Geologic Map of the Sutter Buttes

The center of the Buttes is many small domes of andesite lava (purple), the sediments of a one-time crater lake (light blue) and red spots marking domes of rhyolite, all of Pleistocene age about 1.5 million years old. Uplifted older rocks surround this igneous core, then a nearly circular ring of volcanic deposits (light purple) and the young sediments of the present valley. In the 1920s, volcanologist Howel Williams named the center the Castellated Core, the upturned rocks around it the Moat, and the outer ring the Rampart. Williams, a native of Wales, surely was familiar with the architecture of castles.

The rhyolite domes of the Castellated Core were emplaced first, followed by the andesite domes. Many of these were accompanied by explosive eruptions, creating the Rampart. All this appears to have happened within the space of about 200,000 years. Erosion of the volcano dissected the Rampart and exposed the older upturned soft shales, which eroded easily forming the Moat.

The larger tectonic story of the Buttes is still problematic. Hausback (as do I) favors the explanation that the Buttes are cousins of a string of volcanics that progressed northward with the change in the relationship of the Pacific and North American plates from subduction to transcurrent motion. This change created the San Andreas fault, and the former subducting slab, cut off at its top, opened a "window" to the hotter mantle as it continued to sink. I hypothesize that the slab window exposed a particularly fertile piece of the mantle that yielded an upwelling of magma here. Other slab-window volcanics are in the San Francisco Bay area and include the Clear Lake and Sonoma Volcanics, the lavas of the Oakland Hills, and the Quien Sabe Volcanics near Gilroy.