A large sinter cone along the riverside, Mortar Geyser's activity is closely related to that of Fan 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 no their activity match each other most of the time. Recently new vents have been formed by Mortar eruptions, breaking apart the old cone structure. Three actively boiling vents exist in the large cone, with run off spilling into the river. Eruption intervals can range from days to months.

Mortar Geyser has an average temperate of 169.3°F (76.3°C), an average pH of 5.5, 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.