Knowing that lots of people would like to learn how geysers work, I’ll take a stab at explaining the requirements and the mechanism as simply as possible.
What is required to create a geyser and what’s happening when it erupts? All geysers have four requirements: water, heat, the right kind of rock, and a system of conduits and reservoirs (plumbing) that includes one or more constrictions to keep water and steam from flowing freely to the surface. The water pooled above such constrictions acts like a lid to maintain pressure on water below. Such constrictions differentiate geysers from the much more common hot springs. Although no one has yet been able to see exactly what this plumbing looks like, it must be something like this diagram
drawn by my husband Bruno Giletti for Yellowstone Treasures. The countless different shapes the underground plumbing may take must account for the great variety in the patterns and timing of Yellowstone’s geyser eruptions—what I call their personalities.
Whether the water comes from the vent in the distinctive cone of Beehive Geyser
or from the beautiful geyserite-ringed pool of Great Fountain Geyser, 
the mechanism is the same.
Deep below the geyser’s vent or pool seen at the surface is a kind of rock that is rich in silica. In Yellowstone this rock, called rhyolite, was deposited in huge amounts when the caldera erupted. Silica is the primary element in the hydrous silicon dioxide (technical name for geyserite) that is deposited both along the conduits of the geyser’s water passages and all around its surface vent.
Also deep below a geyser are (1) a source of heat—in Yellowstone it is the still extremely hot volcanic rocks more than two miles below the surface—and (2) the water that has seeped into the earth from snowmelt and rain (meteoric water), and voila!—the geyser erupts.
As water gradually fills the reservoirs and is heated from below, it becomes superheated and forms steam bubbles, and the water pressure increases. Eventually, the steam pushes some water out of the vent, and steam—which requires something like a thousand times more space than water—is also released. When the pressure is thus sufficiently reduced, the steam in the conduit can blow the water column out of the vent. In many geysers, when the water supply is exhausted, the remaining steam continues for the steam phase of the eruption.
I have gleaned some ideas for this post from the sidebar my husband Bruno Giletti, a geochemist, wrote in Yellowstone Treasures (pages 88 and 89) and from T. Scott Bryan, a former ranger and also a geologist and author of books about geysers, including his Geysers: What They Are and How They Work. But as early as the mid 19th century, German scientist Eberhard von Bunsen and others already had it pretty well figured out. You can find out more about the study of the geyser mechanisms here in my September 2013 post.
