Old Faithful has never in recorded time erupted every hour on the hour, as you may have heard. Here are some figures given for average expected intervals between eruptions: 60 to 65 minutes, 1870 (Langford’s journal); 65.5 minutes, 1964 (Haynes Guide); 78 minutes, 1990 (Bryan’s Geysers ofYellowstone); and 92 minutes, 2008 (visitor center records). These figures do not show the range of intervals, which has always been on the order of 0.5 to 1.75 hours. Seismic activity can affect Old Faithful. Following the strong earthquake at Borah Peak, Idaho, in 1983, the average interval between eruptions became greater, at least for a while.
Geologists suspect that the geyser may someday seal itself off: a probe sent down its conduit a few years ago revealed a very narrow throat about 22 feet (6.5 m) down, and there’s another constriction about 70 (21 m) feet down. Silica may continue building up there, but how fast is anybody’s guess. On the other hand, the geyser’s water may actually be eroding away the narrow section. No one knows for sure. In recent years, there’s been a rumor that the geyser is losing its power or becoming less regular or much less frequent. The truth is that the average interval between eruptions has lengthened by a few minutes, but the average height has remained about the same for at least 130 years, varying from 106 to 184 feet (32-56 m), averaging about 145 feet (44 m).
Eruptions range in length from 1.5 to about 5 minutes. Predicting Old Faithful’s Eruptions Ask a geyser gazer the rule for predicting Old Faithful’s eruptions on a summer day and you would hear: If there is a big crowd on the benches, the eruption will occur within about 15 or 20 minutes. A more precise prediction method involves timing the duration of an eruption from the first heavy surge of geyser water through the last splashes of water above the cone. The longer an eruption lasts, the longer the interval to the next eruption. For example, a 2-minute eruption results in about a 55-minute interval; 3 minutes, a 72-minute interval; 4.5 minutes, a 93-minute interval.