Ear Spring, located on Geyser Hill in the Upper Geyser Basin, prior to the recent eruption. Old Faithful can be seen steaming in the background/Yellowstone Volcano Observatory

A rare and surprising eruption of Ear Spring in the Upper Geyser Basin of Yellowstone National Park was a treat for visitors who witnessed it, but also signaled a change in the basin's geothermal plumbing that has prompted a closure for the time being of part of the basin.

Not since 1957 has the normally placid hot pool sent a plume of water 20-30 feet into the sky, tossing with it rocks and other debris that had fallen or tossed into the pool over the years. Though unusual, geologists don't view that eruption last Saturday as a signal of "impending volcanic activity," the park staff said Wednesday afternoon in a release.

And yet, the outcome of the current changes on Geyser Hill in the Upper Geyser Basin is uncertain, said the staff at the Yellowstone Volcano Observatory. The two most likely changes going forward are:

* The area of thermally heated ground may expand and continue to cause changes in hydrothermal activity that persist for years, much like the thermal activity of 2003 in the Norris Geyser Basin. This could necessitate rerouting of the current boardwalk configuration.

* A small hydrothermal explosion could occur in the area, forming a crater a few feet across and ejecting rocks and hot water up to a distance of hundreds of feet, much like that which occurred at Porkchop Geyser in the Norris Geyser Basin in 1989.

Although potential impacts to Old Faithful Geyser are unknown, Geyser Hill and Old Faithful have separate hydrothermal plumbing systems and behave independently, the observatory noted.

Ear Spring after an eruption of water and debris on September 15, 2018. The surrounding bacterial mats have been destroyed and large rocks are strewn around the area surrounding the spring/Yellowstone Volcano Observatory

Hydrothermal activity at several features on Geyser Hill has changed since the eruption of Ear Spring. Most notably, a new feature has formed west of Pump Geyser and north of Sponge Geyser directly under the boardwalk. The feature erupted overnight between September 18 and 19 and continues to pulse water as a small spouter. An approximately 8-foot diameter area of surrounding ground is "breathing" - rising and falling by about 6 inches every 10 minutes. Several other thermal features are more active than usual, including geysering and boiling of Doublet Pool and North Goggles Geyser.

"Changes in Yellowstone's hydrothermal features are common occurrences and do not reflect changes in activity of the Yellowstone volcano," the observatory said. "Shifts in hydrothermal systems occur only the upper few hundred feet of the Earth's crust and are not directly related to movement of magma several kilometers deep. There are no signs of impending volcanic activity. There has been no significant increase in seismicity nor broad-scale variations in ground movement."