Volcanics of Napa Valley

The success of the wine industry in Napa is largely due to the variety of soils created from weathering of different types of rocks in the valley, especially volcanic rocks.

The volcanics of the Napa Valley are the Sonoma Volcanics and were erupted over a period of 5 to 3 million years ago. New interpretations by the US Geological Survey have indicated the presence of a volcanic caldera in East Napa.

A caldera is a large circular depression which forms when a volcano violently erupts and collapses into its own magma chamber. The semi-circular basin is about 7 km (4.4 miles) in diameter with downtown Napa roughly on the western edge.

In Spring ‘06, Geology 10 student Kris Kuehnert investigated this caldera and collected volcanic rocks for thin-section analysis. Samples were taken from Skyline Park along the eastern flank of the caldera.

The volcanoes of Napa are called composite cone or stratovolcanoes. The eruptions follow a two stage process: the first stage is a violent explosion with many rock fragments and ash thrown in the air (pyroclastic); the second stage consists of lava flows on top of the pyroclastic flows. In cross-section, this creates a stratified layering to the volcano with repeated eruptions.

Some types of pyroclastic rocks found in Napa include volcanic breccias (a welded rock with large angular rock fragments, indicating a violent eruption) and volcanic tuffs (a welded ash flow with smaller inclusions).

The types of lava flows found in Napa vary from the very black basaltic lavas to the very light-colored dacites and rhyolites. The basaltic flows contain large amounts of iron and when iron weathers it oxidizes to produce a red-colored soil. Many other types of geologic units weather and mix to create as many as 30 different soil types throughout the valley.

The unique mineral content of each of the soils impart characteristics of taste and aroma to the grapes that grow in that particular region.  Along with the soils, the temperate climate of the valley is also critical in producing world-class wines.

Kris is working on his AA degree and plans to major in geology at the University of Nevada at Reno or Sacramento State.

-Article and Photos

by Mark Feighner