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Geysers of the World   

Geysers of Yellowstone   



  Fan Geyser
Feature Type: Geyser
Geyser/Spring Type: Cone geyser

Upper Geyser Basin
Morning Glory

[GOSA Transactions Articles]

The start of an eruption by Fan and Mortar Geysers is truly spectacular. Both geysers, involving about a dozen vents in total, start with nearly explosive power. The main vent of Fan is the largest, often shooting its strongly angled jet clear across the trail. Those that are so inclined can receive a warm shower during the eruption (the water cools as it travels through the air.) The other vents of Fan geyser are arranged along a nearly straight fracture in the sinter formation. These vents erupt at various angles and strengths, forming a fan shaped wall of water. Nearby Mortar also joins in. While it is rare to see both the lower and upper vents of Mortar erupting at full force together, play usually switches back and forth between the two during the eruption, Mortar often reaches 60 feet or more in a wide vertical jet. The start of Fan and Mortar is one of the most spectacular sights in Yellowstone.

Fan and Mortar Geysers are known to have acted separately during the early years of the park. Even though this is known, the concerted behavior has been so prominent during the last few years that some were surprised when the lower vent of Mortar Geyser started having small, 10-25 feet, eruptions on its own late in 1997. These small eruptions have been termed, "Lower Mortar Minors".

Fan and Mortar Geysers' intervals are quite erratic from month to month but in the short term they are often stable enough so that people are willing to sit and wait for an eruption. Intervals range from about 3 days to weeks. The geyser is known to go dormant for months at a time. Some years pass without any eruptions. Fan and Mortar often slow down or stop in the Spring and early summer leading some to theorize that high river levels may cause water to find its way into the geyser, which abuts the Firehole river. It is thought that this influx of water, assuming that there is such and influx, may quench the geysers and keep them from erupting.

Durations of an eruption last about one hour. During this time, the geysers will pause and restart a number of times. The initial part of the eruption, about the first ten minutes, is usually the strongest. Heights for Fan Geyser can reach 125 feet with a horizontal throw of over 200 feet. Mortar Geyser can reach over 80 feet.

What to look for:
The signs to look for at Fan and Mortar are constantly changing. Prior to an eruption of Fan and Mortar, the entire complex goes through a series of "cycles". These cycles involve minor play from the vents of Fan and Mortar's Lower vent. The cycle is taken to start with the "river vent", the vent of Fan closest to the river. The cycle then proceeds in a somewhat clockwise direction through the first three or four vents of Fan and then over to lower Mortar. What to look for during these cycles changes from year to year and it is best to ask someone in Yellowstone for more information. Cycle lengths can vary from 15 minutes to an hour or more.

The first thing to know about Fan and Mortar is that to the untrained eye, and sometimes to the trained eye, it often seems that Fan and Mortar can erupt anytime it wants to. It doesn't seem to want to follow the rules. Before many, but by no means all, eruptions, the first strong signs that the eruption is imminent is strong steady play from all of the river end vents of Fan, splashing from Fan's main vent and splashing and high water in Lower Mortar. In these cases, Fan usually starts the eruption with Mortar quickly joining in. Sometimes, Lower Mortar starts the eruptions. More rarely, Upper Mortar starts the eruption. In any case, those present are in for a treat.

Electronic Monitor Files
Fan and Mortar Eruptions for 2009.txtFan and Mortar Eruptions for 2010.txt
Fan and Mortar Eruptions for 2011.txt 

Some of the temperature data used to derive the eruption times and durations used in this section were collected by Ralph Taylor under a National Park Service research permit, and the remainder was collected by personnel working for the Geology Department of the Yellowstone Center for Resources (including Ralph Taylor). The loggers are a combination of loggers owned by the NPS and Ralph Taylor. Analysis of the raw temperature data to extract the eruption data was performed by Ralph Taylor. The eruption time files on this website may be used provided that Yellowstone National Park is credited for the temperature data and Ralph Taylor is credited for the eruption times.

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The above map shows the various vents of the Fan and Mortar Complex.
From: "Why Fan is Mortar: An Analysis of the History of the Fan and Mortar Geyser Complex", Paul Strasser, GOSA Transactions Volume IV, 1993, pp 73-118

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This picture taken by Ralph Taylor shows the explosive start of an eruption of Fan and Mortar.

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This picture, taken by Mike Newcomb, focuses on the Upper and Lower vents of Mortar Geyser.

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This picture, taken by Mike Newcomb, focuses on Fan Geyser.

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This picture shows a "Lower Mortar Minor" eruption. These eruptions only involve Lower Mortar and "Bottom" (or Arch) Vent. The eruption can reach up to 25 feet though 10 to 15 feet is more normal. They usually last just a few minutes. Historically, Fan and Mortar are known to have erupted independently but for the past few decades this type of activity has not been seen. It was a complete surprise in 1997 when Lower Mortar started having these solo eruptions. The eruption, while not tall, produces a large runoff.

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This picture, taken by Ralph Taylor, focuses on Upper Mortar Geyser.

Please note - this site is currently under constuction. Please visit for more information.  Last update 01-29-2017

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