File:Geyserite breccia (Holocene; West Thumb Geyser Basin, Yellowstone, Wyoming, USA).jpg

English: Geyserite breccia in the Holocene of Wyoming, USA. (July 2012)

Geyserite (a.k.a. siliceous sinter) is a rare, friable to solid, chemical sedimentary rock composed of opal (hydrous silica, a.k.a. opaline silica: SiO2•nH2O). Geyserite forms by precipitation of hydrous silica from hot spring or geyser water. At the Yellowstone Hotspot Volcano in Wyoming, the source of the silica is subsurface rhyolite lava flows of late Cenozoic age. Superheated groundwater moves along fractures in the rhyolite and leaches out silica. When hot spring or geyser water reaches the surface, the water cools and evaporates, resulting in precipitation of very thin layers of opal on any substrate. Numerous precipitation episodes (for example, from repeated geyser eruptions) results in the buildup of geyserite.

The above photo shows a piece of Holocene-aged geyserite breccia in very shallow water near Yellowstone Lake's shoreline at the West Thumb Geyser Basin. This rock appears to have formed by shallow burial and cementation of lakeshore geyserite sediments (apparently dominated by angular to subangular pebbles and granules). The lithified geyserite sediments have since been eroded and reworked, resulting in geyserite breccia blocks sitting on the shallow lake floor. This is a rare type of breccia.

Locality: southwestern shores of Yellowstone Lake's West Thumb, Lower Group, West Thumb Geyser Basin, Yellowstone Hotspot Volcano, northwestern Wyoming, USA