Lake Mead’s water crisis is exposing volcanic rock from eruptions 12 million years ago

By Rachel Ramírez | CNN

The falling water levels of Lake Mead in recent months have revealed several shocking things – previously sunken boats, old warships and human remains. Now, scientists are reporting a new discovery in Lake Mead’s dry bed: rocks laced with volcanic ash that rained down on southern Nevada in explosive eruptions about 12 million years ago.

The record-low water levels are exposing sedimentary rock not seen since the 1930s, when the Hoover Dam was built and Lake Mead was filled. Beneath these rocks, researchers from the University of Nevada in Las Vegas found ash deposits from volcanoes in Idaho, Wyoming and California.

“We knew these ash units existed, but we were surprised to find so many when Lake Mead’s water level dropped,” said Eugene Smith, professor emeritus of geology at UNLV and co-author of the study.

Droughts in the West caused by climate change and overexploitation of the waters of the Colorado River have pushed Lake Mead levels to unprecedented lows. As of September, the lake’s water level was just 1,045 feet above sea level, or about 27% of full capacity.

Scientists are using the low levels to study sediments that haven’t been uncovered in nearly a century.

Smith’s research team found white to gray volcanic ash meandering through the formerly submerged rocks. They took samples to their lab to pinpoint the source of the ash, but it was not from a single eruption.

They found evidence of multiple volcanic eruptions millions of years ago in places such as the Snake River Plain-Yellowstone area — an area of ​​dormant volcanoes that stretches along the Snake River across Idaho into what is now Yellowstone National Park — and eastern California. They also found ash from eruptions just 32,000 years ago — not that long ago on the geologic timescale.

Jake Lowenstern, a research geologist at the US Geological Survey who is not involved in the study, told CNN that examining past volcanic eruptions can help paint a picture of future risks.

The recent discovery at Lake Mead may be one of the “better” accumulations of volcanic ash from that era, Lowenstern said, and it will be “important in enabling us to reconstruct the geological history of the region and to understand the frequency of large volcanic eruptions and how they impacted.” the Southwest.”

Ash from even moderately explosive volcanic eruptions can travel hundreds of kilometers and cover areas several meters away with heavy material. Recent eruptions have shown that a few millimeters of wet ash can disrupt power transmission. And if inhaled, the tiny but sharp grains in the ash pose a significant health risk.