Geysers erupt when an underground reservoir becomes superheated by subterranean magma, building up enough pressure to reach temperatures well above the boiling point. When the water works its way to the surface, it releases the pressure, causing a sudden, massive boilover. This ejects all the water in the reservoir to the surface in a boiling plume, creating a geyser.
Geysers tend to occur in geologically active areas, especially those near active volcanoes. Each geyser requires a regular supply of water to refill its reservoir, access to magma a few miles below to provide the necessary heat, and a specific geological construction in order to build up the pressure necessary for eruption. The cone-shaped structure maintains pressure as the water heats, increasing the boiling point and ensuring a massive explosion once the pressure is relieved. Without this overpressure, a heated reservoir may simply turn into a hot spring.
Yellowstone Park in Wyoming is home to one of the most active geyser fields, containing about half of the known geysers in the entire world. Yellowstone boasts of the world's tallest geyser as well as the famous Old Faithful, which erupts every 91 minutes. Other major geyser fields are in Russia, Chile, New Zealand and Iceland.