Great Fountain Geyser sits in the middle of one of the prettiest sinter formation in the park. The sinter forms a series of terraced concentric reflective pools around the geyser's 16' diameter vent. Even if the geyser isn't erupting, it is worth driving past to see the pools. They make a very pretty picture at sunset.
Great Fountain is a fountain-type geyser. Its interval range from 9 to 15 hours but its short term average interval is usually stable enough that the eruptions can be predicted to within an hour or two. Great Fountain's maximum height ranges from about 75 feet to over 220 feet. Its duration is usually about one hour but durations of over two hours have been seen.
Great Fountain is a major geyser but many people, when hearing it erupts to over 200 feet, are disappointed by its more common maximum of less than 100 feet. For this reason, Great Fountain has earned a number of pejoratives, such as The Great Flounder. The smaller eruptions of Great Fountain are still large and pretty but pale in comparison to the truly large eruptions. It is the hope of seeing one of the rare but spectacularly large eruptions that tends to bring the geyser gazers back to Great Fountain time after time.
Great Fountain erupts in a series of distinctly spaced bursts. The first group of bursts lasts about ten minutes. Then there is about a five minute quiet period followed by another five or so minutes of activity. These quiet and active episodes continue until the end of the eruption. There can be over seven active periods in the eruption. Usually the first period of activity is the tallest and strongest, with the first and sometimes the second burst being the strongest of the eruption. Sometimes, the largest burst will occur during the third active period. This especially seems to happen when the first period has been uninspiring. Rarely, large bursts will continue long into the eruption. There have even been a few reports of 100 feet plus bursts in the fifth and sixth periods.
Great Fountain has two types of truly spectacular behavior. A Superburst is an exceptionally tall burst of water, over 150 feet. Some superbursts have reached 230 feet. Superbursts, when they occur, are usually the first burst of the eruption but they have been know to sometimes occur later in the eruption. A Blue Bubble occurs when a calm and still pool of water is domed up by a large expanding steam bubble. As the steam bubble rises and expands, the entire 16 foot wide pool of water is lifted and domed outward creating a beautiful blue bubble. Once the steam reaches the surface the water explodes outward and upward. Blue bubbles most commonly occur at the start of the eruption but the have been known to occur at the start of other active periods, especially the third. A fair number of blue bubbles result in a superburst but not all.
Great Fountain sometimes goes through a Wild Phase. During a wild phase, Great Fountain seems to forget how to end an eruption. Ten to fifty foot play continues for hours to days. Once the play finally ends, Great Fountain usually takes a few days to recover before returning to "normal" eruptions. Interestingly, wild phases mainly occur late in the year.
|Great Fountain begins overflowing from its crater into the basins around the vent about 70-100 minutes prior to the eruption. Great Fountain has been known to start overflowing and then stop a short time later. These false overflows can not be used to predict the eruption. Once the overflow starts, you need to wait a few minutes to be sure that the overflow continues. Once you are sure that the overflow isn't going to stop, then you can use the 70-100 minute window to predict the geyser accurately.
About 30-60 minutes after the overflow starts, small bubbles will appear in an arc along one edge of the pool. This boiling slowly grows in intensity and the arc continues to grow until it finally forms a circle of boiling completely around around the edge of the pool. The boiling continues to periodically grow until there is a one meter tall boil. For timing purposes, this is the official start of the eruption. The bursting play may start immediately after the one meter boil or it may wait for a few minutes before starting. The time between the first one meter boil and the first burst is called the pause. If you are fortunate, the geyser may go completely quiet during the pause. While it may be nerve wracking to see the pool quiet like this for as much as nine minutes, it is a good omen. Blue bubbles form best when the pool is calm and some people feel that a quiet pause is also a good omen for an exceptionally tall eruption. While I can understand the logic behind the blue bubble claim, I'm not sure about the height claim.
The first few bursts usually pour out a lot of water. It is fun to watch the wave of water as it spreads out and then tumbles over the concentric terraces of the sinter formation.
Determining the end of the eruption is hard. Just when you think it has had its last active period, it will throw in another. But, eventually it does end.
The geyser refills quickly. Noisily boiling water can be seen a few inches down in the pool shortly after the eruption. If you've just arrived at the geyser, be careful not to confuse this post play boiling for the preplay boiling. This can be problematic when conditions are steamy. The post play boiling occurs while the geyser is below overflow. The preplay boiling occurs after the geyser is in over overflow. The post play boiling will continue for a couple hours after the eruption. Finally the pool will become calm again and then start to slowly rise as it prepares for the next eruption.