California's Death Valley comes by its morbid reputation honestly, but not for the reason you might think. True, this stark desert holds the record for the hottest, driest spot in North America. Scientists now say it also poses a different threat: spectacularly explosive volcanic eruptions.
A new study from geochemists at Columbia University's Lamont-Doherty Earth Observatory reveals that around the year 1300, a volcanic explosion in the northern part of the valley ripped a half-mile-wide hole in the overlying sedimentary rock, blasting out superheated steam, volcanic ash and deadly gases. Study co-author Brent Goehring, now at Purdue University, described this dramatic event yesterday in a press release:
...this would have created an atom-bomb-like mushroom cloud that collapsed on itself in a donut shape, then rushed outward along the ground at some 200 miles an hour, as rocks hailed down. Any creature within two miles or more would be fatally thrown, suffocated, burned and bombarded, though not necessarily in that order.
In the new report, published in the 18 January issue of Geophysical Research Letters, Goehring and his colleagues suggest such an event created Ubehebe (pronounced YOU-bee-HEE-bee) volcanic crater, the youngest and largest of a dozen similar craters in northern Death Valley.
What's more, conditions may be ripe for a repeat performance.
Geologists had long assumed that Ubehebe and its sister craters were thousands (or even tens of thousands) of years old. Those supposed ages would put the eruptions at the end of the last ice age, when the U.S. southwest was considerably wetter than it is today. And that made perfect sense, considering all geologic clues suggest the magma mixed with water, which is what made them so explosive.
Called phreatomagmatic eruptions, such events usually occur where water is abundant-near the edge of a lake, say, or at the bottom of the ocean:
But when the Lamont geochemists used a new-fangled isotope technique to date the volcanic craters, they turned out to be surprisingly young. They ranged from 2,100 years old to 800 years old-meaning they formed long after California had dried out.
That left the researchers with only groundwater to blame. Indeed, the present-day water table is relatively shallow, probably only 150 meters below the floor of Ubehebe crater. "This and the youth of the most recent activity suggest that the Ubehebe volcanic field may constitute a more significant hazard than generally appreciated," the researchers concluded.
This bold, new statement stems from the remarkable ability of quartz-rich pebbles, which now litter the desert soil around the craters, to track the time since an explosion ripped them out of the ground. Ever since that moment, cosmic rays striking certain oxygen atoms within the quartz grains have been creating radioactive beryllium-10, a so-called cosmogenic nuclide with a known rate of decay.
Measuring beryllium-10 is the basis for a dating technique similar to one Dutch scientists used recently to hone in on the date of a prehistoric tsunami based on the grains of sand the storm washed to shore:
If you want some really technical detail about the beryllium-10 dating method, go to the Lamont-Doherty Earth Observatory Cosmogenic Nuclide Laboratory.
Death Valley's half-mile-wide Ubehebe Crater formed just 800 years ago. (Photo courtesy Brent Goehring/Lamont-Doherty Earth Observatory)