Ground water is everywhere. When it is heated up it gains energy which creates pressure if it is in a chamber. The release of this energy in nature is what causes geysers to erupt. These natural phenomena happen because there is a heat source near ground water and a blockage that prevents them from erupting until the pressure is great enough to release the energy stored
(figure one). Much like a tea kettle when it water is being boiled.
Figure 1
These marvels of nature are rare and have two main forms cone geysers and fountain geysers. The main difference between the two is the surface where they erupt. Cone geysers erupt in from a cone with a narrow stream, while fountain geysers erupt below a pool of water and bubbles through. In figure two we have an example of a cone geyser, and in figure three we have an example of a fountain geyser.

Figure 2
Figure 3
Eruptions of geysers happen because the groundwater cannot escape until the heat and pressure of the groundwater is great enough to go through the blockage which keeps the energy circulating underground. This is what makes geysers easily predicable. Due to a need for a certain amount for pressure, geologists know that certain conditions must be met for the geysers to release the water that they are holding. This allows geologists to be able to predict when the

geyser is going to erupt to a certain degree, which is done by guessing using the observations of the previous intervals and past characteristics of eruptions (such as pre-eruption characteristics).
What makes the geysers harder to predict is if there are earthquakes, causing a change in the plumbing systems in which the ground water flows. Because of an earthquake the timings of the eruptions may become longer or shorter depending on if routes collapse or of if blockages are cleared. Another reason for a change in eruption schedule would be because the water table has changed. If the water table has lowered there is less of a chance that the geyser will erupt on time, because the geysers need water to cause their majestic overflows. The ground water may change due to construction, or due to drought/ heavy rain/ snow melt. All of these factors can contribute to one of the most beautiful spectacles that this world has to show.
Citations
United States. National Park Service. (n.d.). National Park Service. Retrieved December 4, 2015, from http://www.nps.gov/yell/learn/nature/geysers.htm
United States. National Park Service. (n.d.). National Park Service. Retrieved December 4, 2015, from http://www.nps.gov/featuers/yell/ofvec/