The Sutter Buttes are the remains of an extinct volcano which erupted between 1.60 and 1.35 million years ago.

The volcano is almost perfectly circular, about 10 miles in diameter. Its highest point, 2117 feet above sea level, is South Butte.

Before the formation of the Sutter Buttes, the land was flat and composed of layers of sandstone, shale, gravel beds, and marine deposits. Most of these sedimentary layers were eroded from the rising Sierra Nevada mountains to the east, the Coast Range to the west, and the Klamaths to the north. Beneath these layers, melted rock, or magma, pushed its way upward and extruded at several different sites, solidifying into large domes.

The first few of these volcanic necks are identified as rhyolite, a light colored rock. Later, more voluminous extrusions cooled into a dark colored rock called andesite. Most of the large, high crags such as South Butte, North Butte, West Butte, and Twin Peaks, are examples of andesite domes.

Pushing through the earth's crust, these extrusions forcibly uplifted, stretched, and cracked the pre-existing sedimentary layers, first arching them into a high dome. As the lava domes breached the surface, they released great volumes of hot, pressurized volcanic gases and steam with great explosive force. These detonations partially fragmented the domes into boulders and large rocks and created numerous small craters within the core area. Fragmented lava blocks and ash were washed away from the high domes by heavy rainfall running off the mountain.

The Sutter Buttes erupted during a Pleistocene glacial period. There were probably no glaciers in the Buttes, but it was a cooler, more rainy climate than today. The outwash of the frequently hot fragmental products of the explosive eruptions accumulated as the peripheral deposits. There is evidence of a deep central crater lake which filled in with this detritus to a depth of 1000 feet.

At last the volcano quieted down, and the long process of erosion began to carve away the softer sedimentary layers, leaving a natural structure that resembles a castle. Exposed now are the central peaks, a castellated core. Immediately surrounding the castle are several large valleys that form a topographical moat caused by the erosion of the sedimentary layers. Beyond the moat the ramparts radiate out in all directions, the remains of ash and debris flows now covered with grasses and oaks. Locally, the cluster of peaks is touted as the "World's Smallest Mountain Range."