An important question is whether the Sutter Buttes are related to the Cascade Range, which extends from the volcanoes of southern British Columbia to Lassen Peak in northern California, or to the Coast Range volcanic trend, which includes the Berkeley Hills volcanics, the Sonoma Volcanics, and the Clear Lake Volcanics (the youngest of these, with eruptions from 2.1 million years to 10,000 years ago). The Sutter Buttes do not fall on the geographic trend of either the Cascades or the Coast Range. However, the composition, texture, and age of the volcanic rocks suggest that the magmas that formed the Buttes are most likely part of the magmatic system of the Coast Range volcanoes.

But why did eruptions occur at this isolated location in the middle of the Sacramento Valley, far from other volcanoes? Geophysical studies have revealed a prominent north-south linear “ridge” that has anomalously high magnetism and gravity deep beneath the Sacramento Valley and directly under the Sutter Buttes. These geophysical features may be the expression of a buried, inactive fault that separates granitic and metamorphic rocks of the Sierra Nevada to the east from oceanic rocks of the Coast Range to the west. Additional faults intersect this deep crustal fault to provide possible conduits for Sutter Buttes magmas to rise to the surface. The Sutter Buttes contain the only lavas to erupt at the surface in the Great Valley, but rhyolites similar to those at the Buttes are found in the subsurface just to the west, along the Willows Fault.

The Sutter Buttes stand as a remarkable geographic and geologic feature of California’s Great Valley, and they remain a subject of scientific interest and research. One curious feature is the presence at the surface in the castellated core of a huge block, one-quarter mile across, of crystalline rock (Cretaceous granite) characteristic of the Sierran basement. How was this block carried upward to the surface? Or is it perhaps the tip of an upfaulted sliver of the basement deep below? This is one of many puzzles remaining for geologists to solve in future studies.

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