Yellowstone Is One of the World’s Largest Magmatic Systems – And It May be Missing a Key Volcanic Gas

Venture too close to the hydrothermal vents scattered around Yellowstone National Park and you’ll be greeted by a powerful stench reminiscent of rotten eggs. These calderas are a (literal) hotbed of harmful gases like hydrogen sulfide and carbon dioxide – but one gas is surprisingly missing.

The absence of sulfur dioxide in Yellowstone appears to be a mystery. Other volcanoes, like Etna in Italy and Santorini in Greece, release tons of it every year.

According to Jennifer Lyn Lewicki, a research geologist at the United States Geological Survey’s (USGS) California Volcano Observatory, there’s really no shortage of sulfur dioxide in Yellowstone. Instead, it undergoes conversion underground, turning into dissolved hydrogen sulfide and sulfate ions before reaching the surface.

Writing in the Yellowstone Volcano Observatory’s Caldera Chronicles, Lewicki explains that this is good news for residents of neighboring states, because the presence of sulfur dioxide could signal volcanic unrest.

Significant volcanic gases

Magma is not just molten rock. It contains several dissolved gases, including water vapor, carbon dioxide and sulfur dioxide.

As magma flows toward the surface, the decrease in pressure causes gases to exsolve from the liquid magma and continue to rise toward the surface, eventually escaping into the atmosphere – a process known as outgassing.

Basically, different gases have different levels of solubility. Sulfur dioxide has relatively high solubility levels and often escapes from the magma only when it reaches shallower depths.

Learn more: One of the oldest organisms on the planet thrives in Yellowstone’s scorching hydrothermal features

Where did the sulfur dioxide go?

Essentially, the absence of sulfur dioxide in Yellowstone comes down to two factors: the depth of Yellowstone’s magma chambers and interference from hydrothermal systems.

Unlike Hawaii’s Kīlauea, which stores magma in shallow reservoirs only a mile or two (1 to 5 kilometers) away) Beneath the surface, the Yellowstone magma system lies deep underground.

There are two chambers, the shallower of the two being located between 5 and 17 kilometers below the surface. The deeper (and much larger) of the two is 20 to 50 kilometers underground and is 4.5 times the size of the first. This depth means that very little of Yellowstone’s sulfur dioxide escapes from the magma.

The fact that Yellowstone also contains a large hydrothermal system including thousands of hydrothermal features, from hot springs to geysers, means that any escaping sulfur dioxide interacts with the water before reaching the surface.

Through a process called “washing,” water converts sulfur dioxide into several other substances, including hydrogen sulfide, dissolved sulfate ions, and elemental sulfur.

Predicting volcanic unrest

While sulfur dioxide is often used as a monitoring tool, helping scientists track volcano activity levels, its absence in Yellowstone may well be a good sign.

According to Lewicki, a sudden change in sulfur dioxide levels could indicate a change beneath Yellowstone’s surface and could indicate that magma has moved closer to the surface, causing water to evaporate. This could signal volcanic unrest.

But even if an eruption were to occur, the USGS predicts it would most likely be a hydrothermal explosion – a relatively minor phenomenon that occurs every few years or so. As for a so-called “super-eruption,” volcanologists doubt whether there will ever be another catastrophic eruption of the type that occurred 2.08 million years ago, 1.3 million years ago, and 0.631 million years ago. At least for now, the magma present in Yellowstone appears to be too widely distributed to threaten an eruption, according to Nature.

“Most volcanic systems that have a supereruption do not have them multiple times. When supereruptions occur more than once in a volcanic system, they are not evenly spaced in time,” according to the USGS. “The rhyolite magma chamber beneath Yellowstone is only 5-15% molten […] so it is unclear whether there is enough magma beneath the caldera to fuel an eruption.

Learn more: Yellowstone bison meets tragic end at Hot Spring, showing danger of hydrothermal features

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