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I focused (only semi-intentionally) on geysers yesterday. Today was mostly spent looking at other hydrothermal features: prismatic springs, travertine, and paint pots. I also saw my first buffalo (ok, bison) today.
The hot springs here come in a surprising variety of colors, and it turns out that those colors have a surprising variety of sources. The water in very hot pools will usually look blue, as you'd expect a pool of pure water to. This blue is caused by the absorption of red light wavelengths and the transmission of blue ones, and is the same reason a lake or glacier will appear blue. There's a fair amount of yellow as well, caused by deposits of sulphur. Some of the greens are the result of the combination of those two effects, but many of the other shades of green and most of the reds and oranges are actually caused by thermophilic bacteria that thrive in very hot water.
Travertine is limestone deposited when dissolved calcium carbonate comes out of solution. This often happens when water from a hot spring reaches the surface and cools. There are large terraces of travertine in the northwestern part of the park near Mammoth Hot Springs, where I had a late lunch.
Heading back south towards my campsite, I stopped at a couple of sites devoted to "paint pots", which are like bubbling hot springs except that, instead of being full of water, they're full of mud. Some of the thermophilic bacteria in the region metabolize the hydrogen sulfide present in the volcanic vents into sulfuric acid, which dissolves the stone into clay. The clay bubbles more slowly than water and is fun to watch and listen to. Here's an audio sample, recorded with my phone. The hissing in the background is actually mostly steam from nearby vents, not just a cheap microphone.
I did get to see one unexpected geyser, Fountain Geyser happened to be erupting while I was checking out the nearby paint pot. It has a period of roughly 8.5 hours, but was running an hour or two late.
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