Yellowstone National Park - geysers

Constrictions in underground plumbing can cause geysers, where hot water erupts under pressure. Because the water doesn’t circulate freely, it doesn’t reach the surface where heat could escape. The deepest water can exceed the surface boiling point. Increased pressure exerted by the enormous weight of the water and rock prevents the water from vaporising. Small steam bubbles rise and expand until they can no longer pass through the tight spots.

Eventually the pressure builds up to the point that the bubbles actually lift up the water above, causing the geyser to overflow. This decreases pressure on the system, allowing violent boiling of the hot water and production of steam. The steam forces water out of the vent in a superheated mass, and eruption begins.

Water is expelled faster than it can enter the geyser’s plumbing, so that the heat and pressure gradually decrease. The eruption stops when the water reservoir is exhausted or the gas bubbles diminish enough that they can rise without ejecting the water.


This one just happened to go off as I walked past. A ranger told me that its eruption pattern is unpredictable and brief, so you have to be lucky to catch it in action.


This geyser, on the other hand, is totally predictable. It has been erupting full time, 24 hours a day, seven days a week for decades. Constant eruption is called a “wild” state, but this one has been at it for so long that it appears to be its normal state now. The only time it calms down for a few minutes is when a nearby large geyser erupts and steals some of its water supply.

I tried staring fixedly at the geyser for a while to test the theory that a watched pot never boils. The geyser haughtily ignored my efforts. Perhaps, with so many other tourists around, it didn’t notice that I was watching. Or perhaps “wild” geysers just have no respect for the power of old folk sayings. Or, possibly, watched pots do boil after all. They just take a little bit longer.