In addition to a heat source, geysers need a constant supply of water and an underground plumbing system that holds the water supply as it heats up. We'll examine each of these separately and then explore how they work together to cause geysers to erupt.
- Water supply: Clearly, a geyser needs a water supply. What might not be so apparent, however, is where the water in a geyser's eruption comes from. The answer depends on the geyser. Many geysers are located near rivers and pull water from them. Others seem to depend more on rain and snow filtering miles beneath the ground for their water supply.
- Plumbing system: A geyser's plumbing system is a series of fissures in the Earth that start at the geyser's mouth and run miles beneath the Earth's surface, where the system connects to the geyser's heat source. Each geyser's plumbing system is different: Some consist of a single long, vertical shaft, while others connect to large pockets of water beneath the surface. Certain types of soil are much more suitable for the plumbing system's formation, particularly soils that contain a high concentration of rhyolite, a volcanic rock that contains minerals that seal the geyser's plumbing system [source: National Park Service]. This mineral lining is crucial to the geyser's formation, as geysers operate under tremendous pressure and the plumbing system must be able to contain this pressure for the geyser to function.
- Heat source: We already know that geysers occur in areas of high geothermal activity, but where does that energy come from? The water in geysers is heated by magma that lies around 3 miles (4.8 kilometers) beneath the surface of the Earth. While that might seem like a long way down, magma underneath a geyser field is actually relatively close to the Earth's surface. There are different reasons why the magma is so close to the Earth's surface, though many geyser fields are located on the edges of the Earth's tectonic plates. These plates, which compose the Earth's lithosphere, are constantly in motion, creating faults and generating tremendous energy. This activity can cause earthquakes and volcanoes, and it can also create heat sources for geysers.
In the next section, we'll learn why the boiling point of water is so critical to understanding how a geyser erupts.