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Published on Nov 29, 2010
Retired Sonoma State University geology professor Dr. Rolfe Erickson and Sonoma Land Trust stewardship staff hiked the slopes of Little Black Mountain in fall, 2010 to collect rock specimens for zircon dating. According to Dr. Erickson, "Little Black Mountain is a small magmatic body of solidified rhyolite that by previous dating methods was considered to be 100 million years old — much younger than surrounding Franciscan metamorphic rocks thought to be about 160 million years old by fossil dates. Recent zircon dating shows the Franciscan to be 83 million years old, and the Little Black Mountain rhyolite date is now too old for the body to be a simple intrusion. We will use the new zircon dates to further resolve this conflict. Another feature of this study is very accurate chemical and mineralogical analysis to characterize this type of rock more precisely." Dr. Erickson has studied the geology of the Cazadero, California area for over four decades and produced multiple papers on the topic.