Yellowstone pool provides insights into hydrothermal systems

A small pool just north of Yellowstone National Park’s famous Old Faithful geyser is helping University of Utah researchers to gain new understandings of the fluctuations of energy in the park’s extensive hydrothermal system.

Doublet Pool is a pair of hydrothermal pools connected by a small neck, situated on Geyser Hill in Yellowstone National Park. Together, the pools would fit into half a tennis court. Unlike the famous Old Faithful, which erupts periodically into a towering spout, Doublet Pool thumps rather than erupts. About every 20–30 minutes, the ground shakes and the water vibrates.

A new study by University of Utah and published in Geophysical Research Letters shows that the interval between episodes of thumping reflects the amount of energy heating the pool at the bottom, as well as in indication of how much heat is being lost through the surface.

“By studying Doublet Pool, we are hoping to gain knowledge on the dynamic hydrothermal processes that can potentially be applied to understand what controls geyser eruptions, and also less predictable and more hazardous hydrothermal explosions,” says Fan-Chi Lin, a study co-author and an associate professor in the department of geology and geophysics at the university.

“We knew Doublet Pool thumps every 20–30 minutes, but there was not much previous knowledge on what controls the variation [or that] the thumping interval varies,” says Lin.

The thumping lasts about 10 minutes and is caused by bubbles in the plumbing system that feeds water to Doublet Pool, heated by a magma system beneath Yellowstone. When those bubbles of water vapour reach the cool upper reaches of the hydrothermal conduit, they collapse suddenly, hence the thumping noise.

A similar process happens in geysers which excites “hydrothermal tremor”, says Lin, but this occurs deeper in the hydrothermal system and ends with the geyser releasing pressure through a narrow opening as an eruption. Doublet Pool does not have a plumbing structure that enables pressure accumulation and hence no eruption occurs.  

To better understand how hydrothermal systems work, Lin and his colleagues set up geophones around Doublet Pool in seven deployments between 2015 and 2021. In winter 2021 and spring 2022, they lowered temperature and water-level sensors into the pool itself.

Using principles of heat transfer, the authors calculated the amount of heat and the heating rate needed to initiate thumping at Doublet Pool.

“And as we know how to calculate the heat being removed from the wind, we can estimate the heating rate at the base,” says Lin.

The heating rate for Doublet Pool works out to around 3–7 MW of energy. For comparison, it would take about 100 household furnaces burning at the same time to heat up Doublet Pool enough to thump. (This is also equivalent to more than US$5,000 worth of energy daily, which highlights the potential of geothermal energy.)

Knowing the heating rate, scientists can use the silence interval as a measurement of how much heat is coming into the pool, since more heat means a shorter interval.

“A better understanding of the energy budget will also improve our understanding of how much energy from the Yellowstone volcano is released through these hydrothermal features,” says Lin.

More information on the study can be found at the University of Utah.

Photo by amanderson2 at Flickr

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