New data redefine Southern California as volcanically active
Using new techniques developed at UCLA and the University of Texas in Austin, a UCLA geologist has found evidence that a chain of small volcanoes, known as the Salton Buttes located at the edge of the Salton Sea, has erupted much more recently than previously thought.
Geologist Axel Schmitt.
This new research results, said UCLA Earth and space sciences professor Axel Schmitt, have redefined Southern California as volcanically active. According to the Smithsonian Global Volcanism Program, which sets widely held scientific standards for this field, a volcano is considered "active" if it has erupted within the last 10,000 years.
"We have evidence of a very recent eruption in the Salton Trough," Schmitt said. His research was published this month in the journal Geology
. "The previously known eruption age (widely recognized by archaeologists) is 30,000 years with large uncertainties. Our result is 10 times younger and 50 times more precise." According to the new research, the Salton Butte volcanoes were actually formed between 2,010 and 2,950 years ago.
The volcanoes were formed by eruption of sticky lava which, in places, turned into obsidian during cooling, Schmitt explained. The lava, he said, provided a source for obsidian, including an obsidian arrowhead found in the area that he was able to examine through a collaboration with the Cotsen Institute of Archaeology at UCLA. Using an ion microprobe, his research team determined the age of lava samples taken near the Salton Sea.
The Salton Sea was created in 1905 when the Colorado River overran an irrigation canal system and filled the basin.
The Salton Sea, a body of salty water in Southern California’s Imperial Valley, is known more for its ramshackle buildings and piles of dead, rotting fish than its picture-postcard views. But it has always been a place of intriguing geologic activity that has long fascinated Schmitt. The Imperial Valley is filled with evidence that the North American continent "is trying to pull itself apart, to the point where Palm Springs might have been beachfront property," he said.
The Salton Sea itself was created by a flood in 1905 when the Colorado River overran the Imperial Valley irrigation canal system and filled the basin, which is 226 feet below sea level. It sits between the San Andreas and the San Jacinto Faults. There have been major earthquakes in the Imperial Valley, proof of its intense geological activity, said Schmitt, who studies crustal evolution and geothermal processes.
This obsidian arrowhead was found near the Salton Sea.
"Young volcanic rocks can be difficult to date with conventional methods, one reason being that plant remains suitable for carbon dating are absent in a desert environment," Schmitt explained. So to determine the age of the obsidian, Schmitt used zircon, a silicate mineral that contains uranium. The age of a volcanic eruption can be determined by measuring the decay of this uranium, he explained.
"Zircons are the sturdiest and most resilient, datable materials. Diamonds don’t last forever, but zircons do; the oldest zircons are 4.4 billion years old," Schmitt said. "When we looked at samples from the Salton Sea area, they were easy to date using this method.
To perform zircon dating, scientists needed the right machine. Fortunately, Schmitt is also affiliated with the national ion microprobe facility at UCLA. Founded in 1992 and funded partially by the National Science Foundation and the Keck Foundation, the lab houses the nation’s first high-sensitivity, high-resolution ion microprobe.
Used by researchers from around the world, the ion microprobe focuses a beam of highly energized ions onto an object and collects the signal given off as a result. An analysis of the signal helps determine the composition of the object, in this case, the amount of uranium decay in the zircon within the arrowhead sample, Schmitt said.
"Archaeologists have debated for a long time why obsidian from the Salton Buttes is absent in older prehistoric deposits. We offer an explanation of why this is the case, that is, the obsidian did not exist until 2,500 years ago."
Previously, archaeologists theorized that the absence of obsidian materials was due to the fact that the obsidian was covered by water from Lake Cahuilla, the Salton Sea's predecessor. This new information, however, shows that the late prehistoric obsidian use from Salton Buttes is due to a very recent eruption, which was likely witnessed by humans in the area, the geologist said.
A relatively recent eruption, combined with an accumulation of magma, means that there is potential for future eruptions in the region, according to the geologist.