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Earth Notes: Volcanic Ash in Lake Mead

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UNLV College of Science Cryptotephra Laboratory for Archaeological and Geological Research
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UNLV researchers Rachael Johnsen, left, and Andrew Litto look for ash layers near the boat ramp to what used to be Las Vegas Bay. Sediments are beigy-orange, and ashes are very thin lighter-colored material within this sedimentary unit.

This is Earth Notes…

Lake Mead is lower than it’s ever been, the result of decades of drought and warmer temperatures caused by climate change. The sinking water levels have revealed a different sort of catastrophe; layers of volcanic ash preserved in stone.

Scientists from the University of Nevada-Las Vegas discovered half a dozen of those ash layers and continue to find more. They haven’t been visible since the dam was built in the 1930s.

The scientists wanted to know if the ash came from nearby volcanoes. In their laboratory, they separated small glass sherds out of the ash for chemical analysis. The elements within those sherds act as a kind of fingerprint, unique to the volcano that produced the ash.

The results were a surprise: none of the ash layers were older than 12 million years—that’s young, by geologic standards. Some were only a few tens of thousands of years old. That means they didn’t come from local volcanoes, which went extinct long before that.

The scientists matched the ash layers to volcanoes further afield in Nevada, and to eruptions that happened in the Long Valley Caldera of California and the Snake River Plain of Idaho. The ash traveled for hundreds of miles before falling out of the sky.

In modern times an ashfall could choke cars and ground airplanes, and pose a hazard to people’s health. Lead researcher Eugene Smith says the research is still ongoing, but the findings point to a need for cities to plan for how they would handle a volcanic eruption, even if it’s not nearby.

This Earth Note was written by Melissa Sevigny and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.

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Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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