A geyser is a geothermal feature that periodically emits a fountain of hot water and steam. Geysers are spectacularly rare. There may be something like 1,000 in the world. They are unusual because there must be a specific confluence of factors to produce them.
An underground heat source is needed, to bring the water past boiling temperature. There must be ample water to trickle down to the heated area. In the heated area, there must be a reservoir with a constricted entrance, to trap the water while it is heating. There must be a vent, a fissure leading to the surface, to carry the superheated water upwards once it is hot enough to make a fountain of steam and hot water.
The underground heat source for a geyser is a pool of magma, molten rock from a hot spot in the earth, also associated with other kinds of volcanic activity. Yellowstone, which has the largest number of geysers of any location in the world, is actually the site of an enormous ancient volcano. It almost certainly will not erupt again in our lifetimes, but its huge underground pool of magma powers the famous geysers as well as hot springs and other volcanic features. Hawaii sits above a hot spot. Another site is the Valley of the Geysers in Kamchatka, where geysers are found among hot springs, mud pots, and fumaroles.
A hot spring is a source of water heated by magma, but because the fissure that brings the water to the surface is not constricted, no pressure builds up and there is no geyser. A fumarole emits gases and vapor; it is a hot spring that boils off any water it may contain before it reaches the surface. Fumaroles that contain bad-smelling sulfur compounds are called solfatara, from a word for sulfur in Sicilian. A mud pot is a hot spring with a high ratio of dissolved minerals to water. Mud pots are usually pale or tan; if colored by minerals or organisms they are called paint pots.
Without a water supply, there can be no geyser. Several geysers worldwide have been "turned off" by geothermal power projects that did not replace the water they used. Iceland uses geothermal power extensively, but the water it withdraws is replenished. The Geysers geothermal plant uses recycled water from the city of Santa Rosa to recharge the water table. Though the area had long been called The Geysers, it contained only fumaroles and hot springs until geothermal energy was produced by drilling wells that created artificial geysers.
The determining factor for a geyser is really the constriction in its vent. It holds the water in place until it is so charged with heat energy that it must burst into a fountain. Water percolates down from the surface and flows into a reservoir where it begins to heat. As it heats, pressure from cooler water above it seals it into a chamber. It cannot escape.
The water heats to above boiling temperature, but it does not boil, because the pressure above it is too great. Just as in a pressure cooker, the boiling temperature rises. The temperature increases, until the water is superheated. Finally, it becomes hot enough that some water forces its way through the narrow place in the vent. Instantly, it expands, forcing the water above it higher. This relieves the pressure on the water in the reservoir, and it too becomes steam. Steam takes up at least 1,500 times the space that an equal amount of water occupies.
This immense expansion is what causes the fantastic fountains of a geyser. After the explosion, water begins to trickle down into the reservoir again. It heats again as it refills. If the water flow is constant, the geyser will erupt again at regular intervals. At Yellowstone, the geyser called Old Faithful is somewhat predictable because it has its own isolated water supply. The geysers surrounding it mostly share their water supply. Therefore, the eruption of one may throw off the recharging of another, and their eruptions are less regular.
Geyserite, or siliceous sinter, is associated with geysers, as you might guess from the name. It is dissolved out of the igneous mineral rhyolite underground and deposited in and near the vents of the geyser. It may help to constrict and harden the opening above the reservoir of a geyser. Cone geysers form mounds of geyserite above ground, and erupt within them. Fountain geysers may explode from underwater.
Thermophiles and Hyperthermophiles are life forms associated with geysers. These are prokaryotes, organisms without a cell nucleus. They live where nothing else that we know of can, and thrive there. Thermophiles have been useful to humans; for example, they are used to produce enzymes for PCR, which is used in genetic research. Some of these organisms produce the strange colorings found at many geothermal features.
Geysers are rare because the circumstances that produce them are rare. They are short lived, in geologic terms, because they depend on hot magma, which may cool, on a water supply, which may be diverted, and on a tight-necked reservoir, which may alter with time or circumstance. Geysers are one of the great wonders of nature, predictable to a degree, yet still astonishing in their displays of grace and power.