A smaller cone geyser with three main vents along the Firehole River, Fan Geyser is constructed of small geyserite that is tinted orange from thermophiles. The water spouts from the small vents and the outflow channel runs directly into the Firehole River. Fan Geyser eruptions tend to fan out, instead spraying out in a defined jet, hence the name. Fan Geyser's activity is closely related to that of Mortar Geyser's. In the early history of the park, Fan and Mortar were separate geysers but at some point the plumbing of the features changed and now their activity match each other most of the time. Eruption intervals can range from days to months.

Fan Geyser has an average temperate of 172°F (77.8°C), an average pH of 8.2, and an average conductivity of 1800 uS/cm.

Geysers

Geysers have constrictions in their plumbing systems that prevent water from moving freely to the surface where heat would escape. Water beneath the constrictions creates a buildup of steam. Eventually the steam pushes water past the constrictions and the geyser erupts.

Upper Geyser Basin

The majority of world’s active geysers are in the Upper Geyser Basin, including Old Faithful. Only four other places in the world have large concentrations of hydrothermal features: Russia (Kamchatka), Chile, New Zealand, and Iceland.

The heat for the hydrothermal features comes from Yellowstone’s volcano. Molten rock or magma may be as close as 3-8 miles (5-13 km) underground. Rain and snow supply water that seeps down several thousand feet (more than a kilometer) below the surface where it is heated.

Underground cracks form a natural plumbing system. Hot water rises through the plumbing to produce hot springs and geysers.

Use Caution in Hydrothermal Areas

• Stay on boardwalks and designated trails.
• Hydrothermal water can severely burn you.
• Never run, push, or shove.
• Supervise children at all times.
• Do not scratch hydrothermal mats.

You are responsible for your safety.

Think safety, act safely. Yellowstone is a dangerous place.