A thousand years ago, a volcanic eruption in what is now northern Arizona sent a plume of gas 24 kilometers into the sky and lava flows streaming as far as 11 kilometers. It caused local populations to flee their homes and abandon nearby farms.
Now a new analysis of gases from Sunset Crater’s single, catastrophic eruption is surprising geologists and challenging long-standing assumptions about how such basaltic eruptions occur.
Since volcanoes like Sunset Crater, called basaltic scoria cones, usually erupt only once, they don’t receive much of the attention their more active counterparts see. But the new research showed this eruption liberated disproportionally high amounts of carbon dioxide and sulfur for the volume of material released, suggesting these scoria cones might be more impactful than previously thought.
“What we’re finding here is that some of these volcanoes may actually be able to erupt in a similar manner to something like Mount St. Helens,” said Chelsea Allison, lead author on the new paper and a postdoctoral researcher at the City College campus of the City University of New York. “It’s really important that we can understand how that’s happening…so that in the future, if such an event were to occur, we’d be prepared for what might happen.”
Discovering What Underscores Scoria Cones
Although the 1064 eruption of Sunset Crater rivaled the 1980 awakening of Mount St. Helens in terms of ejected material, Sunset Crater (located in the San Francisco volcanic fields spanning northern Arizona) is a much smaller volcano. Geologists at Arizona State University, led by Allison during her Ph.D. work, wanted to figure out why the scoria cone erupted in such a violent manner to help better understand the risks associated with these types of volcanoes.
“[Scoria cones] are the most common volcanic landforms,” Allison said. “There are just tons of these little, tiny pimples all over Earth’s surface, and they’re also really common throughout the entire solar system.”
Building on previous analysis of Sunset Crater’s ancient fissures and lava flows, the scientists studied tiny bubbles of gas trapped inside crystals in the lava. Less than a tenth of a centimeter long, these bubbles are trapped inside pockets known as melt inclusions found in crystals that form well before an eruption occurs, providing a tiny time capsule of what gases were present in the magma before they emerged and escaped into the atmosphere.
Using Raman spectroscopy, the scientists were able to analyze the amount and density of gases in the bubbles by comparing them to known standards created in the lab. The Sunset Crater bubbles were found to be primarily carbon dioxide.
Their results, published in Nature Communications in January, showed the carbon dioxide was trapped at a depth of around 15 kilometers and likely played a critical role in propelling the eruption to such explosive heights. The scientists aren’t exactly sure where the carbon dioxide came from, but it could be a result of long-brewing subduction in the region.
Regardless of their origin, the high levels of carbon dioxide surprised the scientists and provided insight into what a similar eruption might be capable of. The Sunset Crater eruption sent 11 megatons of carbon dioxide and 6 megatons of sulfur dioxide into the atmosphere, making basaltic cone eruptions a possibly overlooked source of atmospheric aerosols.
The new findings come 5 years after similar research first started showing the importance of melt inclusion bubbles in active volcanoes and is likely only the tip of the iceberg of research to come.
“If these bubbles are important in these monogentic systems—where they only erupt during one main phase of activity—and they’re important in these really active systems, they are probably important everywhere,” said Penny Wieser, a volcanologist at Oregon State University who was not involved with the study. “That kind of means we really need to go back and reevaluate the data that’s been collected in the last few decades.”
—Mara Johnson-Groh ([email protected]), Science Writer