A geyser is a hot spring that periodically erupts, throwing water into the air. Though that sounds simple, geysers are extremely rare. As of August 2008, the total of active geysers on earth numbered approximately 1000.

Conditions must be just right for geysers to occur. Three components must be present for geysers to exist: an abundant supply of water, an intense source of heat, and unique plumbing. Water is common in nature, heat can come from volcanic activity, but the plumbing is critical. For water to be thrown into the air, geyser plumbing must be water- and pressure-tight. Geyser scientists and observers have identified the volcanic rock rhyolite as being particularly effective at hosting geysers. Rhyolite is high in silica, which can deposit a water-tight seal along the walls of the geyser plumbing. Most of the geyser fields in the world are found in rhyolite or similar silica-laden rocks (like ignimbrite). The mixture of water, volcanic heat, and plumbing is exceptional at Yellowstone National Park. Over one-half of the world's geysers are located within the park's boundaries.

It is increasingly apparent that geysers must possess a fourth characteristic to exist: remoteness. Within the last fifty years, volcanic heat and abundant water have been increasingly harnessed to turn turbines for electricity production. Geothermal energy can be produced at any site where volcanic heat and water are readily available. Unfortunately, geyser fields are ideal for this type of energy production. Geothermal energy production steals the geysers' water, and destroys geyser activity (for example, Wairakei, New Zealand). A growing threat to geysers stems from mineral extraction. Hot groundwater may precipitate gold or other valuable minerals, and extraction may require removing the geyser plumbing itself. For example, in May 2003, mineral exploration at South America�s second largest geyser field (Puchuldiza, Chile), caused cessation in the field�s geysers. Few realize the actual rarity of geysers. As a result, many geyser fields have been destroyed and many others are being threatened.

How do geysers work?

The following is an excerpt from Scott Bryan's GEYSERS OF YELLOWSTONE, 3rd edition, copyright 2001. It is reproduced here for educational purposes. Scott Bryan's book not only describes each Yellowstone geyser in detail, but also includes descriptions of geyser fields worldwide. It is probably the best book on geysers out there. Buy it or check it out!

The hot water, circulating up from great depth, flows into the plumbing system of a geyser. Because this water is many degrees above the boiling point, some of it turns to steam instead of forming liquid pools. Meanwhile, additional, cooler water is flowing into the geyser from the porous rocks nearer the surface. The two waters mix as the plumbing system fills.

The steam bubbles formed at depth rise and meet the cooler water. At first, they condense there, but as they do they gradually heat the water. Eventually, these steam bubbles rising from deep within the plumbing system manage to heat the surface water until it also reaches the boiling point. Now the geyser begins to function like a pressure cooker. The water within the plumbing system is hotter than boiling, but "stable" because of the pressure exerted by all the water lying above it. (Remember that the boiling point of a liquid is dependent upon the pressure. The boiling point of pure water 212 degrees Farenheit (100 degrees Celsius) at sea level. In Yellowstone the elevation is about 7,500 feet, the pressure is lower, and the boiling point of water is only about 199 degrees Farenheit (93 degrees Celsius).

The filling and heating process continues until the geyser is full or nearly full of water. A very small geyser may take but a few seconds to fill whereas some of the larger geysers take several days. Once the plumbing system is full the geyser is about ready for an eruption. Often forgotten but of extreme importance is the heating that must occur along with the filling. Only if there is an adequate store of heat within the rocks lining the plumbing system can an eruption last for more than a few seconds. Again, each geyser is different from every other. Some are hot enough to erupt before they are completely full and do so without any preliminary indications of an eruption. Others may be completely full well before they are hot enough to erupt and so may overflow quietly for some time before an eruption occurs. But, eventually, the eruption will take place.

Because the water of the entire plumbing system has been heated to boiling, the rising steam bubbles no longer collapse near the surface. Instead, as more very hot water enters the geyser at great depth, even more and larger steam bubbles form and rise toward the surface. At first, they are able to make it all the way to the top of the plumbing system. But a time will come when there are so many steam bubbles that they can no longer simply float upwards. Somewhere they encounter some sort of constriction or bend in the plumbing. To get by they must squirt through the narrow spot. This forces some water ahead of them and up and out of the geyser. This initial loss of water reduces the pressure at depth, lowering the boiling point of water already hot enough to boil. More water boils, forming more steam. Soon there is a virtual explosion as the steam expands to over 1,500 times its original, liquid volume. The boiling rapidly becomes violent and water is ejected so rapidly that it is thrown into the air.

The eruption will continue until either the water is used up or the temperature drops below boiling. Once an eruption has ended. the entire process of filling, heating, and boiling will be repeated, leading to another eruption.