This eroding, lichen-encrusted rock mass is an old geyserite mound in the Fountain Group of Yellowstone's Lower Geyser Basin. An inferred vent occurs atop the small knob at lower right. Geyserite is a specific type of siliceous sinter - both are varieties of opalite, or opaline chemical sedimentary rocks, and produced by precipitation of opal (hydrous silica) from hot spring water. Other types of sinter include calcareous sinter (typically called travertine) and ferrous / ferric sinter, the rarest kind. Siliceous sinter is typically whitish to grayish and has alternating layers of solid opal and porous, vuggy opal.
Locality: old, eroded geyserite mound on the western side of the Fountain overlook platform, Fountain Group, Lower Geyser Basin, Yellowstone, northwestern Wyoming, USA
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