Fee and mileage charge: +/- $35, as standard for club; milage at.09 to .14 per mile depending on price of gas.
Equipment needed: Screen, extension cords, and table.
Length of program: 1 to 1 1/4 hour.
Advance notice needed: 1 month.
Area traveled: Fresno, San Luis Obispo and north.
THE FORMATION OF THUNDEREGGS - Reflecting a widespread bias toward "Rockhounds" and — in contrast to the situation in Europe — toward agate and other "cryptocrystalline" quartz as minerals, in the United States few geologists have taken an interest in the origin and formation of thundereggs. Yet these objects, by other names, were being commented upon in this country as early as 1893 and in Europe in the 18th century.
The development of thundereggs is a complex process, distinctly different from the process that forms amygdaloidal nodules. Thundereggs occur, presumably worldwide, in acid volcanics and often display great beauty in cross section. Much work on their formation has been done in recent decades in Germany. Some experimental results have been achieved.
A good paper by Daniel Kile, a USGS geologist in Denver, was published in 2002. Perhaps of most interest is a 385 plus page study by the "Geode Kid" based on his very extensive observations while mining thundereggs in a variety of localities throughout the western U. S. A worldwide gallery of specimens is presented.
John Stockwell received his B. S. in geology magna cum laude at Yale in 1957. He was employed as an exploration geologist by BP Alaska and later Sohio from 1974 -1981, working mainly on and in Alaska. After 1981 he remained in the Bay Area and eventually retired in 2002 from teaching high school earth science and chemistry. He now considers himself an amateur geologist who happens to have had some formal training and professional experience.
He is a member of the board (K-12 programs) of the Northern California Geological Society, past president of North Bay Field Trips, and is currently Field Trips North chairperson for the California Federation of Mineralogical Societies. The past 10 years he has taken a considerable interest in thundereggs, collecting extensively and reviewing problems of their formation. These studies have led to a developing interest in acid volcanics. Mr. Stockwell resides in Berkeley and is a member of the San Pablo Bay Gem and Mineral Society.
CFMS Program Manual and Directory, October 2009