File:Dual eruption of Big Anemone Geyser (center) & Little Anemone Geyser (left background) + Improbable Geyser eruption (upper right) (14 August 2010) 5.jpg

English: Geysers are hot springs that episodically erupt columns of water. They occur in few places on Earth. The highest concentration of geysers anywhere is in Yellowstone’s Upper Geyser Basin (northwestern Wyoming, USA).

Big Anemone Geyser is a frequently erupting small geyser in the southern Geyser Hill Group. It is immediately adjacent to Little Anemone Geyser - together they make up “Anemone Geyser”. Big Anemone has a nearly circular basin with a central vent and lacks a well-defined border. Encircling the cream-colored to grayish, smooth geyserite-floored vent area is abundant, well-formed, closely spaced nodules of grayish geyserite (columnar geyserite & pseudocolumnar geyserite), similar in appearance to cave popcorn (coralloids). Some small biscuit-like masses of grayish geyserite are present on the northern side of the geyser, often with apical puckered structures. The geyserite biscuits are in and along a small, irregularly-shaped, eruption splash pool. The diameter of Big Anemone Geyser, as measured across its nodulose geyserite area, is about 2.5 meters. The diameter of the feature, as measured from the outer edge of its colorful, encircling microbial mats, is about 4.5 to 5 meters.

Big Anemone’s geyserite is especially well-formed and attractive. Geyserite is a friable to solid chemical sedimentary rock composed of opal (hydrous silica, a.k.a. opaline silica: SiO2•nH2O), It forms by precipitation of hydrous silica from hot spring water. Geyserite is the dominant material at & around Yellowstone hot springs and geysers (the Mammoth Hot Springs area is a major exception to this). The silica in the geyserite is ultimately derived from leaching of subsurface, late Cenozoic-aged rhyolitic rocks by superheated groundwater. Rhyolite is an abundant rock at Yellowstone.

Big Anemone Geyser has frequent, moderately low to medium-sized, splashing eruptions. Eruption durations are about half a minute long. Intervals between eruptions starts range from about 7 minutes to almost half-an-hour. Eruption activity at adjacent Little Anemone Geyser has a delaying affect on Big Anemone’s eruption frequency. Eruption cessations at Big Anemone Geyser are impressive, rapid pool drains. Most of the erupted water ends up back in its own vent. Some eruption splash water enters Little Anemone’s basin or its runoff channel.