Why does a geyser erupt in bursts?

A geyser's pool can look almost quiet before a column of hot water bursts upward. The surprise is not simply that water underground is hot. A geyser also needs natural plumbing that lets heat build up and allows a change in pressure to trigger rapid boiling.

Rainwater or melted snow can seep into underground cracks and spaces. Hot rock transfers energy to this water. A geyser's passages include narrow sections that restrict circulation, so hot water cannot always escape as freely as it does from an ordinary hot spring. Water above also presses down on water deeper in the system.

That pressure changes when boiling begins. Under greater pressure, liquid water can become hotter before turning into steam. Deep water can therefore remain liquid at a temperature that would make it boil closer to the surface. The water is hot, but its surroundings are still pressing hard enough to limit boiling.

Eventually, boiling and expanding steam can push some water out of the opening. With less water weighing down from above, pressure below falls. More of the hot water can now boil. The new steam takes up far more space than the liquid water that formed it, pushing out still more water and reducing pressure further. This reinforcing sequence helps turn the first overflow into a fountain.

The eruption ends as stored water and energy are depleted. Water can refill the passages and begin heating again. A geyser's rhythm comes from that cycle of filling, heating, pressure change and release. Different underground shapes produce different rhythms, and changes in those hidden passages can alter a geyser that once seemed predictable.