File:Improbable Geyser erupting (14 August 2010) 10.jpg

English: The temperature scale is in degrees Fahrenheit.

Improbable Geyser is currently a perpetual spouter in the southern Geyser Hill Group of Yellowstone's Upper Geyser Basin. It is located 14 meters southwest of Big Anemone Geyser. Improbable Geyser erupts from a pool about 3 to 4 meters in diameter. A continuous, irregularly-scalloped geyserite border decorated with small nodules encircles Improbable Geyser’s pool on its northern, eastern, and southeastern sides. 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.

The southwestern side of Improbable Geyser’s pool lacks a border and has irregularly-distributed, small mounds of geyserite. This borderless portion of the feature represents the start of its overflow channel. Eruption runoff drains to the southwest and west into the Firehole River.

Improbable Geyser currently engages in low to moderately low perpetual spouting behavior from a vent in the northwestern part of the pool. This feature first had eruptions in fall 2005. Slightly violent vent enlargement events resulted in geyserite debris occurring around the geyser. A runoff channel from the geyser to the Firehole River first developed at this time. Reported activity of Improbable Geyser during October and November 2005 involved eruption columns up to an estimated 20 feet high.