Old Faithful: How It Works

Illustration by Mika Grondahl

A geyser is a natural hot spring that erupts with a gush of steam and water. The most famous of these natural wonders, known for its predictability and beautiful plume, is Old Faithful in Yellowstone National Park. The average interval between its spoutings is 78 minutes. Here’s what sets Old Faithful and other geysers off.

A geyser is the product of heat, water, and some plumbing. The heat comes from partially molten rock, or magma. Underground water is supplied by rain and melting snow that seep through a vast system of cracks and fissures. These same passages provide escape hatches for steam and water exploding upward. Mineral deposits narrow these tubes into nozzles at points.

1. Magma forms from the melting of the continental plates as they slide over one another. Magma is much closer to the surface at geyser sites than elsewhere.

2. Magma heats the rock above it, which heats the water that comes into contact with the rock. The water temperature can soar to 660