A team of scientists from UC Berkeley set out to explain why geysers erupt. They found that boiling water in side chambers underground were the necessary ingredients.
The looping chambers trap steam from the hot water. Escaped bubbles from trapped steam heat the water column to the boiling point. When the pressure from the trapped steam builds enough, it blasts, releasing the pressure. As the entire water column boils out of the ground, more than half the volume is this steam. The eruption stops when the water cools below the boiling point.
Michael Manga, a UC Berkeley professor of Earth and Planetary Science explained that the new understanding of geyser mechanics comes from studies over the past few years of geysers in Chile and Yellowstone, as well as from an experimental geyser he and his students built in their lab. Most of the research inside Yellowstone was at the Lone Star geyser in the backcountry about 3 miles southeast of Old Faithful.
The video below that shows a test tube "geyser" with clear looping chambers underneath. You can watch it slowly bubble until the pressure is reached, creating a blast.
The detailed findings of the study have been published in the February 2015 issue of the Journal of Volcanology and Geothermal Research.
Manga plans to continue the study and will be doing research in Yellowstone in the Fall of 2015.