A geyser must have a nearly vertical underground tube that connects with side chambers or porous rock, where water can accumulate and act as a reservoir. A geyser must also have a constriction or narrowing near the surface in the natural plumbing system. This constriction acts as a check valve, like the workings of a pressure cooker. Water does not reach the surface in a fumarole. Only steam and gases echo up its throat, causing it to hiss and roar.
Geysers maintain a delicate balance between water and steam.
A slight change can upset an eruption. A pre-eruptive splash may trigger a major eruption or cause a delay. Man can also have this same effect. Coins, sticks, stones, handkerchiefs, or soap thrown into a Yellowstone thermal feature can cause it to erupt prematurely, or more likely, cause it to clog, wither, and die. Earthquakes also play a major role in upsetting the delicate balance of geysers.