Now that we understand how the key components of a geyser work, let's figure out how these parts work together to create such spectacular eruptions.
The process begins as water migrates to the geyser's plumbing system through fissures in ground. Since the plumbing system is miles deep, the water at the bottom of the geyser's plumbing is under incredible pressure from the water above it. Think of the entire system as a giant pressure cooker.
Pressure cookers work by creating a sealed, pressurized enclosure that helps to cook food much faster than cooking it in a normal pot. Pressure cookers can do this because water's boiling point rises as it's pressurized. Think of what actually happens when water boils: Water starts to churn and bubbles of air begin to surface. If more pressure pushes down on the water, water needs more and more energy (in the form of heat) to overcome that pressure and start pushing bubbles to the surface. This explains why water inside a pressure cooker can reach temperatures of more than 125 degrees Celsius (257 degrees Fahrenheit), while the boiling point of water at standard pressure is only 100 degrees Celsius (212 degrees Fahrenheit).
Now consider the massive pressures placed on water within a geyser's plumbing system. Water in such a system can reach incredibly high temperatures (and store incredible amounts of energy as a result) before it starts to boil. As the magma at the base of the geyser transfers heat throughout the system, more energy gets trapped in the water. Eventually, pockets of water begin to reach their boiling point and become turbulent. This turbulence pushes a relatively small quantity of water out of the opening of the geyser, decreasing the amount of pressure on the water remaining in the geyser. With this sudden pressure drop (and corresponding drop in boiling point), the water in the geyser flashes into steam. The steam quickly expands to 1,500 times the volume of water and this expansion violently pushes water and steam from the mouth of the geyser in an eruption. These eruptions last as long as the water in the geyser remains hot enough to push water out of the geyser opening. Eventually, the entire system will either run out of water or the water will cool down enough for the eruption to stop. The cycle, of course, starts all over again.