Michael Manga and his colleagues report on their findings in the February issue of the Journal of Volcanology and Geothermal Research. They spent years studying geysers in Chile and at Yellowstone National Park (which is home to half of the world's 1,000 geysers). They found that geysers like Old Faithful really were just that, erupting with shocking regularity.
Manga's model isn't quite so reliable. But it does follow the same principle of periodic eruptions that his measurements suggest are due to kinks in the geysers' pipes. These underground twists and turns trap steam, forcing it to slowly bubble out and heat the water column above.
"Most geysers appear to have a bubble trap accumulating the steam injected from below, and the release of the steam from the trap gets the geyser ready to erupt," Manga said in a statement. "You can see the water column warming up and warming up until enough water reaches the boiling point that, once the top layer begins to boil, the boiling becomes self-perpetuating."
These bubbles of steam cause the mini-eruptions (called preplay) that can be seen at Yellowstone. And when the big eruptions occur -- shooting the entire water column out of the ground -- most of what actually comes out is steam. Onlookers see a huge stream of water because that steam quickly condenses as it leaves the ground.
All of this is fascinating stuff (clearly), but Manga is actually more interested in volcanoes. Volcanic eruptions seem to be similar in some ways to geyser eruptions, but it's much easier to drop instruments and cameras into almost-boiling water than it is to drop them into liquid magma.