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Northern California, like the Pacific Northwest of North America, falls along the Pacific Ring of Fire, the world’s most extensive 40,000-km (c.25,000-mile) arc of volcanoes and earthquake zones that circles from Patagonia to New Zealand around the Pacific Ocean Basin. California may no longer be famous for volcanoes, though the last 160 million years – since the Farallon Plate floating under the Pacific slammed into the North American Plate - have witnessed a spectacular progression of volcanic activity up and down the coast. The amazingly varied and complex soils of California wine country, too, are the result of being in a zone of highly active geology. 

The complex story of volcanism that directly affected northern California picks up some 25 million years ago, when the North American and Farallon Plates were met by the north drifting Pacific Plate somewhere near San Diego. At this point subduction of the Farallon Plate was partially replaced with translation, one plate sliding laterally past another, giving rise to more volcanism and a series of unstable faults that still haunt California to this day. The San Andreas Fault is the most famous, and this triple junction – the convergence of three tectonic plates – is responsible for its notable instability, and spectacular damage potential. The junction reached northern California some seven million years ago. 

The volcanic activity in Northern California must have been apocalyptic during this era, if the remnants of diverse volcanic ashes, tuffs, pyroclastic material, lahars and abundant lavas have been properly read. Wave after wave of effusive and explosive eruptions from multiple elongated fissures threw up huge curtains of basalt lavas, andesite breccias and rhyolite tuffs, and added thick strata of powdery white volcanic ash to the landscape.