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“This crazy-ass geological formation gives us, quite literally, the perfect weather to ripen Bordeaux grape varieties….”

Coombsville is a distinctive appellation defined by an unusual geological formation called a caldera – a crescent-shaped ridge that creates a half-bowl shape. It was formed approximately 8 million years ago during the height of Napa Valley’s volcanic activity and is essentially a volcanic vent, half of which collapsed a few million years later to create the crescent-shaped formation you see today. This formation is particularly useful for our vineyards, as its perfect half-bowl shape collects the cool marine air that drifts in from the San Pablo Bay and allows our vines to slowly ripen over a longer period of time. If hang time means anything, we’ve got the holy grail here, and we have the caldera to thank.

Volcanos and Napa Valley

Northern California sits on three tectonic plates that come together in a spot known as the Mendocino Triple Junction. Currently the Pacific plate is subducting, while the Western plate is moving parallel to the coast of North America. As the plates move, they create a void or space in the earth’s crust, which makes the magma below begin to well up into a magma chamber. When the pressure in the magma chamber exceeds the holding capacity of the rock above it, it’s forced up and out, causing what we know as a volcanic eruption. 

Around eight million years ago, the tectonic plate movement responsible for the formation of the San Andreas fault moved northward into what is now Napa Valley. This northward movement created a period of volcanic activity that lasted for about five million years, until all volcanism in Napa came to a halt two or three million years ago. (The Clear Lake region is now the site of the fault’s volcanic activity, but it’s currently dormant.) During the active period in Napa, a number of volcanic vents formed, including one in the area that’s now recognized as the Coombsville AVA. Debris from this vent created what we now call Mount George.

Contrary to popular belief, the mountain we see today wasn’t the actual volcano – it’s just a pile of debris created by the lateral (sideways) blast of a nearby eruption. You’ll see some proof of this when you drive out to the Petrified Forest and notice all the petrified trees laying in an east-west orientation – indicating that a lateral blast blew them down, and then buried them under debris to petrify. The other vent is located under Stag's Leap Wine Cellars. They discovered it when they were digging their cave, and found a big mass of congealed magma from the neck of the extinct volcano vent.

When you're inside our Caldwell cave tasting room, you can check out the layers of ash that formed during the last major volcanic eruption.




There are at least two other vents that we know of in Napa Valley, the first of which formed Mount St. Helena. Contrary to popular belief, the mountain that we see today wasn’t the actual volcano, but is rather a debris pile that was formed from the lateral (sideways) blast of a nearby eruption. You’ll see some proof of this when you drive out to the Petrified Forest and note that all the petrified trees were blown down in an east-west pattern, indicating that the lateral blast blew them down and left them to petrify under the debris. The other vent is located under Stag's Leap Wine Cellars, and was discovered when they were digging their cave and found as a mass of congealed magma from the neck of the extinct volcano vent.


Formation of a Caldera

When an active volcano dies (because the plates continue to move underneath it), the heat source shifts, the magma chamber empties, and the surrounding ash and volcanic rock debris collapses inward under its own weight. The circular or semi-circular formation that results is called a caldera.

Geologists measured the specific gravity in and around the area of the Coombsville Caldera, and then modeled it to identify the circular rock formations below as a collapsed caldera dating back three to five million years ago. The neck of the volcano looks to have been about half a mile wide. Back then, sea level was about 300 feet lower than it is now, so what’s now the Napa River would have been a much larger body of water that washed significant amounts of debris into what’s now San Pablo Bay.