Yellowstone Has a Hot Spring Ringed Like a Rainbow, and the Rings Are Alive

From the wooden boardwalk it looks like something a child drew with every pen in the box: a wide pool of deep blue, ringed with green, then yellow, then a blazing orange that runs off across the ground in long wavy streaks, all of it under drifting steam. This is Grand Prismatic Spring, in the Midway Geyser Basin of Yellowstone National Park in the United States, and it is the largest hot spring in the park. The geologist who described it in a report of 1871 wrote that he had named it Prismatic on account of its brilliant colouring.

A rainbow that is alive

The colours around the edge are not paint, and they are not simply minerals. They are alive. The National Park Service explains that the rings are made of thermophiles, heat-loving microbes far too small to see one at a time, growing in such numbers that they form mats of colour. Which colour you get depends on warmth. Colourless and yellow kinds grow in the hottest water, while orange, brown and green ones grow where it is cooler. The water cools as it flows outwards from the middle, so the microbes sort themselves into bands, rather like a thermometer you can read from a distance.

The brightest oranges lie in the shallow channels that carry the overflow away. Physicists from Montana State University, working with a colleague from Germany, describe those outflows as typically less than 10 centimetres deep, and give the temperature of the pool itself as 63 to 74 degrees Celsius.

Why the middle is blue

The blue is another matter. The same team visited in the summer of 2012 with cameras, handheld spectrometers and thermal imagers, built a simple model of how sunlight travels through the pool, and published it in the journal Applied Optics in 2015. In shallow water, what you see is light bouncing off the mat on the bottom. In very deep water the bottom hardly counts. What comes back to your eye is sunlight scattered by the water itself, and clear water scatters blue light the most. The model produced the whole run from yellow at the edge through green to blue in the centre from a single yellow mat, simply by making the water deeper.

• 370 ft the widest diameter, longer than an American football field
• over 120 ft the depth sounded in the early 1990s, an uncertain figure
• 63–74 °C the temperature of the water in the pool
• < 10 cm the depth of the channels where the orange mats grow

What people often get wrong

People tend to assume the colours are all chemistry, or else that the blue must be one more kind of microbe, or only the sky reflected. In fact one spring shows colour made in two different ways: living things at the rim, and the physics of deep clear water in the middle. The study found that the change from yellow to green to blue follows depth, not temperature. The sizes need a caution too, because good sources disagree. The National Park Service gives 200 to 330 feet across, roughly 60 to 100 metres, and more than 121 feet deep. The United States Geological Survey gives a widest diameter of 370 feet, about 113 metres. The physicists worked with 76 by 85 metres and a depth of 50 metres.

How to measure a nearly boiling pool

The depth has its own story. In the early 1990s, the Geological Survey recounts, a park geologist and a boat-builder designed a tiny boat called Little Dipper, 8 feet long and 4 feet wide. Crews rowed it out to the near-boiling centre several times. A line let down there gave a depth of over 120 feet, about 37 metres, though the Survey adds that the figure is uncertain, because the line may have folded on its way down.

Visitors today stay on the boardwalk, which is kinder both to them and to the mats. It is, after all, how the residents manage it: every microbe in the place has picked the distance from the middle that suits it best, and stayed there.