File:Fan Geyser & Mortar Geyser eruption (1-18-1-52 PM, 6 June 2018) 2.jpg

English: Foreground erupting feature projecting into river = Mortar Geyser

Background feature with hose-like eruption column = Fan Geyser

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Geysers are hot springs that episodically erupt columns of water. They occur in few places on Earth. The highest concentration of geysers anywhere is at the Yellowstone Hotspot Volcano (northwestern Wyoming, USA).

Fan and Mortar Geysers are major features in the Morning Glory Group of Yellowstone’s Upper Geyser Basin. They are located on the northeastern bank of the Firehole River, downstream from Riverside Geyser. Mortar Geyser is just upstream from Fan Geyser. Mortar is an eroding cone-type geyser with three principal vents: Upper Vent, Lower Vent, and Bottom Vent (= formerly Arch Vent). Fan Geyser is also a cone-type geyser and consists of a series of vents oriented along a northeast-southwest fracture zone that goes through nearby Spiteful Geyser. From southwest to northeast, the principal vents of Fan Geyser are: River Vent, High Vent, Gold Vent, Angle Vent, Main Vent, and East Vent.

Fan Geyser is the more active of the two features. Eruptive activity at Fan currently takes several forms: "garbage", regular cycles, event cycles, and (major) eruptions. "Garbage" refers to disorganized low spouting from Fan's various vents. Regular cycles involve organized low spouting and splashing that starts with River Vent, progresses to High Vent and Gold Vent, and continues to Angle Vent. Eventually, the low spouting ends. Event cycles are characterized by slightly more unusual activity, including one or more pauses and resumptions in River Vent’s erupting, more energetic and higher spouting from High Vent, Gold Vent, and Angle Vent, and small splashing in the Main Vent. Major eruptions of Fan Geyser are almost always in concert with nearby Mortar Geyser (there have been exceptions in the past). Eruptions occur after an energetic event cycle and involve high spouting from East Vent, Main Vent, and all other vents. Eruption columns extend in various directions, resulting in a fan-like spray. Spouting often reaches the trail or past the trail. Eruptions are about 30 to 40 minutes in duration, and include one or more relatively brief pauses.