Explanation: In this masterfully crafted sample of artistry, four key steps to the production of a hydrothermal eruption are seen. It is important to note that this is not what all geyser systems look like. In fact, it's impossible to know for sure what any truly look like (and they are probably much more complex than the above diagram). Regardless, the above diagram does illustrate the variables that all come together.
1: In the first box we see a chamber that is partially filled with boiling water and steam. Water flowing through the rocks slowly fills the chamber as it makes it's way toward box 2.
2: The chamber has filled to a point where the gas has been trapped in the chamber. Water continues to enter, raising the water level in the conduit (at right). As the water is above the boiling point, steam continues to build, raising the pressure inside.
3: The steam has built to a point where a small pocket can now escape. The sudden release of pressure causes the water to flash boil, propelling it upward violently.
4: The eruption continues until the chamber is sufficiently emptied. Box 1 maybe skipped, going instead to box 2.