Doublet Pool. Yellowstone National Park, November 2004.Doublet Pool is one of the most attractive thermal features in the Upper Geyser Basin, if you find it - it's tucked away on Geyser Hill, across the Firehole River from the lodges, stores, and benches for viewing Old Faithful. I don't mean that it's really hidden away, but just that the folks who don't bother walking through the basin won't get to see it.
Doublet has been known to erupt slightly with a bit of bubbling, and on rare occasion even throw some water a couple feet in the air. Usually, though, it's just another of the steaming hot pools, with that clear blue (bacteria-free) water that the hottest pools have, and the red bacteria mats in the shallow areas where the water is merely warm and can sustain thermophilic life. It's called Doublet because there are two small pools connected by the narrow channel that you see in the photograph.
I don't entirely understand how the scalloped edges form. Silica is precipitating out of the water, slowly building up the mass on the sides of the pool. But why the round scallops? There's no sign of water draining into the pool and cutting channels; those would look like gullies instead of scallops, anyway. My best guess is that the edges begin jagged and random, but as the silica accumulates, it does so at equal distances around any pointed surface (recall that a circle is defined as the set of points equidistant from a given point). The tendency would be to grow the rounded scallops out from the edges of the wall and ever farther into the pool; pointy edges can't help being a passing phase. Or maybe there's some entirely different reason for that shape.
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