[Illustration: SEISMIC GEYSER, showing rock rubble blown out during an explosive thermal eruption. Note the trees that have been killed by the heat and eruptive activity. According to George D. Marler of the National Park Service, this geyser developed from cracks caused by the Hebgen Lake Earthquake of August 17, 1959. (Fig. 51)] [Illustration: OLD FAITHFUL IN FULL ERUPTION. The interval between eruptions averages about 65 minutes, but it varies from 33 to 96 minutes. The time lapse between eruptions can be predicted rather closely, mainly on the basis of the length of time involved in the previous eruption. If an eruption lasted 4 minutes, for example, this means that a certain amount of water emptied from the geyser’s chambers and that a certain length of time will be necessary to recharge the system for the next eruption. But if the previous eruption lasted only 3 minutes, less time will be needed for recharge, and the next eruption will occur sooner. (The above discussion is based primarily on many years of observation and study of Old Faithful by George D. Marler and other observers of the National Park Service; photograph courtesy of Sgt. James E. Jensen, U.S. Air Force.) (Fig. 52)] Mudpot activity differs from season to season throughout the year because of the varying amounts of rain and snow that fall upon the surface to further moisten the mud. Accordingly, mudpots are commonly drier in late summer and early fall than they are from winter through early summer. Fumaroles Fumaroles (from the Latin word _fumus_, meaning “smoke”) are those features that discharge only steam and other gases such as carbon dioxide and hydrogen sulfide; hence, they are commonly called “steam vents.” Usually these features are perched on a hillside or other high ground above the level of the flowing springs. In many fumaroles, however, water can be heard boiling violently at some lower, unseen level. Thermal explosions A few features present in the Yellowstone thermal areas display evidence that extremely violent thermal explosions occurred in the past, particularly during Pinedale Glaciation, about 15,000 years ago. Such explosion features, of which Pocket Basin in Lower Geyser Basin is a good example, appear as craterlike depressions a few tens of feet to as much as 5,000 feet across surrounded by rims of rock fragments that were blown out of the craters. The underground mechanism causing the explosions was similar to that of geysers, but in these special cases the energy remained bottled-up until a very critical explosive stage was reached.