A helicopter survey mapped the plumbing under Old Faithful and Grand Prismatic as deep as two miles

A helicopter towing an electromagnetic instrument on an 80-foot-wide hoop has produced the first park-wide picture of the plumbing beneath Old Faithful, Grand Prismatic Spring and the rest of Yellowstone’s thermal features. The airborne survey, published by U.S. Geological Survey scientists and collaborators in 2022, imaged the ground to depths of 1 to 2 miles, far below anything a surface observer can see.

Help yourself and the planet: The Cool Down makes it easy to help yourself while helping the planet, with smart, simple changes to save more and waste less in your everyday life.

Get The Cool Down →

We may earn a commission if you sign up.

The result answers a question that visitors and volcanologists have both asked for decades: why do hot springs, geysers and mud pots appear where they do? The short answer is that the water follows hidden cracks, and the survey is the first to trace those cracks across the whole park instead of one basin at a time. Because the instrument covers ground and not just a single spring, it can compare areas with very different thermal behavior using one consistent measurement.

An 80-foot hoop beneath a low-flying helicopter

The method is an airborne electromagnetic and magnetic survey, described in the USGS national news release on the study as an electromagnetic instrument on an 80-foot-wide hoop beneath a low-flying helicopter. The instrument measures differences in electrical conductivity and magnetic properties between wet and dry rock. Hot, salty or fluid-filled ground conducts electricity well, while dry, cold or solidified rock does not, so the contrast draws a map of where water and heat move below the surface.

The survey flew over the park for weeks. USGS likened the output to a CAT scan of Yellowstone, and the team built more than 100,000 models spanning the entire park from the data. Carol Finn, a USGS research geophysicist and the study’s first author, said it was also one of the largest and highest-resolution airborne geophysical surveys of its kind. Paul Bedrosian, a co-author who led the modeling, worked with scientists from the USGS, Virginia Tech and Aarhus University in Denmark.

Faults, fractures and shallow horizontal pathways

The central finding is that most thermal features sit above buried faults and fractures that carry fluids upward. The park holds more than 10,000 active hydrothermal features, and the release names five that illustrate the pattern: Old Faithful, Grand Prismatic Spring, Mammoth Hot Springs, Mud Volcano and Morning Glory Pool. Each is the visible end of a pathway, and the survey shows those pathways as real structures with measurable depth and shape rather than as a diffuse zone of warm ground. For a visitor standing at the boardwalk rail at Grand Prismatic, the colored rings are the surface expression of a plumbing system that reaches a mile or more down.

Shallow horizontal pathways add the second ingredient. They bring colder groundwater into the system, where it mixes with hot fluids rising from below. Constrictions in the pathways can cause boiling, degassing or conductive cooling, and that variety of outcomes helps explain why the park has everything from turquoise pools to erupting geysers and bubbling mud.

Mike Poland, the scientist-in-charge of the USGS Yellowstone Volcano Observatory, said that without the new geophysical data there had been no way of knowing these pathways existed. The observation matters because the pathways were invisible to ordinary mapping, even though the features they feed are among the most photographed landscapes in the country.

Three factors that shape the springs

Bedrosian summed up the controls in the release with the line “It boils down to three factors.” The first is the location of the underlying magma, which supplies heat. The second is the location of fractures and faults, which supply routes for fluid. The third is the elevation of the surface relative to groundwater, which sets where water can reach the surface and what happens when it does.

That framework turns a collection of famous landmarks into a system with rules. A feature appears where heat, a plumbing route and a favorable elevation line up, and it behaves differently if the route narrows or if cold water enters. The same logic helps scientists reason about which areas of the park might change over time, although the release does not offer forecasts and the survey is a snapshot of structure, not a prediction of eruptions.

Public models and survey data on ScienceBase

The Nature news article covering the work appeared the same day as the USGS release, March 23, 2022. Researchers who want to test the interpretation can start with the published models on ScienceBase and the original survey measurements in a second ScienceBase record, both listed by USGS.

For current conditions in the park, as distinct from the structure beneath it, the Yellowstone Volcano Observatory publishes monitoring updates and is the place to check for status. The survey itself does not change that monitoring picture: it supplies a map of the plumbing against which future observations can be read.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

Help yourself and the planet: Get The Cool Down →

We may earn a commission if you sign up.