They've thrilled visitors to Yellowstone National Park for well over a century -- simmering ponds of brilliant, rainbow colors with unforgettable names like Grand Prismatic Spring and Sapphire Pool.
Yet not all of Yellowstone's famed thermal pools were always the riot of color they are today. In fact, long before this volcanic region of Wyoming, Montana and Idaho was made a national park in 1872, the now yellow, green and blue Morning Glory Pool was most likely a pond of deep blue.
In research published Friday in the journal Applied Optics, scientists used computer modeling to turn back the clock and examine what Morning Glory and other magma-heated pools probably looked like before humanity left its calling card.
"As a result of coins, trash, and rocks thrown into the pool over time, the vent has become partially blocked, leading to a lower temperature and altered color pattern," wrote the study's authors.
The prism-pattern colors that visitors see today are primarily caused by vast communities of microbes that collect in "mats" and thrive in the pools' hot waters, which range between 140 and 194 degrees Fahrenheit.
"It is now understood that these mats are comprised of complex communities of microbes, primarily thermophilic cyanobacteria (often called blue-green algae) and other thermophilic bacteria and archaea," wrote lead study author Paul Nugent, a computer engineering researcher at Montana State University, and his colleagues.
Different communities of microbes lend different colors to the pool. Because the different microbes prefer different temperatures, their arrangement causes the concentric patterns of yellows, greens, browns and oranges.
The vivid blues however are the result of light scattering within the water, and signify greater depths at the center of pools like Morning Glory.
"The pool center -- though presumably covered by the yellow mat -- appears deep blue, indicating that the pool is deep enough that backward-scattered sunlight from the water is the dominant component of upwelling light," wrote Nugent and his colleagues, Joseph Shaw, an optical science professor at Montana State University, and Michael Vollmer, a professor of experimental physics at Brandenburg University of Applied Sciences, in Germany.
The researchers based their model on the shape and depth of each pool, its temperature and the angle of observation, among other factors.
Based on historic temperature records and visual accounts, the researchers knew that Morning Glory was hotter in the past, and therefore not as inviting to the microbes that inhabit it today.
However, as the fissures in the earth that transmit heat from concealed magma were filled with detritus, the temperature dropped, the researchers say.
"Before the vents were partially plugged, Morning Glory Pool ... was reported to be primarily blue in color," authors wrote.
It was a much bluer pool that greeted members of the Hayden Geological Survey as they explored the park area in 1871.
In its report, the survey team, which was led by surgeon and geologist Ferdinand Vandeever Hayden, remarked that "nothing ever conceived by human art could equal the peculiar vividness and delicacy of color of these remarkable prismatic springs."
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