Note: This experiment shoots boiling water into the air, and should not be attempted at home without proper training and safety equipment!
Geysers are one of nature’s most incredible spectacles. In the most powerful eruptions, water can be shot over 300 feet–or the length of a football field–into the air!

The Castle Geyser in Yellowstone National Park erupts. Water droplets in the air produce a picturesque rainbow against the blue sky. Photo credit: Brocken Inaglory
But just how do these things work? Geysers require some very specific conditions, which means that they only exist in certain locations. In fact, over half of the world’s geysers live in Yellowstone National Park. To find out why, let’s look at how one of these actually work!
When a geyser erupts, it is going through something known as a hydrothermal explosion, which sounds awesome, if a little complex. Luckily for us, the key to this is something familiar: boiling water. Even more fortunate, most of us have never seen a violent eruption of steam from the stovetop while cooking dinner.
The key to this is something called superheating. Most of us are familiar with the phases of matter. In everyday life, many of the things we see exist solely in a liquid, gaseous, or solid state. However, any material can be in any of these states depending on its heat and pressure. Below is the phase diagram for water. On the left is pressure which gets higher as we move up the diagram, and on the bottom is temperature, which increases as we move right.

Phase diagram for water. In geysers, high pressure can push water’s boiling temperature from 212℉ to almost 500℉, well above the temperature required to burn wood! Image from Pearson Education.
To make a geyser, we need to have superheated water, which simply means water that is boiling at higher than the normal boiling point. You can see on the diagram, increasing the pressure–moving up on the diagram from the normal boiling point–means that water stays in a liquid state until a higher temperature. Bingo! I think we just found out how geysers work!

There is one more piece of the puzzle: How do we increase the pressure? By capping the boiling vessel, we don’t allow the air out. Capping the flask doesn’t instantly increase the pressure. Instead, as the water boils at the original pressure, liquid water turns into water vapor, which takes up about 1500 times as much space as in ts normal form. To fit in the fixed volume available to it (thanks to the cap) it has to become more tightly packed, which increases the boiling point slightly. This process repeats, causing the pressure to continue to rise and pushing the boiling point higher and higher!
As soon as the flask is open to the air again, we get our hydrothermal explosion: all of the compressed water vapor expands to its normal size flinging the surrounding water into the air, and water above the boiling point follows suit!

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