Old Faithful and the Physics of a Plumbing System

The members of the eighteen seventy Washburn-Langford-Doane expedition watched the geyser erupt twice in a single afternoon. It had erupted reliably both times. The expedition leader, Henry Washburn, named it Old Faithful on the spot, and the name stuck. What he saw as exceptional reliability was, he had no way of knowing, the product of a plumbing system that had been under construction for hundreds of thousands of years.

Old Faithful sits above a narrow underground conduit, a channel of fractured rhyolite that connects the surface to a superheated water reservoir far below. Water seeps in, heats against the magma chamber beneath the park, and builds pressure until it cannot stay liquid at depth. It flashes to steam. The steam drives the column upward. The geyser erupts. Then the system refills and the cycle begins again.

In the early twentieth century, the average interval between eruptions was roughly sixty-five minutes. Today the average is closer to ninety minutes. The change correlates with the nineteen eighty-three Borah Peak earthquake in central Idaho, a major event that rattled the hydrothermal plumbing across the region. Old Faithful's interval lengthened after the quake and has never fully returned to its previous pace. The geyser that visitors see today is not quite the same plumbing system that Washburn named.

The ranger station posts the predicted eruption time on a board inside the visitor center, typically accurate within ten minutes in either direction. What looks like clockwork is actually the park's seismic history, expressed one eruption at a time.