College of Liberal Arts and Sciences at Iowa State University

Fascinated by rocks

As a child Carl Jacobson was enthralled with rocks. Not much as changed in adulthood.

Rocks became an obsession to Carl Jacobson while he was growing up in New York.

"I would attend summer camp in upstate New York," he recalled. "And I was fascinated by fossils, minerals -everything that had to do with rocks. Most kids when they reach high school move onto other interests. That excitement never left me."

That same excitement followed Jacobson to the State University of New York at Binghamton where he graduated with a bachelor's degree and then on to the University of California, Los Angeles (UCLA) where he got his Ph.D.

"Point is I just like rocks," said the professor and chair of the Department of Geological and Atmospheric Sciences.

It was at UCLA that Jacobson first became interested in the San Andreas Fault in southern California, perhaps the most well known geological fault in the world. Los Angeles sits on the west side of the fault, with San Francisco on the east side.

The San Andreas Fault is the cause of the major earthquakes in these two California cities.

Most of Jacobson's research is centered on the application of structural geology, metamorphic petrology, and radiometric age dating to understanding the Late Cretaceous through Tertiary tectonic evolution of southern California and southwestern Arizona.

"Any geologist who works in southern California sooner or later has to deal with the San Andreas fault," Jacobson said. "It has so much impact on the geological aspects of that region."

The San Andreas fault continues to slide, so much so that if it continues at its same pace San Francisco and Los Angeles could be side-by-side on a map in a ten million years or so.

Geologists estimate that the land along the fault line has moved or "slipped" some 300 kilometers in the last five million years. While this seems like a large amount of movement, there are some researchers who propose that a similar fault system active 50 to 80 million years ago may have shifted blocks of western North America even greater distances - up to 2500 kilometers.

This theory is based on the analysis of minerals that can record the orientation of the Earth's magnetic field at the time a rock forms. Because the orientation of the magnetic field varies with latitude, this method can be used to determine whether a certain region has moved northward or southward with time.

This "paleomagnetic" technique provides some evidence that rocks currently in British Columbia, Canada, initially were positioned adjacent to what is now southern California and Baja California, Mexico.

It's a theory that Jacobson doubts, but is researching.

"I don't believe in the theory," he said. "Paleomagnetism can be a great tool, but it doesn't always yield straightforward results. I'm trying to find distinctive rocks in both locations to test this hypothesis."

Jacobson is able to date a mineral called zircon, which is found in rock formations in both Baja and British Columbia. Zircon dating is useful because the North American continent grew progressively southward over hundreds of millions of years.

If British Columbia really started out in Mexico, it should contain younger zircons than if it is "native" to Canada.

"On the other hand, if zircons from British Columbia and Baja turn out to have the same ages, then there is a greater probability that the two regions in North America were once upon a time much closer together," Jacobson said.

"This study is really a historical analysis of southern California," he continued, "by which we can maybe better understand the evolution of North America."