Oat Hill Mine Trail

The Sonoma Volcanic field has been extinct for 3 million years. The volcanic vents did not form distinct volcanoes as one might think. The eruptions occurred along ancient fault zones and magma flowed out through pipe-like openings. The initial eruptions were mostly pumice - a frothy form of volcanic glass. After explosive eruptions andesite lava flowed, and was followed by more explosive activity that deposited huge volumes of rhyolite lava and volcanic ash. The final episode in the Sonoma Volcanic field formed a "rhyolite-like" welded tuff unit that capped Mt. St. Helena.