Castle Geyser (Castle Group, Upper Geyser Basin, Yellowstone Hotspot Volcano, northwestern Wyoming, USA).
Castle Geyser is the namesake & largest member of the Castle Group in Yellowstone’s Upper Geyser Basin. It has a large, gray, asymmetrical, terraced geyserite mound that is relatively old, estimated to be near-latest Pleistocene to mid-Holocene in age. Minor eruptions are water-phase only, and last less than 15 minutes. Major eruptions have a longer-than-15 minutes water phase, which gradually changes to a mixed water-steam phase and then a moderately noisy steam phase.
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