The Enigma of Sulfur Dioxide's Disappearance at Yellowstone
When we think of active volcanoes, it's easy to envision dramatic eruptions and massive clouds of gas soaring into the sky, particularly clouds rich in sulfur dioxide (SO2), a gas notorious for creating volcanic smog. These emissions can even be detected from space! Volcanoes like Kīlauea in Hawaii are known to release hundreds to thousands of metric tons of SO2 daily, leading to visible haze that affects air quality throughout the islands. Other active volcanic sites, such as Mount Etna in Italy and Ambrym in Vanuatu, are also significant sources of SO2, and monitoring these gas emissions is crucial for volcanic observatories.
However, there's an intriguing paradox regarding Yellowstone: despite being located above one of the largest magma systems on Earth, it releases almost no detectable sulfur dioxide. This fact raises questions, especially since Yellowstone is among the top emitters of carbon dioxide (CO2) in volcanic systems globally. The park's hydrothermal features, known for their billowing steam and the distinctive rotten-egg scent due to hydrogen sulfide (H2S), characterize places like Norris Geyser Basin and the Mud Volcano area. With all this volcanic activity, one might wonder: where does the SO2 go?
To unravel this mystery, we need to look at the depths of Yellowstone's magma. Comprehensive geophysical investigations have uncovered that Yellowstone's magmatic system is composed of two primary reservoirs. The upper rhyolitic chamber is situated between 4 and 17 kilometers (approximately 2.5 to 10 miles) beneath the surface, while a larger, deeper basaltic reservoir extends from around 20 to 50 kilometers (about 12 to 30 miles) down.
This depth is significantly greater than that of many actively erupting volcanoes. When magma rises toward the Earth’s surface, the decrease in pressure allows dissolved gases to escape, similar to how bubbles form when you open a soda bottle. Different gases will separate based on their solubility in the magma at various depths. Carbon dioxide, which has low solubility, tends to bubble out when located at depths of 40 kilometers or more. In contrast, sulfur dioxide typically exsolves at much shallower depths, often just a few kilometers from the surface.
In Yellowstone's case, because the shallowest magma resides several kilometers underground, any minor amounts of SO2 that do escape must travel a long distance through the crust before reaching the atmosphere. During this journey, a critical interaction occurs: the gases come into contact with water.
Yellowstone National Park is home to one of the largest hydrothermal systems in the world, boasting over 10,000 thermal features spread across more than 100 thermal areas. This extensive network of hot water, steam, and altered rock lies above the magma chamber and creates a highly reactive chemical environment. As magmatic gases like SO2 ascend through this water-saturated system, they undergo a process termed "scrubbing."
"Scrubbing" involves chemical reactions between volcanic gases and water (or rock) that effectively remove certain gases before they can reach the surface. When SO2 encounters liquid water in Yellowstone's hydrothermal system, it dissolves rapidly and undergoes a chemical transformation known as disproportionation. Through a series of reactions, the SO2 is converted into H2S, dissolved sulfate ions, and sometimes even elemental sulfur, which forms the yellow deposits found around many of Yellowstone's thermal features.
Interestingly, the absence of detectable SO2 at Yellowstone is beneficial for monitoring volcanic activity. If the Yellowstone Volcano Observatory were to suddenly observe an increase in SO2 emissions, it would signal a significant shift—indicating that magma had moved closer to the surface and created dry gas pathways through the hydrothermal system. This scenario suggests that water may have been displaced due to the heat of the rising magma, both of which are signs of heightened volcanic unrest.
For the time being, the Yellowstone Volcano Observatory focuses on monitoring the gases that do escape, primarily CO2, which emerges from the magma at deeper levels and isn't easily scrubbed by water, along with H2S, which can originate from various processes, including the transformation of the elusive SO2. A multi-GAS station currently situated at Mud Volcano continuously tracks these gas concentrations, enabling scientists to observe any changes in the behavior of this volatile volcanic system.
So, the next time you find yourself in Yellowstone inhaling that unmistakable sulfurous scent wafting from a steaming fumarole, remember: you’re witnessing the result of a remarkable natural chemical process. Instead of missing, the SO2 has simply been transformed deep underground during its passage through one of Earth's most fascinating hydrothermal systems. What else about Yellowstone's geothermal wonders surprises you? Let's discuss!
