The Science of Sustainability

Help Find the Healdsburg Tektites

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Professor Rolfe Erickson holds part of his collection of Healdsburg tektites in this 2003 photo. Photo by Jean Wasp, Sonoma State University

A scattering of mysterious stones in Sonoma and Solano counties appears to be a newly documented population of tektites, a team of local geologists argued last week. Their poster presentation at the San Francisco meeting of the American Geophysical Union laid out new evidence about these unusual rocks.

Tektites are what you might call a semi-meteorite—they are Earth material that has been melted in large cosmic impact events and splashed into nearby outer space. As they fall back through the atmosphere, the glassy drops gain the blowtorched appearance of meteorites. You can buy them at any well-equipped rock shop and any of the Bay Area's rock and mineral shows. They are distinct in many ways from real meteorites, which are pieces of rock or metal from heavenly bodies, mostly asteroids.

Tektites are found scattered in large oval territories called strewnfields, each of which represents a single impact event. Until now, only five strewnfields were known: in the southeastern U.S., in Indochina, in central Europe, in Ivory Coast and in Central America. Three of these have been linked to a known impact crater by dating and chemistry. In last week's presentation, Professor Emeritus Rolfe Erickson of Sonoma State University and three coauthors made a wide-ranging case that we have our own Healdsburg strewnfield, "only a very small part of which has been identified to date."

Erickson first issued a call to the Bay Area's rockhounds in 2003, when the photo above was made. Since that time he has acquired more specimens and scraped the money together to have some tests made. Among these were age dating, done in Berkeley by the highly sensitive argon-argon method, and detailed chemical analyses. Their chemistries are very close to identical, putting them in the rhyolite category, and the four argon ages are statistically the same at about 2.823 million years.

That alone doesn't rule out volcanic rocks like obsidian, of which the North Bay has plenty, or the volcanic blobs called Apache tears. But their shapes and surface appearance are quite unlike these candidates and exactly like the tektites I own.

Freshly gathered healdsburgites from Dry Creek Valley. Rolfe Erickson photo

Erickson's team described the pits and grooves in these specimens and concluded, in their best scientific language, that "the clasts look like tektites." Additionally, the stones have no sign of weathering, which is quite unlike obsidian of that age, and their smooth surfaces show that their rounded shapes could not have come about by erosion in a stream. See more detail in the poster's summary posted on the meeting website. Erickson says that the full poster will be posted soon at the Sonoma State library site; I'll add a comment here when that happens.

They did not identify a candidate for an associated impact crater, but the date narrows things down a great deal, and the chemistry, they said, "suggests a continental origin." It is possible that a 2.8-million-year-old crater around here could be almost obliterated. The Coast Range is a highly active area, with the Earth's crust being shredded, lifted and dropped by various strands of the San Andreas fault system. And in the Cascades, a crater could have been buried long ago by volcano complexes, where it might look exactly like an ancient volcano in gravity surveys.

The strewnfield is still poorly documented, and careful watchers of the ground, like me and you, can help in this task. I would be keeping my eyes open wherever sediments of late Pliocene age might be found. They would have to be above sea level, where later sea-level fluctuations would not bury them, in basins that are not being actively filled today. In such elevated areas of sediment deposits, like the Livermore basin and southern Santa Clara Valley, these odd pitted pebbles might stand out. Erickson finds his examples in and around the Dry Creek Valley, in agricultural fields and roadcuts. Let's see where else we can find them.

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Andrew Alden

About the Author ()

Andrew Alden earned his geology degree at the University of New Hampshire and moved back to the Bay Area to work at the U.S. Geological Survey for six years. He has written on geology for About.com since its founding in 1997. In 2007, he started the Oakland Geology blog, which won recognition as "Best of the East Bay" from the East Bay Express in 2010. In writing about geology in the Bay Area and surroundings, he hopes to share some of the useful and pleasurable insights that geologists give us—not just facts about the deep past, but an attitude that might be called the deep present. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • http://www.facebook.com/brian.c.burrer Brian C Burrer

    If I remember correctly Healdsburg Glass is full of microliths; this fact is hard to resolve and still call them tektites.

  • Aubrey Whymark

    The first picture appears to show genuine tektites in the tray (indochinites and moldavites). The second picture, genuinely showing Healdsburg Glass, looks like obsidian. They very superficially, to the untrained eye, resemble tektites. The surface sculpture is quite unlike that of a tektite in detail, with no diagnostic tektite-like features. The shapes also do not fit with tektites. To date there is zero proof these are tektites and the authors have not done any diagnostic tests. I'd be extremely surprised if these were anything other than obsidian: In fact if these are tektites I'll eat one!

  • Bob V

    No worry about having to eat one, Aubrey. Our colleague, Mendy Ouzillou, has tested several specimens in a kiln and none of them melted like a tektite – they all boiled into a froth just like moisture-laden obsidian. Such an easy test to conduct, yet this folly still persists 10 years later. Regrettable.

    • Andrew Alden

      Bob and Aubrey, you can satisfy yourselves by reading Erickson's presentation, which is now online. Let me know what you think.

    • http://www.facebook.com/brian.c.burrer Brian C Burrer

      Bob It seems you have confused the results of Mendy's experience with some other test results. The two previous tests whose data was shared with me showed a low (70 ppm) water content. I discussed Mendy's test with him in a phone conversation yesterday. Healdsburg glass did not foam or melt for him at 1700 F, the maximum temperature of Mendy's kiln.
      This glass is very curious as it shares unusual traits with tektites. If we ultimately accept is as a type of tektite it will mean some revision in the definition of tektites.

  • Bob V

    The late Darryl Futrell taught me a simple field test to distinguish tektites from obsidian. Hold-up a sliver or thin-edge of a specimen to the sun or bright light. In transmitted light a tektite will always be brown to yellowish to greenish. Obsidian will be gray to purplish. This test has never failed me. There are only two obsidian localities in the entire world where this field-test might be problematic. Hope the tektite fakers never find-out about those localities.