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Measuring the heat output of a hydrothermal area is not easy—Earth’s surface is often too noisy for accurate measurements to be made easily. But the floor of Yellowstone Lake is a thermally quiet environment and provides a unique opportunity to assess heat flow in one of Yellowstone’s most dynamic hydrothermal areas.
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Rob Harris, Professor in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University.
Yellowstone is hot. The many thermal features within the 631,000-year-old Yellowstone Caldera, including geysers, hot springs, mud pots, and fumaroles, reflect the enormous quantity of heat being released from the magmatic and hydrothermal systems. Measuring this heat and its variation across the region is important for understanding the energetics of Yellowstone. Heat flux is most directly estimated through observations of the thermal gradient and thermal conductivity. The thermal gradient reflects the increase of temperature with depth, and thermal conductivity is a material property describing the ease at which heat can flow through specific material. The product of these two quantities yields the conductive heat flux.
In continental settings like Yellowstone, determinations of heat flux usually require deep boreholes because heating and cooling of the Earth’s surface due to daily, seasonal, and longer temperature changes at the Earth’s surface affect thermal gradient measurements at shallow depths. These boreholes are expensive and undesirable in environmentally sensitive areas, so they aren’t used at Yellowstone. Instead, researchers use indirect methods to estimate the flux of heat, like satellite thermal data. The bottom of Yellowstone Lake, however, offers a stable thermal environment without the noisy on-land temperature variations and where short probes can be used to map variations in heat flux directly.
Heat flux measurements on the floor of Yellowstone Lake were done in 2016–2018 as part of a large, multidisciplinary project called Hydrothermal Dynamics of Yellowstone Lake (HD-YLAKE; https://hdylake.org), which was funded largely by the National Science Foundation with additional support from the National Park Service, the U.S. Geological Survey, and the Global Foundation for Ocean Exploration. HD-YLAKE researchers borrowed techniques used on the ocean floor to make many closely spaced heat flux measurements in Yellowstone Lake. The primary focus of the measurements was the Deep Hole vent field, east of Stevenson Island in the deepest part of lake (about 120 m, or 400 ft).
Measurements by the HD-YLAKE team were designed to measure the total heat flux through the Deep Hole vent field, and to map spatial patterns of heat flux in and around the vent field. The primary tool was a 1-m (3-ft) probe that was inserted into the lake-bottom sediment by the remotely operated vehicle (ROV) Yogi deployed from the research vessel Annie. The probe has five precision thermistors along its length for measuring the thermal gradient with depth, and a heating wire used to measure thermal conductivity. Once the probe is inserted into sediment, the thermal gradient is determined by measuring the temperature along the length of the probe, and the thermal conductivity is estimated by heating the probe for a short period of time and monitoring the decay of heat as the probe cools.
The overall heat flux around Yellowstone is estimated to be about 2 W/m2, about 30 times greater than the global average heat flux of about 0.065 W/m2. Measurements around the Deep Hole vent field range between 69 and 0.84 W/m2. Not surprisingly, lower values are found outside the vent field, and higher values are found inside. The median value for measurements within the Deep Hole area was 13 W/m2. The larger flux of heat within the vent field is due to fluids that focus heat as they rise through the sediments at the lake floor.
As might be expected, some of the largest values of heat flux are in the core of the vent field, where hot fluids are discharged into the lake. Fluids discharging through the vents have a mean temperature of 132° C (270° F). Fluids can exist at these high temperatures due to the pressure (about 12 times normal atmospheric pressure) of the overlying lake water. The total heat output of the vent field is about 30 MW—enough to power about 20,000 homes—making it among the highest of any hydrothermal field in Yellowstone. The large quantity of heat released through the Deep Hole vent field is likely due to the high boiling temperatures associated with the elevated pressure found on the lake floor and the character of the hydrothermal fluids rising through the Deep Hole vent field.
These data, published earlier this year (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020JB021098), offer a fascinating view of the dynamics of the Deep Hole vent field—a hydrothermal area that is among Yellowstone’s most dynamic—and emphasize the many interesting geothermal features at the bottom of Yellowstone Lake.
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