Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Wendy Stovall, Deputy Scientist-in-Charge of the Yellowstone Volcano Observatory and William Keller, a Geologist with Yellowstone National Park.

Last week some changes occurred in Yellowstone National Park's most most famous hydrothermal area, the Upper Geyser Basin, which hosts Old Faithful. A normally docile hot pool erupted water 20 to 30 feet high! A new thermal feature appeared beneath the boardwalk! Parts of the basin were closed to the public! News of the changes made great headlines, and it also serves to remind us that volcanic landscapes are dynamic and constantly changing places.

Geyser Hill, located across the Firehole River from Old Faithful, hosts dozens of hot springs, geysers and fumaroles. Astute observers noticed unusual behavior earlier in the month at a few normally calm hot pools. Some were putting out excess water while others were boiling. On Saturday, September 15, a rare water eruption of Ear Spring ejected rocks, coins, soda cans, and lots of other human-derived debris (including a cinder block). Hot water is still boiling and overflowing from the spring, occasionally still spouting up to 2-feet high. The spring's surrounding orange-to-yellow bacterial mats have been killed by the hot-water overflows.

According to T. Scott Bryan's "Geysers of Yellowstone" 4th Edition (2008), "...true Eruptions (of Ear Spring) are extremely rare. One reported as 15 feet high took place during 1957. The 1959 earthquake produced a few eruptions perhaps 2-feet high, and additional action of that size was observed in 1986, 1992 and 2004..."

Several features on Geyser Hill have shown changed behavior since the eruption:

• A new thermal feature has formed directly beneath the boardwalk, west of Pump Geyser. The feature started as a crack and has grown into a spouter with water erupting for 5 minutes at 15 minute intervals. Wooden planks have been removed from the boardwalk for easier observation.
• Doublet Pool has had boiling surges up to 2 feet high, resulting in overflow that is killing some of its bacteria mats.
• An unnamed geyser on the Observation Point side of the boardwalk that hasn't been known to erupt recently began erupting to 10-15 feet or higher.
• Lion Geyser, which typically erupts many times per day, went quiet for 3 days
• North Goggles erupted for the first time since August and now erupts every 12-20 minutes.
• A feature on the Solitary Geyser/Observation point trail very near the boardwalk is splashing water up to a foot onto the trail.
• Pump Geyser has less water than prior to Ear's eruption, to the point that its bacteria mats are dry.

Changes in Yellowstone's hydrothermal system are common. Similar changes occurred at Norris Geyser Basin in 2003 when new thermal features formed, hot springs began erupting water, and increased ground temperatures caused vegetation to cook. The superintendent of the park decided to close the area to keep people safe, and eventually boardwalks were reconfigured to accommodate the changes to the thermal ground.

For the public's safety some boardwalks and trails in the Geyser Hill area have been temporarily closed. Closure signs are posted. For those visiting the area, additional information is available at the Old Faithful Visitor Education Center during business hours. The boardwalks around Old Faithful remain open.

Although the outcome of current changes on Geyser Hill are uncertain, Yellowstone National Park geologists are monitoring the situation closely. The area of thermally heated ground may expand, or a small hydrothermal explosion could occur, much like that which occurred at Porckchop Geyser in 1989. We will keep you up to date with changes as they occur.