LAS VEGAS (KLAS) — Not only have Lake Mead’s dwindling water levels exposed human remains and old relics but now decades-old sedimentary rocks containing volcanic ash are being seen at the lake, according to a recent UNLV study.

The study found that ash from volcanoes in Idaho, Wyoming, and California rained down on Southern Nevada as many as 12 million years ago and became part of the rocks, which haven’t been seen on Lake Mead’s shoreline since the 1930s.

“Ash from even moderately explosive eruptions can travel hundreds of miles from the source, blanketing entire areas with anywhere from a centimeter to several meters of the heavy material,” said UNLV emeritus professor of geology Eugene Smith. “Although the Las Vegas valley is currently very far away from any active volcanoes, we can and will have ash from these volcanoes fall over Southern Nevada in the future.”

UNLV researchers found that samples taken from exposed ash layers at Lake Mead and near Henderson were most likely from outside the area because local volcanoes capable of producing eruptions large enough to deposit so much ash became extinct roughly 12 to 13 million years ago.

According to the study, four main possible source areas include:

  • The Snake River Plain-Yellowstone hotspot track, active for the last 15 or so million years, which has migrated from the northern Nevada/Oregon/Idaho junction to the area of Yellowstone National Park.
  • The Southwest Nevada volcanic field, primarily active between 7.5 to 13 million years ago, located about 90 miles northwest of Las Vegas. It produced about 10 supereruptions over its lifespan.
  • Volcanoes of Walker Lake, famous for producing earthquakes in western Nevada and southeastern California, with volcanic fields aged from 8.5 million years old to presently active.
  • The Ancestral Cascades, including Mt. St. Helens, which extends from northernmost California through Oregon and Washington and into British Columbia.

Most of the ash layers found at Lake Mead are between 6 million and 12 million years old, but some are as recent as 32,000 years old, indicating current potential health threats from volcanic activity, according to researchers.

Volcanic ash can contain volcanic glass, debris, and crystals from the magma chamber that feeds the eruption, the study said.

“The ash layers we study come from volcanoes long extinct. However, studying them has helped us determine just how often the Las Vegas area was inundated with ash over time and may help us prepare for future events from active volcanoes far from us,” said lab manager Racheal Johnsen.