The heat-loving organisms in Grand Prismatic Spring get credit for making it the most colorful hot spring in Yellowstone National Park. From above, Grand Prismatic is a large oval splash of cornflower blue on the ash white landscape of Middle Geyser Basin, edged by flowing, arm-like strands of yellow, orange and red.
“It’s just dazzling on a sunny day,” said Lee Whittlesey, park historian.
As the water passes through fissures underground it picks up silica, one of the components in glass. When the hot water cools at the edges of the springs and runs over the rim, silica is deposited, creating the white rock cone around this spring, as well as others in the park. The springs at Mammoth differ. The water there contains calcium carbonate.
Although Grand Prismatic is the largest hot spring in North America, it is only the third largest in the world. New Zealand has the two largest hot springs.
But the Kiwis’ hot springs can’t compete with Grand Prismatic for colorful grandeur.
In the center of Grand Prismatic, where the pool is deepest and too hot for bacteria, the water is an azure blue that looks like the sky has been reflected below. As the water cools toward its edges, different thermophilic bacteria find comfort zones where they make their living. From blue the spring fades to light blue, then green, yellow, orange and red.
“What you generally see around nonacidic springs and geysers are cyanobacteria - oxygen-producing bacteria that use photosynthesis,” said park naturalist Carolyn Loren.
Not all bacteria are the infectious kind people fear. There are thousands of others that are simple organisms important to the Earth’s ecosystem.
The color given off by Grand Prismatic’s bacteria is a pigment that acts as a sunscreen. By the time the water gets to the edge of the spring, it has cooled to about 147 degrees.
As the colors of the bacteria are reflected off the steam rising from the pool, Grand Prismatic’s hues are revealed in a gauzy tapestry of light, as if the Northern Lights were shimmering during the day.
Osborne Russell, a noted trapper who kept a diary of his travels, is one of the first Euro-Americans credited with writing about the feature in 1839, Whittlesey said. The Langford Expedition, one of the first to visit the park in the 1870s, produced detailed accounts of the natural wonders they saw. That prompted the formation of the Hayden Expedition in 1871, including artist Thomas Moran, who made a watercolor sketch of Grand Prismatic’s vibrant colors.
A young mineralogist on the expedition, Albert C. Peale, wrote in letters to his hometown newspaper, The Philadelphia Press, that the park’s hot springs in general were incredible.
He wrote, “I wish I could give an idea of the exquisite beauty of some of these springs. They rival any fairy palace that can be imagined. ... It seems as though the cavity were lined with a portion of the heavens above us.”
Peale was uncertain that Moran’s painting was accurate, since he thought the colors were “a little too bright.” So in another trip to Yellowstone in 1878, he made a point to visit Grand Prismatic, where he “confirmed the truthfulness of (Moran’s) pictures. ... The colors cannot be exaggerated.”
Author Rudyard Kipling, who visited the geyser basin in 1898, is credited with immortalizing the area by calling it “hell’s half-acre.”
He wrote of the basin, “There was a plain, blistered, peeled, and abominable, and it was given over to the sportings and spoutings of devils who threw mud, and steam, and dirt at each other with whoops, and halloos, and bellowing curses.”
An 1899 tourist guide described Grand Prismatic as “so dazzling that the eye cannot endure it.”
It is Peale who is credited with naming the spring Prismatic in 1878 “on account of the brilliant coloring displayed in it.” According to Whittlesey, the “grand” got tacked on later. The spring also got tagged with at least 10 other names over the years, including Boiling Coral Well, Prismatic Lake and Emerald Lake.
No matter what the spring has been called, it remains “very striking,” Loren said.
“The colors there are particularly bright and beautiful,” she said. “It’s helpful if it’s sunny and not too cool. It’s easier to see the colors when it’s not so steamy.”
The wind direction can also heighten or spoil the springs’ sulfurous display, Loren said.
Grand Prismatic is a popular feature, she said, noting that the parking lot for the Midway Geyser Basin is often full in the summer. How many of those visitors, gazing in wonder at Grand Prismatic’s hazy rainbow, know that it’s a bunch of bacteria doing the colorful work?
“It’s a spectacular feature, no doubt about it - world class,” Whittlesey said.
Visit the rainbow
A half-mile boardwalk accesses the east side of Grand Prismatic Spring as well as Excelsior Geyser, which was once the largest geyser on Earth.
The parking area for the boardwalk is about 11 miles south of Madison Junction or five miles north of Old Faithful. There’s also a parking area farther south that accesses the Fairy Falls trail. Hikers and bike riders can take this route to access a hill overlooking Grand Prismatic Spring and then continue on to the Fairy Falls trail turnoff. No bikes are allowed on the 1.6 mile trail to Fairy Falls, but bikers can ride to the Fountain Flat Road and the Nez Perce Creek picnic area, a five-mile route one way that is partially paved.
Contact Brett French at french@billingsgazette.com or at 657-1387.
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