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Caldera Chronicles

Caldera Chronicles is a weekly article written by U.S. Geological Survey Yellowstone Volcano Observatory scientists and colleagues.

March 18, 2019 Article Link

Exploring thermal areas in Yellowstone's remote southwest corner

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Sara Peek, Hydrologist with the U.S. Geological Survey.

Why are there hydrothermal areas in Yellowstone? Fundamentally, it's due to the presence of water (from rain and snow), heat (from magma), and suitable geology (porous rocks). Water that percolates into the ground is heated indirectly by the underlying magma and migrates back to the surface along faults and at the terminations of volcanic flows. Volcanic gases travel to the surface alongside the water. The highest concentrations of thermal features are near the center of the park: for example, the Upper Geyser Basin, where Old Faithful is located, and Norris Geyser Basin. However, there are over 10,000 thermal features scattered throughout the park, and to get a full picture of the Yellowstone hydrothermal system, we are attempting to make a more complete accounting of water and gas chemistry of these features.

Mules loaded with sampling equipment and supplies for a mission to collect gas and thermal water samples from the Bechler River area in the southwest part of Yellowstone National Park (Click image to view full size.)
Mules loaded with sampling equipment and supplies for a mission to collect gas and thermal water samples from the Bechler River area in the southwest part of Yellowstone National Park. USGS Photo taken on September 18, 2017, by Shaul Hurwitz.

For the last two years, scientists from the USGS and the National Park Service travelled to the southwest corner of the park to study remote thermal areas around Boundary Creek and the Bechler River. We planned our trips by looking at visible and thermal infrared satellite imagery, which reveals the locations of thermal areas. Even Google Earth is a great tool for exploring these regions!

Before we headed out to Yellowstone, we prepared our equipment in the lab. Gas collection bottles were hooked up to a vacuum line to pump out all the air, so that we could get samples consisting solely of volcanic gas. We also packed clean plastic sample bottles and filters for collecting water, and equipment for measuring water temperatures and stream flow rates.

The thermal areas in the southwest corner of the park are quite remote. To get there, we packed all of our equipment and supplies onto mules, then hiked for long distances (up to 13 miles from the road) to set up a base camp.

On work days, we carried equipment in our backpacks and hiked a few miles, mostly on trails, to sample thermal areas. Equipment we needed to pack for the day might include, for example, up to eight glass bottles and several pounds of tools, not to mention tubing, field books, a GPS unit, and other necessary equipment.

Features in the southwest corner consist of a mix of thermal waters and gases from deep reservoirs, and cooler waters (with associated dissolved gases) resulting from rain and surface water interacting with hot rock. Finding the ideal site for gas sampling can be tricky: a pool with strong bubbling action is certainly releasing a lot of volcanic gas, but the gas can contain air and water. A site with "steaming ground" can be harder to identify, but the sample will likely contain more volcanic gas. Ideally, we would sample gas from hot, vigorous fumaroles, but there are not many of them in this area, whereas bubbling pools are common.

An unnamed but charismatic thermal feature alongside Ferris Fork, near the Bechler River (Click image to view full size.)
An unnamed but charismatic thermal feature alongside Ferris Fork, near the Bechler River. The center of the pool is roiling with gas bubbles, but not boiling! The pool is only about 180 degrees F (81 degrees C), and all the gas is bubbling up from underground. The white color is due to the silica-saturated water depositing sinter as it flows out and cools, while the red and green patches are cooler areas where algal mats thrive. USGS photo taken on September 17, 2018, by Sara Peek.

When we returned from Yellowstone, we analyzed the gases we collected. The gas discharging from the thermal areas is mostly steam, which is visible on a cold morning. Except for steam, more than 90% of the gas consists of carbon dioxide (CO2). The remainder of the gas in the southwest corner of the park is mostly nitrogen. Most (but not all) of the CO2 originates in the magmatic system beneath Yellowstone, but most of the nitrogen has a different origin; when rainwater percolates into the ground, it contains dissolved air, including both nitrogen and oxygen. As groundwater flows through the hydrothermal system, the oxygen reacts with the surrounding rock, while the nitrogen does not. Notably, the gases emitted in the southwest corner of the park contain very little hydrogen sulfide (H2S), unlike gas sources in some other active areas of Yellowstone.

We will use the data we collected to improve the understanding of hydrothermal activity in Yellowstone's southwest corner. In this ongoing work, we are investigating whether the chemical compositions of gases and thermal waters are similar to those in other areas and what these chemical compositions might tell us about the entire Yellowstone hydrothermal system.