Geysers are impressive geothermal events where hot water from deep within the earth is periodically pushed through a small vent to the surface, creating a natural fountain. The water is sprayed from the opening up to several hundred feet in the air along with steam and various gases, including carbon dioxide, oxygen, and hydrogen sulfide. Geysers can erupt as frequently as every few minutes to as rarely as once every several years, and the eruptions can last from minutes to hours. Some geysers are so well studied that their eruption times can be reliably predicted down to the minute. Geysers are not permanent features because the heat source beneath the earth that they rely on is constantly shifting. New geysers can spring up from nowhere while active ones may suddenly stop erupting and even go dry. The term “geyser” comes from “Geysir,” the name of one of the geysers in Iceland. “Geysir” in turn came from the Icelandic word “giosa,” meaning “to gush.”
Roughly one thousand geysers are documented around the world and most are located within about fifty “geyser fields,” geothermally active areas that contain numerous geysers. Geysers are located in Mexico, South America, Japan, and Africa, and the largest geyser fields are located in Russia, Iceland, and New Zealand. However, the area with the most geysers is Yellowstone National Park in Wyoming, which is home to about five hundred geysers. Yellowstone Park is also the location of the most famous and reliable geyser, Old Faithful, as well as the tallest geyser, Steamboat, which erupts once every several years to heights of 300 to 400 feet.
Conditions Needed to Form a Geyser
It's obvious that geysers need a certain combination of conditions in order to form since they only occur in a few areas of the world. Geysers need three things, a source of water, a source of heat, and a reservoir system under the ground. The first two are relatively simple to obtain. The water comes from groundwater boosted by rainfall or snowfall. Additionally, some geysers obtain water from nearby rivers. All documented geysers are located over a shallow, volcanic heat source to obtain heat. This heat is needed to boil water and cause eruptions. However, the most important condition needed to form a geyser is the underground reservoir system. This system typically consists of a thin pipe leading from the surface opening to a chamber that holds most of the water. Most geysers have a system built into a special type of volcanic rock called rhyolite. This rock contains a high concentration of silica, which helps to create a water- and pressure-tight seal around the reservoir. Additionally, there must be several areas of constriction within the chamber and pipe. These constrictions help build up pressure in the reservoir that forces the water and steam to the surface.
If the conditions are not properly met, different geothermal features will form instead of geysers. For example, if the reservoir system is not constricted or the pipes are too large, if the temperature is too low, or if there is too much water then the geyser cannot erupt and a hot spring forms. If the water is too acidic or if the rock is too permeable then a mud pot forms. If there is enough heat but not enough water then a fumarole, or steam vent, forms.
Geysers can be generally classified as either fountain or cone geysers, depending on the structure of their reservoir system and opening:
- Cone geysers have a single, straight pipe underground that is connected to a chamber that holds water. The opening at the surface of these geysers has a cone around it that was created from minerals in the water. Cone geysers typically have long, high, and predictable eruptions with a continuous spray of water and steam. Old Faithful in Yellowstone is an example of a cone geyser.
- Fountain geysers have a reservoir system very similar to cone geysers, except the opening is within a filled pool like a hot spring instead of inside a cone. Eruptions from these geysers tend to be shorter in time and height than cone geysers. The eruptions also tend to not be a continuous spray, but instead are broken up by periods of inactivity. Great Fountain Geyser in Yellowstone is an example of a fountain geyser.
Additionally, there are some geysers that don’t fit in either category. For example, some geysers have an eruption similar to cone geysers but their opening is flat instead of cone shaped. Other geysers may have multiple chambers in their reservoir system instead of one, causing a series of eruptions.
How a Geyser Erupts
The eruption of a geyser is a complex, stepwise process involving heat and pressure. In the first step, cool ground water in the reservoir chamber mixes with boiling water brought up from deep within the earth. The chamber slowly fills with hot water and both the temperature and pressure in the chamber gradually increase. The high pressure in the chamber prevents the water from boiling at its normal boiling temperature and the water gets superheated. At a certain temperature above the boiling point some of the water starts to turn to steam and tries to escape through the pipe to the surface. However, the steam cannot readily escape because of various constrictions in the system. The steam builds up as bubbles that eventually make it to the surface along with a small amount of water. This loss of water causes the boiling point of the remaining water in the chamber to lower, which immediately converts much of the water to steam. This drastic increase in volume forces the steam and water out to the surface, causing an eruption. As the geyser erupts there is a steady decrease in temperature and pressure in the chamber. Once both fall below a certain level the eruption stops, the pool slowly refills itself, and the entire process starts over again.
The magnitude and frequency of geyser eruptions are affected by various conditions. For example, increases in rainfall seem to shorten eruption times while decreases in barometric pressure causes more eruptions. Additionally, tides also seem to affect geysers. It’s thought that low tides squeeze the reservoir system, restrict water flow, and inhibit eruptions while high tides open the system, allowing water in and causing more eruptions. The most impressive event that affects geysers is earthquakes. A 1959 earthquake in Yellowstone caused all the geysers in the park to erupt simultaneously and raised the temperature of the water in geysers and hot springs several degrees. Dormant geysers suddenly became active while active geysers permanently changed their eruption patterns. Earthquakes do not need to be directly near the geyser to have an effect either. An 8.5 magnitude earthquake in Alaska was found to have an effect on the eruption pattern of Old Faithful in Yellowstone.
A special class of geysers can erupt even when the water is below the boiling temperature. This is because the geysers contain a higher level of gases such as carbon dioxide or hydrocarbons. The gas lowers the hydrostatic pressure of the geyser so the water can boil at temperatures lower than the boiling point. Such geysers that are driven by gas instead of temperature are called “gassy geysers.” These geysers can be found in oil- and gas-producing regions around the world. Their eruptions tend to be much less predictable than those from typical water geysers.