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Water levels in Lake Mead hit record lows this year as the crushing drought in the western US continues, revealing relics like a World War II-era boat and multiple sets of human remains from decades ago.

Now, geologists at the University of Nevada, Las Vegas (UNLV) have now documented something much older – volcanic ash from eruptions millions of years ago, stemming from volcanoes all over the west.

“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,” Eugene Smith, a geologist at UNLV and one of the researchers who worked on the study, said in a press release.

Much of the rock around Lake Mead is sedimentary – formed by layers that deposit over millions of years, creating a striated record of Earth’s geologic history.

Some of those layers are made up of ash spewed by volcanic eruptions. The UNLV scientists determined that many of the layers date back between six and 12 million years, while some are as young as 32,000 years old.

Lake Mead was formed in the 1930s after the construction of the Hoover Dam on the Colorado River along the border of Nevada and Arizona, creating the US’s largest reservoir.

And as water levels have decimated the lake, parts of the landscape not seen since the mid-20th century have risen above the water, allowing these geologists to study the rocks.

“We knew that these ash units existed, but we were surprised to find so many as the Lake Mead water level lowered,” Dr Smith told CNN.

Volcanoes that could have produced the ash range from nearby in southwest Nevada, where volcanoes are more dormant, to the Snake River-Yellowstone volcanic hotspot in Wyoming, which is still active.

And understanding these prehistoric eruptions could be important to help us prepare for volcanic eruptions in the future.

“The ash layers we study come from volcanoes long extinct,” Rachel Johnson, a geologist at UNLV and another one of the researchers, said in the release.

“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.”

Dry conditions in the southwest US have been driven by the region’s ongoing “megadrought” fuelled by the climate crisis, which has crippled water supplies for more than 20 years. One recent study found that the past two decades have been the driest in the region for at least 1,200 years.

The Lake Mead area is currently experiencing “severe drought” conditions, according to the US government drought monitor, putting long-term water supplies in the southwest at risk. The reservoir is now at just 28 per cent of its full capacity.

States along the Colorado River are trying to negotiate new terms for water use and conservation but have made little progress in recent months.

Drought in western North America is likely to get even more frequent and severe as the planet continues to heat up, a UN climate science panel has found.

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