Originally it was a small hot spring that occasionally erupted. Slowly it sealed itself closed with geyserite so that pressure build up and in 1985 it started erupting continuously. Pressure continued to built up so much that in 1989 it exploded. The explosion turned the geyser back into a small hot spring that is now it working on sealing itself up again. Old Faithful is one of the largest and most predictable geysers at Yellowstone National Park.
Earthquakes often change the function of a geyser.
In 1959 when an earthquake hit Hebgen Lake, Montana, near Yellowstone, many geysers went off simultaneously, often with heights and durations never seen before. While most of the geysers resumed their normal activity in a few weeks, some were changed forever. Man's activity can also have an effect on geysers. Vandalism of a geyser can kill it. Minute geyser in Yellowstone was close to a 19th century stagecoach stop and travelers waiting for coaches would toss coins into the geyser.