Tiny, weird rocks in Healdsburg, Dry Creek areas are remains of meteorites
Published: Tuesday, January 22, 2013 at 4:53 p.m.
Last Modified: Tuesday, January 22, 2013 at 7:50 p.m.
As a kid, Dry Creek Valley native Paul LeBrett would collect distinctive, olive-sized, pitted gray rocks, not knowing what they were, except they were the perfect size for his slingshot.
"They were just weird rocks," said LeBrett, 58, a Healdsburg resident. LeBrett would look for arrowheads when the ground in his grandfather's vineyards was stirred up, "and these just stood out like sore thumbs."
It turns out those rocks are tektites, part meteor and part earth, and are designated Healdsburgites after where they were first found, said Rolfe Erickson, a Sonoma State University geology professor emeritus who has been studying them for 20 years.
Tektites are the result of strikes by meteors that would have been almost a mile across and hit with enough force to melt the Earth's crust, Erickson said.
The strikes created craters that would be miles wide and "strewn fields" that could cover a thousand square miles with tektites, Erickson said.
The strewn field that was discovered in Healdsburg's Dry Creek Valley is one of only five known to exist in the world, and the result of a meteor that struck 2.8 million years ago, Erickson said.
"The asteroid would have been traveling faster than a rifle bullet and generated tremendous kinetic energy," Erickson said.
At impact, billions of the molten droplets, part Earth crust and part meteor, were slung into space, where they froze and then rained back down to Earth. As they fell, the friction from the atmosphere would char their surface, leaving pits and striations.
"You would have a layer of these objects, tektites . . . for a thousand miles," Erickson said.
The identification of the rocks in Dry Creek Valley and documentation of the strewn field has been the work of Erickson and the late Steven Norwick, also an SSU geology professor, for 20 years.
"It is the most important thing I have ever done, so if you call it the highlight of my career, I guess it is," Erickson said.
Erickson, 76, a geology professor at Sonoma State for 40 years, presented evidence of the tektite strewn field at a meeting of the American Geophysical Union in San Francisco in December.
While meteor strikes are common, it is a rare meteor that is big enough to create the force necessary for tektites, Erickson said.
The only other documented strewn fields are in Chesapeake Bay on the East Coast, and in Central Europe, West Africa and Southeast Asia.
Only in West Africa and Central Europe have geologists also been able to identify the crater associated with the impact. For the others, including the tektites in Dry Creek Valley, the crater has not been found, Erickson said.
Tektites had been collected by Dry Creek residents for decades, unearthed as grapegrowers turn over their fields and also next to where roads were cut into the side of the hills.
Erickson said he first learned about them from Healdsburg resident Kate Moore.
"She knew they were something funny and had talked to a rock hound, who thought they were tektites," Erickson said. "I took the call, I'm the one who handles odd rocks. She had a box with 100 of them in it."
The tektites were dated to 2.8 million years by the UC Berkeley Geochronology Center.
The tektites themselves are ovoid glass clasts, made up of silicon and potassium, their surface covered with small deep pits and striations, and most not much bigger than an olive.
Erickson said 4,000 to 5,000 have now been found in an area from Warm Springs Dam to the Russian River.
The challenge now is to find tektites outside the Dry Creek Valley to document exactly how big the strewn field is. He has heard from people who think they found tektites in the Central Valley and in Los Angeles.
"I had no idea there was even such a thing as a tektite when I started out as a geologist, and now I have gone into planetary surface studies," Erickson said.
LeBrett has collected about 1,500 tektites and still searches for them.
"It became a passion, and now with their new designation, called Healdsburgites, there may be interest in people wanting to go out and find them," LeBrett said.
You can reach Staff Writer Bob Norberg at 521-5206 or email@example.com.
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