GOSA (The Geyser Observation and Study Association) Midway Geyser Basin  

Midway Geyser Basin:

Midway Geyser basin is actually a small extension of the Lower Geyser Basin but because of the way the road approaches the area, it appears to be separate. Midway is home to Yellowstone's two largest features: Grand Prismatic Spring and Excelsior Geyser.

Circle Pool: [Picture]

Circle Pool is the large circular pool located next to Flood Geyser. The pool has a diameter of about 106 feet and a temperature near 159 Fahrenheit.

Excelsior Geyser:

When it was active in the 1880's, Excelsior was the largest geyser in the world. Some of its fountain- type eruptions reached 300 feet tall and 300 feet wide. It poured out so much scalding water that it was said to greatly increase the flow of the Firehole river for a half hour after the erption. The river became so hat that horses refused to cross within a quarter mile down stream. Excelsior was active as a major geyser from 1878 to 1890 (there are hints of possible eruptions in 1891 and 1901.)

After 95 years of dormancy, Excelsior awoke in 1985, erupting during a 46 hour period from September 14 to 16. These eruptions did not reach the magnitude of the 1880's eruptions. Most were relatively small at 30 feet but a few were as much as 75 feet tall and 75 feet wide. All eruptions lasted about 2 minutes with intervals of 5 to 66 minutes. During this activity, the outflow of the geyser greatly increased. Producing extra runoff channels to the river. (extra runoff channels are probably a good indicator to look at.)

Even when inactive, Excelsior still produces a huge runoff, approximately 4500 gallons per minute or 6 million gallons per day.

Flood Geyser: [Picture]

Flood Geyser is a fountain-type geyser. Erupting with bursts to 25 feet at intervals of minutes to a few hours. Commonly, Flood plays frequently with one of three types of eruptions:

Eruption Type Interval Duration
Minor 1-4 minutes 30 seconds
Intermediate 15-25 minutes 2-5 minutes
Major 30-45 minutes 6-8 minutes

At times though, Flood has been known to have intervals of 2-3 hours with durations of only about five minutes.

What to look for:

Flood is most easily seen from the roadside pullout at the top of the rise just south of the main Midway boardwalk area. The geyser is located at the base of the hill on the edge of the Firehole River. Flood actually has a fairly large crater. The distance and the large size of its nearby neighbor, Circle Pool, conspire to make it look small. Flood erupts from a circular crater approximately 30 feet in diameter.

Between eruptions, the water in Flood sits down a few feet. As the eruption nears, boiling in the crater begins. The boiling increases and then the water quickly rises and Flood bursts to life. Unfortunate fishermen have been caught by surprise by Flood's unexpected play. A few have been seriously burned as a result.

Grand Prismatic Spring: [Picture]

Grand Prismatic Spring is the largest hot spring in Yellowstone, though not the world. Grand Prismatic is about 250 by 300 feet in size. It is also, hot and deep, 160 degrees Fahrenheit and up to 160 feet deep. Often, it is surrounded by highly colorful cyanobacteria mats.

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