Grand Prismatic Spring

One of the most famous locations in Yellowstone (probably second to Old Faithful) is the Grand Prismatic Spring. The bast pictures of it are taken from above such as this one (not by me).  Note the scale of the people on the diagram.

There is a boardwalk that goes almost to the edge of the lake / spring, but it is so enormous that you cannot really take the whole thing in. For the New Zealand audience, the Grand Prismatic Spring (GPS) is similar to the Champagne Springs in Rotorua. Amanda in front of the GPS (on the boardwalk)

The bacteria on the outside of the GPS (my guess is that these are slight valleys, and have water for the bacteria when the pool contains more water / paths for runoff (from the boardwalk)

The different colors are from different bacteria, which have different temperatures in which they thrive. The red bacteria survive in the coolest temperatures, with yellow, then green, then blue thriving at successively higher temperatures. I found it curious that the colors map to the energies of photons in the electromagnetic spectrum fairly well -- that is, red is the least energetic to blue which is the most energetic. As far as I know, unlike stars, there is no reason for this to be the case -- it is just a coincidence (after all, we do not tell anything about the temperature of plants from their colors).

The pool is heated from the center, and as it touches the (cooler) land on the outside the temperature changes.  The colors from the bacteria serve as an indirect way to measure the temperature. As bacteria live in a reasonably board range of temperatures the "thermometer" is not particularly precise! Still, this is a good model for my PDE students: the GPS color profile is approximately a steady state solution of the heat equation.

There is a hill to the south of the GPS which allows an elevated view. On our second trip to the GPS we decided to venture up it. The trail is called the Fairy Falls Trail, but we were completely unable to find the eponymous falls. If you plan to do this trail yourself, be warned that the trail is actually quite difficult to find. The hill has many (unlabeled) routes leading up it, and we selected one more or less at random to get the view we were after.

From the fairy falls trail looking over to the GPS: note the reflection off the mist

The GPS as seen from above (on the fairy falls trail). See people for scale.