What lies beneath —
Hot, mineral-rich water creates changes in electric fields that can be mapped.
The vast volcanic caldera at Yellowstone National Park is just the latest in a long string of volcanic sites, all of which seem to be linked to a hot blob of material that may go all the way down to the Earth’s mantle. There’s been a lot of effort put into tracing that hot material, given that some of the earlier eruptions from it have been utterly enormous.
But there’s also a connection between that hot material and the features, like geysers and hot springs, that make Yellowstone a major tourist destination. And those connections are very difficult to trace. But a new study has proposed a map that shows how the hot water of Yellowstone flows beneath the feet of visitors and why it reaches the surface at specific sites.
Mapping the plumbing
We tend to talk about water under our feet as traveling through underground rivers, but that creates a misleading image. In reality, water creeps along as a broad flow through permeable materials, its path shifted by things like faults and hard, impermeable rock, like granite. Tracking it isn’t the simplest thing.
To get a clear picture of Yellowstone, the researchers flew a helicopter across the caldera along a series of lines spaced by as little as 250 meters. Onboard was an instrument sensitive to electromagnetic fields, allowing a measure of the resistivity and magnetic susceptibility of the area under the helicopter. With all that data in hand, the researchers were able to construct a model of the properties down to a few hundred meters below the surface.
Water is a very polar liquid, which will alter the electromagnetic properties of any rocks it flows through. Water that has interacted with the hot rocks beneath Yellowstone will contain plenty of dissolved material, altering the water’s electrical conductivity even further.
We know something about the structure of the rocks beneath Yellowstone from having drilled boreholes at specific locations in the site. There are older consolidated rocks, loose volcanic tuff produced by explosive eruptions, and a variety of clays, which may or may not contain hot water. Information on all these materials helped inform the three-dimensional model that the researchers produced.
Lots going on
As you might expect, there was evidence of broad regions filled by hot water beneath many of the geyser basins in the caldera. But these areas were often not directly below the geyser fields themselves. Instead, they were often capped by rocks and clays that didn’t allow smooth upward flows. In many cases, the hot water moved upward through faults that created gaps in the otherwise impermeable rock—the faults appeared as sharply defined stripes of hot water.
Once closer to the surface, the hot water often ran into a cap of impermeable volcanic deposits. In general, the geyser fields are defined as a gap in these deposits, which lets hot water reach the surface over a relatively broad area.
In addition to feeding geyser fields, some of the hot water moved horizontally along lines of permeable material nearer to the surface. Sometimes, this led to hot water from multiple faults mingling before emerging at a geyser. In other cases, the hot water mixed with groundwater present in the area, creating moderate temperatures that feed hot springs around the caldera. At some sites, this warm water flowed right out of the area covered by the helicopter.
The resolution on this imaging isn’t great, and many of the details (such as the reservoirs and cracks under individual geysers) can’t be resolved. But the imaging does help make sense of some phenomena we’ve already observed. It shows that a site associated with swarms of small earthquakes under Yellowstone Lake is an area where hot water moves toward the surface. The association between some geysers and specific sources might help explain the difference in the mineral composition dissolved in the water at the different sites.
So, while the new work isn’t a complete picture of Yellowstone, it knits together pieces of information we already have and could help direct future efforts at understanding one of North America’s most iconic volcanoes.
Nature, 2022. DOI: 10.1038/s41586-021-04379-1 (About DOIs).