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OK, So It’s Not a Volcano!

By Karen MacNeil
August 2, 2019

Note from Karen MacNeil: My friend the geologist David Howell, a former scientist with the United States Geologic Survey and now a visiting instructor on geology and wines at Stanford University, has become our go-to expert for all questions related to geology. And if I ever make a mistake on geology in print, you can bet I get a note from David. Recently, I suggested that the Napa Valley has a volcano—Mt. St. Helena. Well, David quickly set me straight. Here’s his email to me, reprinted with his permission.

Mt. St. Helena at the northern end of the Napa Valley may (topographically) look like a volcano; it certainly has lots of volcanic rock cropping out of it. And at some point in time, there was a volcano near where it stands today. (The source of the Petrified Forest blowdown). But Mt. St. Helena is not a volcano.

The volcanic rocks around there are about 3 million years old, and volcanos are so poorly made they would never hold up that long. The mountain was up lifted by compressive forces in the last 1 to 2 million years much like the rest of the Vaca and Mayacamas mountains that border the two sides of Napa Valley. The particular area of hard rock around Mt. St. Helena (volcanic rock and also Franciscan meta-greywacke and serpentine) rose high. Erosion and gravity have been carving it up ever since, into a shape just like the Matterhorn in the Alps.

There were surely lots of volcanoes in the Napa Valley in the past. Stag’s Leap Wine Cellars’ cave is cut into the throat of one such volcano. The entire area of Coombsville is a caldera, reflecting most of a collapsed volcano. Volcanism arrived in the Napa area 8 or 9 million years ago, and the last puff of volcanic ash was about 2.8 million years ago.

Volcanism is now centered to the north of Napa Valley in Lake County. There, Cobb Mountain represents the youngest volcano (about 10,000 years old). In addition, Clear Lake itself is some kind of a caldera.

A reasonable guess would be that every 30,000 years, there has been a volcanic eruption in one form or another along this part of California’s Coast Ranges. Thus in Napa, during its episode of volcanism, there were roughly 2000 eruptions that laid down the ash, andesite, and rhyolite lava flows where Harlan, Continuum, Caldwell and many many other top Napa wine estates now harvest their grapes.

Sorry to be a pest. David.