Upper Geyser Basin in Yellowstone National Park has the highest concentration of geysers anywhere in the world. This section of the UNESCO World Heritage-listed park is often overlooked because many people concentrate on Old Faithful, but it’s definitely worth walking through. There are a good number of geyser groups in the basin making for a good few miles of walking. This entry focuses on the Grotto and Riverside Groups and describes only a few of the geysers and pools in the area.
The boardwalk from the Giant Group continues on to the Grotto Group. Grotto Geyser erupts about every six to 24 hours. It has two modes, a short mode with a one to three hour eruption and a longer marathon mode which can last from six hours to more than 26 hours. 75% of the eruptions are short modes. The strange formation stands eight feet tall and was formed when sinter covered fallen tree trunks around the vent.
Next to Grotto Geyser is Rocket Geyser, which erupts in unison with Grotto Geyser. It’s very rare for it to erupt alone. It usually reaches heights of about 10 feet but has been known to reach 50 feet.
Spa Geyser, further up the path to the right, erupts during Grotto Geyser’s marathon modes. The expulsion of water reaches up to 50 feet high.
Continuing on an extension of the path just past Spa Geyser is Marathon Pool. It’s directly connected to Grotto Geyser. During marathon mode eruptions of Grotto, the water level drops in Marathon Pool. The pool erupts to a height of six feet but this is extremely rare.
Along the path and across the Firehole River is Riverside Geyser. It’s neatly placed next to the river, making it one of the most attractive geysers to look at. Riverside Geyser erupts roughly every seven hours and can reach heights of 75 feet. The duration is between 20 and 30 minutes. It’s one of the most predictable geysers in the park.
When the extension intersects with the main path again, to the left are the Chain Lakes. It’s a colorful series of pools with a geyser mixed in.
To the right, across the Upper Geyser Basin’s final bridge over the Firehole River, is a group of three geysers. Spiteful Geyser is the one closest to the path. Its eruptions are highly unpredictable and very rare. They last around five minutes and can reach heights of up to 15 feet.
Fan and Mortar Geysers are along the river behind Spiteful Geyser. Supposedly, watching these two geysers erupt is an incredible sight. Fan Geyser sprays water from seven vents in a fan-shaped patten. The water can shoot across the trail and up to 100 feet in the air. The vents aren’t visible from the path. Mortar Geyser shoots water 80 feet straight up into the air. The two geysers erupt together. Eruptions are unpredictable and the geysers have been known to go dormant for months to years. A short walk beyond these geysers is the popular Morning Glory Pool.