As the climate crisis continues to hit the American West, sunken boats and human remains aren’t the only surprises revealed by record low water levels at Lake Mead. Sedimentary rocks not seen since the 1930s are now uncovered along the constantly changing coastline, and a UNLV study of the deposits has found that many of these rocks also contain ash from volcanoes as far away as Idaho, Wyoming and California, which rained down on southern Nevada as much as 12 million years ago.
While metropolises located directly in the lava flow path of active volcanoes receive a lot of attention, the Cryptotephra Laboratory for Archaeological and Geological Research (CLAGR) team at the UNLV College of Sciences says explosive volcanic eruptions could have an equally significant impact on health and safety. of the population far from the source.
“Ash from even moderately explosive eruptions can travel hundreds of miles from the source, covering entire areas with an inch to several meters of heavy material,” said Eugene Smith, UNLV professor emeritus of geology who specializes in volcanology, geochemistry, and geological mapping and has worked on projects in Africa, Asia, North America, Europe and Antarctica.
“While the Las Vegas Valley is currently very far from active volcanoes, we can and will in the future be dropping ash from these volcanoes over southern Nevada,” Smith said. “Even a few millimeters of ash when wet is incredibly heavy and can take away power and telecommunications lines. It can block roads. It is easily re-mobilized by wind and water. When inhaled, the incredibly small but sharp glass grains in the ash become can cause significant, chronic lung conditions, such as silicosis.”
CLAGR researchers used geological maps to locate and collect samples from multiple ash layers at Lake Mead and from outcrops just south of Henderson. Back in the lab, the samples were chemically analyzed and then compared to ash from known volcanic eruptions in the western US to help determine their age and site origin. Among their findings:
- Although the Las Vegas area has had volcanic activity over time, scientists have determined that the source of the ash layers they found most likely came from outside southern Nevada, because local volcanoes can produce those large enough explosive eruptions. to deposit robust amounts of ash, became extinct around 12 o’clock. million to 13 million years ago.
- The four main potential source areas, each producing distinctive axis, include:
- The Snake River Plain-Yellowstone (SRP-Y) hotspot track, which has been active for the past 15 million years. The SRP-Y is a chain of very large volcanic centers that have migrated over time from the Nevada/Oregon/Idaho northern junction, across Idaho, to the active area of Yellowstone National Park near the Wyoming, Idaho junctions. and Montana.
- The Southwestern Nevada Volcanic Field (SNVF), which was primarily active between 7.5 million and 13 million years ago. Located about 90 miles northwest of Las Vegas, this volcanic field produced about 10 major calderas, or super-eruptions, during its lifetime. The proposed Yucca Mountain repository is located just south of one of the calderas.
- Volcanoes of Walker Lake, which is probably better known for causing major earthquakes in western Nevada and southeastern California, such as the Ridgecrest earthquakes in 2019 and Monte Cristo Range in 2020. However, it also contains several large volcanic fields that occur in ages range from 8.5 million years to active now. The Long Valley caldera and Mono Craters domes near Bishop, California, are the most famous because they are still active, but extinct, 5- to 6-million-year-old volcanoes of the Greenwater Range on the eastern edge of Death Valley in California, as well as one near Silver Peak in central Nevada, also sparked explosive eruptions.
- The Ancestral Cascades – the precursor to the modern Cascade Volcanic Arc – stretching from northernmost California, through Oregon and Washington, and into British Columbia, and includes Mount St. Helens. Most of these volcanoes’ eruptions are not explosive enough to scatter ash in southern Nevada.
- Most of the ash layers the scientists have found are between 6 million and 12 million years old, but some are as recent as 32,000 years old — indicating that potential health threats from volcanic activity are ongoing.
Volcanic ash can contain volcanic glass (think the pumice stones people use to remove particularly stubborn calluses, only then shrunk to almost microscopic size), debris and crystals from the magma chamber that fuels the eruption. Depending on the size and strength of the eruption, the ash, or tephra as geologists call it, can travel hundreds or even thousands of miles from the volcano.
One of the most famous examples of volcanic activity in the US was the catastrophic 1980 explosion at Washington’s Mount St. Helens, which leveled forests, dammed Spirit Lake, and deposited ash over several states — yet considered a small explosive. eruption. Larger eruptions that deposit ash further away could be dangerous to infrastructure such as power lines, roads, railways and buildings, even several states away from the volcano itself, the UNLV research team said.
“Studying the past allows you to plan for the future,” said Racheal Johnsen, CLAGR lab manager and longtime geologist in Southern Nevada. “The ash layers we study come from volcanoes that have long been extinct. However, studying them has helped us determine how often the Las Vegas area has been inundated with ash over time and may help us prepare for it.” on future events of active volcanoes far away.”
Volcanic lightning shoots through the skies over fiery Etna
DISCOVERY OF ASH IN SEDIMENTS AROUND THE LAS VEGAS VALLEY: IMPLICATIONS FOR FUTURE ASFL HAZARDS FROM DISTAL VOLCANOES, Geological Society of America Abstracts with Programs. Full. 54, no. 2. DOI: 10.1130/abs/2022CD-374326 , gsa.confex.com/gsa/2022CD/webp … ram/Paper374326.html
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