A geyser is a vent in rock that periodically puts forth a blast of super-heated water and steam. It is likely that most people already know this. However, the cause of the jet of hot water that makes up a geyser eruption may be a little more mysterious.
Geysers are initially formed when volcanic activity finds or forms vents in the rock. Vents allow volcanic gases to escape into the air from inside the earth. The hot magma is often fairly close to the surface, and without the vents, the gases could easily build up until the rock explodes. This sometimes does happen, in fact. Pictures depicting the moment of explosive eruption on many volcanoes often show rocks and boulders being blasted high in the air by the force of the bottled up gas that was under them until there was a release.
The gases are both corrosive and under pressure. A weakness in the rock allows the gas to escape, though the force and corrosive nature also tends to widen the vent, particularly in softer rock.
Shape of the vent
A common misconception is that these holes in the rock are straight up and down, like a smooth tube. Occasionally, vents are shaped this way, however more often they twist and turn around hard solid rock that lies below the surface. At times, rock will jut out somewhere in the neck of the vent, creating a narrow 'bottle-neck' through which the volcanic gases must pass in order to escape. Such a restriction is needed for a geyser to form.
Presence of water
A geyser also can't form without the presence of ground water. The moisture, trickling down to the obstruction in the vent, creates a water plug. The gases rising from the magma prevent the water from going any farther down, and yet the weight of the water prevents the gases from moving upward. Since the gas mixture from below is extremely hot from the magma, the water at the bottom of the plug begins to super-heat and turn to steam, though it is also prevented from escaping in the same way that the volcanic gases are.
At some point, the pressure from below becomes greater than the weight above, which is preventing the escape of the gas, water and steam. The result is an eruption of the hot water, gas and steam that is seen as the geyser.
If the water flow is constant, the geyser usually has an eruption that is regular, though there will usually be at least some variation. One of the best known geysers of this sort is the Old Faithful geyser of Yellowstone National Park. The amount and length of the constriction can combine to dictate the height of the eruption, too. This can also vary slightly.
While Old Faithful may be well known, it should be pointed out that there are geysers in many places, including in countries other than just the US. Also, if the water flow is interrupted for some reason, the geyser ceases erupting and will return to being a normal gas vent.
It is also interesting to note that since the volcanic gases often contain quantities of sulfur, which combines with water to make sulfuric acid, eventually the geyser will cease to exist as rocks are corroded away. They can even become mud pots, bubbling like boiling pudding, with the dissolved rock, mixed with moisture, being the 'pudding'.
Geysers originally form as volcanic vents. The mechanism that causes the geyser, however, is a combination of a restriction in the vent, water forming a plug and intense heat to super-heat the water, creating enough pressure for the water, steam and gas to escape the confines of the vent, explosively.