mixtures. For example, the Kawah Ijen volcano in Indonesia is known for its yellow sulfur deposits and fumaroles, which produce molten sulfur that burns blue at night. The color palette present in Dallol, Ethiopia, is created by inorganic geologic processes. The volcano was formed by basaltic magma intruded into marine geologic sequence. Magmatic fluids rich in acidic gasses and chlorides boiled the meteoric (atmospheric) water and seawater trapped in the marine sequence, leading to the generation of a hydrothermal system containing hyperacidic springs. The changing colors of the geothermal deposits in this area, ranging from white to green and yellow to red, are caused by oxidized iron. The iron is derived from the hot acidic fluids dissolving the underlying basalt and marine sediments.

Many other deposits are produced from Dallol's acidic brines. When fluids evaporate, any solids left remaining in the mixture form precipitates and evaporites, such as halite (table salt), anhydrite, gypsum, and potash. These precipitates and evaporites can come in any color imaginable, depending on the composition of the mixture. White, gray, tan, brown, and black deposits constitute precipitates, evaporites, carbonates, calcium, silica, hydrocarbons, and other metals. Greens, yellows, oranges, and reds can also indicate precipitates, salts, evaporites, oxides, different types of iron, arsenic, copper, lead, sulfur, other metals, and chlorides.

Microorganisms

Grand Prismatic Spring Microbial Mat. Photo Credit: Leandra Xochitl Marshall.
Grand Prismatic Spring Microbial Mat. Photo Credit: Leandra Xochitl Marshall.
Microorganisms, such as thermophilic cyanobacteria (blue-green algae), bacteria, and archaea, also play a role in the colors displayed by geothermal pools. These microorganisms have been discovered in geothermal areas up to 121 degrees C. In Yellowstone National Park, United States, microbial mats surround many geothermal features in the volcanic caldera. Green, orange, brown, and yellow are evidence of photosynthesis in cyanobacteria, which provide thermophiles with energy and protection from solar irradiance.

Microbe color is sometimes also correlated to temperature. However, the color patterns expressed by microbes are not necessarily the same in every region. Geologic isolation produces differences in the genetics of cyanobacteria and other thermophiles, which can affect their appearances and colors.

How do scientists research the colors of geothermal features?

Blue fire (burning sulfur gas) at Kawah Ijen. Photo credit: Thomas Fuhrmann, Wikipedia.
Blue fire (burning sulfur gas) at Kawah Ijen. Photo credit: Thomas Fuhrmann, Wikipedia.
Computer Modeling - Computer models constructed from programming may incorporate factors such as microbial mat properties using pool temperatures and spectra of nearby microbial mats, pool shapes and depths, light angles from the time of day and the location's latitude and longitude, the spectral irradiance of sunlight and diffuse skylight at the water surface, the viewing angle for water reflectance and observation path length, spectra for all depths of the pool, and color adjustments and adjustments for sunlight. Models were used to reproduce geothermal pool colors in Yellowstone and Japan.

Infrared Thermal Imaging (IR Imaging) - IR imaging is useful for determining the thermal properties of objects. IR imaging allows scientists to view the temperature gradient of geothermal features (microbial mats, water, steam, precipitates, and solid areas) in more detail. IR imaging was combined with spectroscopy to reveal the relation of color and temperature in hot thermal pools in Yellowstone.

Geochemistry and Laboratory Analysis - Laboratory analyses can help identify microbe genetics and colors. Geochemistry and geochemical analyses can reveal the composition of geothermal deposits such as those present in Dallol, Ethiopia.

References

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