How to Get a Date

Using Volcanic Glass to Tell a Soil Layer's Age

May 5, 2009

By Russ L. Hudson

How do archaeologists, anthropologists, or a construction company that has just unearthed an ancient burial site, determine the age of their find? Jeffrey R. Knott, associate professor of geological sciences at Cal State Fullerton, has developed a method of accurate dating based on volcanic eruptions.

Volcanic glass forms when magma rapidly cools as it leaves the volcano. It forms so quickly that the chemicals in the magma have no time to evaporate or change, so the chemical imprint of the volcano is captured in the glass, which is blasted into tiny shards and blown skyward by the eruption along with millions of tons of rock ash. Once in the atmosphere, winds carry the ash and glass thousands of miles as it settles to earth.

That imprint is distinct from all other eruptions of that and all other volcanoes. If an archaeologist finds something in a layer of soil with glass from a known volcanic eruption, that find can be dated quickly and accurately, said Knott. For example, he said, Vesuvius erupted and buried Pompeii in 79 A.D., so anything in the same layer of soil as the shards from that eruption was there in 79 A.D.

To chemically “tag” an eruption, Knott vaporizes the glass with a laser, then runs the vapor through an inductively coupled plasma mass spectrometer, a device that can quickly read up to 30 of the chemicals, even if they are in concentrations of less than one part per million.

By contrast, the most common method today uses an electron microprobe, which can only identify nine of the elements in the glass. Knott’s method detects more than three times the number of chemicals and increases the tagging accuracy exponentially.

Identifying and dating eruptions also helps paleoclimatologists, Knott explained. If the glass deposits rest on rocky soil, it means the climate was dry when the ash settled. If the glass is in fine silt, it means it settled in a lake or inland sea — a wet climate.

“I started searching for a better dating method when I was studying the Death Valley fault zone in the early 1990s as a graduate student,” Knott said. “No one could tell me the age of the volcanic deposits cut by the fault zone.

“Knowing the age would help date the fracture. The dating of volcanic deposits (tephra), was one possibility.

“In an article published in 1991, Andrei Sarna-Wojcicki of the U.S. Geological Survey had argued that more accurate methods of dating tephra were needed to advance the science, and had suggested inductively coupled plasma technology might be the answer.” Knott and Sarna-Woicicki together pursued that line, but the real breakthrough came with the introduction of lasers in the process.

Knott has pursued the laser-aided method since 2005. In 2007, researchers from the University of Toronto, University of Wales and Institute of Geological and Nuclear Sciences in New Zealand reported similar success tagging glass shards with the same method. They, too, were able to detect up to 30 chemicals in a glass-shard at very low concentrations.

“I’ve been sharing what I’ve learned at conferences, in discussions and in articles,” Knott said. “I think a lot of geologists are interested in the process now.”

Continuing the Research

Knott conducts most of his research in and near Long Valley near Bishop. Long Valley was formed 760,000 years ago by an explosive eruption from four miles deep, throwing 150 cubic kilometers of rock and ash into the atmosphere and forming a 20-by-10-mile crater that is now Long Valley. The ash spread over most of the western United States — up to 100 feet deep in Bishop and about 12 feet thick in Death Valley, where Knott has continued to conduct research since his graduate days.

Two grants from the American Chemical Society-Petroleum Research Fund totaling $95,000 have funded Knott’s research since 2005. For the ACS, he maps alluvial fans — the silt, soil and rock washed out of canyons — and how tectonic action affected their construction. He maps faults and volcanic ash deposits, providing valuable information for corporations looking for mineral and petroleum deposits, the bulk of the ACS membership.

While there, Knott pursues his glass dating studies, which is of benefit to ACS because it dates the deposits that interest them.

Steve Okubo (B.S. geology ’08) helped Knott map a portion of the Kit Fox Hills for ACS and collected samples of volcanic deposits in 2007 and 2008. He also worked in Knott’s lab, separating the glass from other material and analyzing it.

"I learned from Dr. Knott one should not always approach a scientific problem, or any problem, in a linear manner if it restricts the knowledge that can be gained,” Okubo said. “A more comprehensive approach is better.

“He could have tried to understand the Kit Fox Hills by mapping alone,” Okubo pointed out, “but, because he incorporated the analysis of volcanic deposits as markers, he is able now to enhance his interpretations and reach a more complete understanding of the hills and how they were constructed.

Okubo is now a geotechnical scientist at Exponent Engineering and Scientific Consulting, which conducts research and analysis for corporations and government bodies on soils, structures, ecological systems, the environment and manufacturing materials. He intends to pursue a master's degree in geology.

“I took his example to heart and apply what I learned to the research problems I encounter working at Exponent Engineering," Okubo said.

Knott earned his doctorate in geology at UC Riverside. His dissertation was on the formation and extent of the Death Valley fault zone.