The World’s Smallest Mountain Range: The Sutter Buttes

Brian Hausback, California State University Sacramento ― April 21, 2007

Submitted by Andrew Alden
Images courtesy Andrew Alden except where noted

This report, prepared in 2024, is adapted from a photo gallery written for About.com in 2007. We have surely advanced our understanding in the interim!

It was a privilege for NCGS members to be escorted through the Sutter Buttes on an unexpectedly cool and gray spring day. Access to this privately owned area was largely controlled at the time by the Middle Mountain Foundation, under whose auspices we paid this visit. (Since our visit, the foundation has morphed into two sister organizations, the Sutter Buttes Regional Land Trust and Middle Mountain Interpretive Hikes.)

The Sutter Buttes aren’t typical flat-topped buttes, but instead are lava domes. They are what old-timers would have called sugarloaf mountains, but butte is the correct term for these steep-sided hills. The locals today sometimes call Sutter Buttes “the world’s smallest mountain range.” The Buttes sit in mysterious isolation in the middle of the flat Sacramento Valley, a magnet of fertility for the indigenous tribes but a puzzle to generations of geologists.

Location map of the Sutter Buttes in Sacramento Valley (USGS image).

This view from space shows the rugged eruptive center of the Sutter Buttes surrounded by a gently sloping apron of volcanic sediments. (NASA image)

The Buttes are widely visible around the Sacramento Valley, but only when the persistent agricultural haze doesn’t obscure them. The rich bottomland around them is dedicated to mixed agriculture. The slopes are rocky and best suited for range. The oak-studded peaks are almost all privately owned. North and South Buttes, at left and right, are the highest points in the range.

South Butte is the highest point in the Sutter Buttes, standing 645 meters above sea level. The lava domes in this range have a fairytale quality.

Our trip leader was Dr. Brian Hausback of Cal State University Sacramento, here showing us a remarkable rock specimen. We began at a parking lot lined with boulders quarried from the Buttes. They were part of the attractions, but first we got oriented with Brian’s latest mapping.

Mounted with magnets on the side of a truck, this was the most up-to-date map of the Buttes’ geology. The center consists of 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 did I) favored 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 (or subducted crust, such as a seamount) 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. I took some pictures and hand specimens.

The heart of the Sutter Buttes consists of highly crystalline porphyritic andesite. The lavas of the Buttes are so packed with crystals that they could barely flow. The larger whitish grains are crystals of sanidine (high-temperature K-feldspar) and the smaller black grains are hornblende and biotite. Andesite is a volcanic rock that is higher in silica content than basalt, which gives it a lighter color and a different set of minerals. A rock like this, with a population of large crystals (phenocrysts) in a fine-grained groundmass, is called a porphyry or, more correctly, a rock with porphyritic texture.

The groundmass can be either gray or red, depending on chemical conditions near the surface. Red rocks were altered at high temperature by steam. This hand specimen shows dark hornblende laths and needles as well as sanidine phenocrysts.

Hausback told us the porphyritic andesite of the Sutter Buttes is on display in the Smithsonian Institution as a museum-worthy example of the type. He also said that the rock was quarried for use lining canals, reinforcing riverbanks and bolstering levees all around the Central Valley.

This hand specimen displays hexagonal biotite crystals as black dots along with sanidine phenocrysts.

The magma also caught and incorporated pieces of other rocks—xenoliths. The dark (mafic) xenolith in this boulder shows signs of melting in its wavy (crenulated) margins.

Rhyolite has even more silica than andesite and takes on a banded appearance during its slow, taffylike flow.

Sutter Buttes rhyolite displays large grains of quartz in a smooth groundmass like sugar crystals in taffy. Rhyolite can be black, gray or red as well as white.

We drove Pass Road across the south side of the Buttes through the Moat. The marine shales here are rarely exposed because they so quickly return to clay.

Large boulders from the pyroclastic deposits of the Rampart crop out south of Pass Road. Many of these have rolled down from higher source beds.

We then entered the Buttes on foot to examine some large-scale volcanic features in the Rampart. As we crossed a saddle, North Butte came into view. We didn’t get much closer to it than this. North Butte is a relatively large andesite dome that reaches a height of 568 meters. Its craggy top appears to date from its eruptive days in the Pleistocene.

It was a short walk and a steep climb to this cliffy outcrop of the volcaniclastic rocks making up the Rampart, on the side of a small hill labeled “Pigeon” on the large-scale topo map.

This view from the scene looking southwest skirts the edge of the Castellated Core and shows the distant Moat with a low section of the Rampart beyond it.

From about 50 meters away, the cliff is seen to be capped by a conglomerate layer above a finer-grained rock displaying cavernous weathering.

We approached and proceeded to geologize under Hausback’s guidance.

The upper layer is a pyroclastic deposit, emplaced as a red-hot landslide, while the lower layer appears to be from a lahar, or volcanic mudflow. We know from modern examples that lava domes are sloppy landforms that often collapse after being built up from the inside by effusive activity. When they collapse, the resulting pyroclastic flows are basically red-hot rockslides, consisting of mostly large particles of hard rock and not much fine-grained material, although the farther they flow the more fine material is generated. The upper deposit here is typical of that kind of “block-and-ash” flow. The other kind of pyroclastic flow is the vulcanian type, resulting from explosive rather than effusive activity. It has finer particles and consists of ash and lightweight pumice more than solid lava. The lower lahar deposit is likely to have come from such a flow, putting the two types together here in one remarkable outcrop.

This boulder’s blocky fracture is a sign that it was emplaced hot and cooled quickly, like hot marbles dropped in water. The same feature is often seen in pillow lava.

We then moved to another spot with a good view of North Butte. Some other day . . . At least none of us got sunburn from our day in the heart of the Sacramento Valley.

NCGS is grateful to Brian Hausback for the rare chance to experience the topography, geology, and atmosphere of the Sutter Buttes.