The Explanation Behind The Vivid Hues Of Yellowstone’s Thermal Spring

Yellowstone National Park has long been a popular go-to destination for nature lovers and adventure seekers alike. For decades, visitors have particularly taken interest in the Morning Glory pool, a hot spring located in the Upper Geyser of the park that boasts vivid hues of yellow and green. But while these colors certainly look pleasing to the eye and are responsible for attracting numerous tourists every year –  Morning Glory didn’t always look this way. In fact, it once was a deep shade of blue – just like the flower it was named after. Over the course of several years, however, tourists, who toss rocks, coins and other debris, have unknowingly transformed the body of water.

All the foreign objects are now collecting at the bottom of the pool, partially blocking the underground heat source and thus lowering the overall temperature of the spring in the process. As result, swaths of photosynthetic microorganisms – referred to as microbial mats – are currently living in the spring, thanks to the cooler, habitable temperatures of the water. This also explains the different nuances of color that are visible. According to Brent Peyton, the director of the Thermal Biology Institute at Montana State University, “[Because the springs are hottest at the center and typically coolest at the edges [a color gradient appears]…”due to the microbes preference of certain temperatures over others.

Another factor is the way water interacts with light at different depths. Intrigued by this phenomenon, Joseph Shaw, the director of the Optical Technology Center at Montana State University, along with a team of researchers, developed a model that could essentially predict the colors of the various thermal springs in the park. In order to do so, the team utilized thermal cameras to measure the surface temperature of the pool, combined with spectrometers to determine the amount of light reflected. The resulting data was then studied together with existing studies of the pool’s physical structure.

Based on the research, which is currently published in the Journal of Applied Optics, it was determined water tends to scatter high-energy light (the blue end of the spectrum) more than lower energy light (warmer tones), which is filtered through absorption. Ultimately, this causes deeper water to appear green, while shallow water is more of a yellowish color.

From this model, the scientists were also able to determine how the Morning Glory looked liked before the contamination. The photograph seen below is a recreation of the deep blue that once filled the spring: