Yellowstone: a groundwater map to reveal the secrets of the caldera

To the long series of volcanic sites that seem to be linked to a hot mass of material that could reach the Earth’s mantle, we also add the huge volcanic caldera of Yellowstone Park. Researchers have long been at work trying to track down this hot material and the volcanoes that are connected to it, as some of the eruptions that originated from it have been nothing short of colossal. Knowing and mapping these sites, therefore, could help us prevent catastrophes related to volcanic eruptions.

Groundwater is difficult to map

Researchers argue that a good way to identify the volcanoes connected to this mass of hot material, both trying to map the connections between it and some geothermal features such as geysers and hot springs. These connections are very difficult to trace and map, but a new study, conducted in Yellowstone, proposes a map of thermal and hot groundwater and how and why these waters reach the surface in some specific points, such as geysers and springs.

Speaking of groundwater, we don’t have to imagine rivers, streams and streams. The situation under our feet is in fact much more complicated. The water creeps through permeable materials, its path is altered by faults and hard, impermeable rocks, and slips through the cracks and fissures in the rocks. Mapping the flow of groundwater is therefore a far from simple task.

Mapping of the waters beneath the Yellowstone caldera

Fortunately for researchers, hot, mineral-rich water like Yellowstone’s hot, thermal water creates gods changes in electric fields which can be mapped. To get a clear picture of Yellowstone, the researchers flew a helicopter over the caldera following a pattern with a series of lines spaced just 250 meters apart.

On board the helicopter was installed an instrument sensitive to electromagnetic fields, capable of measuring the resistivity and magnetic susceptibility of the area under the helicopter. Thanks to this type of data, the researchers were able to create a model of these properties and characteristics in the subsoil of the caldera, up to a few hundred meters below the surface.

For the research team it was possible to create this three-dimensional model also thanks to ours knowledge of the structure of the rocks under Yellowstone. A knowledge obtained thanks to several wells and cores practiced in different points of the site.

The importance of knowing how the water moves under Yellowstone

Thanks to this model, the team of researchers has revealed what it is the fascinating plumbing which feeds the geysers and hot springs that make the park popular with visitors and tourists.

They have in fact been identified the vast regions filled with warm water under many of the geyser basins in the caldera. To our surprise we also learned that often these areas were not directly beneath the geysers, but were often covered with rocks and clays that did not allow the water to flow smoothly upward. In many cases, hot water only moves upward through faults that created cracks in the otherwise impermeable rock: the faults appeared as well-defined strips of hot water.

Once closer to the surface, the hot water often ends up in a cap of impermeable volcanic deposits. In general, geyser fields are defined as an empty space in these deposits, which allows hot water to reach the surface over a relatively large area.

In addition to feeding the geyser fields, some of the hot water moves horizontally along lines of permeable material closer to the surface. This leads in some cases to the mixture of hot water from several faults before it emerges in a geyser. In other cases, the hot water is mixed with groundwater non-thermal springs present in the area, creating moderate-temperature waters that feed thermal springs around the caldera.

What we discovered thanks to the map

The resolution of this first map isn’t great either many details cannot be foundbut it is nevertheless of great help to give an explanation to some previously observed phenomena.

For example, the map showed us that a site associated with swarms of small earthquakes beneath Yellowstone Lake is in fact an area where warm water moves towards the surface. The association between some geysers and specific sources could help explain the difference in the mineral composition dissolved in the water in the different sites.

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