Karen Harpp, assistant professor of geology at Colgate University, provides this explanation:

A GEYSER IN ACTION. Located in Calistoga, Calif., this geyser also goes by the name Old Faithful.

Imagine yourself exploring a desolate, rocky region in Iceland or Yellowstone National Park, minding your own business and admiring the volcanic formations around you. The ground is relatively flat but covered in mounds of yellow and white material with a vague but distinctive smell of rotten eggs. The entire area is mysteriously foggy, framed by trails of vapor rising from cracks in the earth everywhere you look. Suddenly, steam and hot water burst out of the ground in a spectacular fountain that lasts for several minutes, reaching hundreds of feet in the air and making a roar like a freight train speeding right past you. Just as quickly as it began, though, the event ends, leaving small pools of steaming water in a wide circle around the vent.

What you would have witnessed is a geyser eruption, one of natures most impressive displays of hydrothermal energy. They occur where magma lies just below the earths surface, particularly in volcanic regions such as Iceland or New Zealand, and places that have been volcanically active in the past, including Yellowstone in Wyoming. Water from rain or melted snow percolates into the ground through cracks and fractures and interacts with the hot underlying rocks. The water reaches temperatures far above where it would boil on the earths surface (about 100 degrees Celsius), but because there is so much rock above the water (sometimes up to several miles), the water does not boil. Instead it becomes superheated and pressurized. Once enough pressure builds up, the superheated water will overcome the weight of the overlying rocks and burst out of the ground in an explosive steam eruptiona geyser. It basically works like a teapot with a closed lid; only when enough pressure builds up from accumulating hot water and steam is there enough force for the steam to burst out through the top and activate the whistle.

One of the most fascinating aspects of geysers is that once they form, they become self-perpetuating. After the initial eruption of hot water, the pressure on the superheated groundwater is reduced, which causes some of it to flash to steam (when you reduce pressure on a liquid, it becomes easier for the individual molecules to escape into the vapor phase--even without an increase in temperature). Because it is a gas, the steam expands rapidly, causing it to burst upward through the small, tight fractures in the rock, forcing out any hot water that was left behind after the initial blast. Once the channels are empty, the eruption ends and the cycle begins anew. More water seeps into the hot areas along the fractures, heats up and starts building pressure all over again.

Some geysers go through the pressure-building cycle quickly, producing fountains every few minutes. Old Faithful in Yellowstoneone of the most famous geysers in the worldputs on its show approximately every 80 minutes and can reach up to nearly 200 feet in height. If you observe Old Faithful in action for several eruptions, you can begin to see how the pressure-building process plays a role in its behavior. When the eruption is short (less than about two minutes in length), the next blast usually happens within about 45 minutes, a relatively short interval of inactivity for this geyser. But when the eruption is more powerful (up to five minutes long), it will take more time for the pressure to reach the critical level, and you could be waiting nearly an hour and a half for the next event.

Other geysers are not such reliable performers and have not erupted in years, such as Steamboat, also located in Yellowstone. Often, however, these infrequently erupting geysers can be some of the most spectacular when they do finally burst into action. Steamboat, for instance, has been known to produce blasts extending nearly 400 feet in the air, one of the largest in the world.

There are hundreds of geysers around the worldestimates are up to 700and Yellowstone boasts nearly half of them. The name "geyser" comes from the Icelandic word "geysir," for "gush," and the original namesake is found in Iceland, in the Haukadalur Basin. Unfortunately, it only erupts occasionally, which illustrates another important point about geysers. They are ephemeral features, usually only lasting for several thousand years or until a major geologic event occurs. For instance, an earthquake of magnitude 6.1 in 1975 in the Norris Geyser Basin of Yellowstone caused greater water volumes to flow in some geysers and decreased the activity of others by rearranging groundwater paths and fracture patterns throughout the area. Even earthquakes as small as magnitude 4 can change the behavior and eruption intervals of geysers. Moreover, it turns out that earthquakes may be necessary to keep some geysers active. Over time, water conduits feeding geysers can become narrower or even clog as a result of deposition of minerals from the hot water (just like pipes can clog in areas with hard, mineral-rich water). Frequent tremors may break up the deposits or create new fractures, providing alternative pathways for the water to move through the rock and keep the geysers supplied.

Answer originally published on January 7, 2002.