Twin Buttes (California)

The Twin Buttes are two volcanic cinder cones located in the Cascade Mountain Range in Shasta County, California. They are part of the Bidwell Spring chain and lie within a region that was active in the Quaternary. Formed during the Pleistocene between 25,000 and 15,000 years ago, the volcanoes erupted lava flows that coursed toward the Burney Mountain lava dome. These lava flows cover an area of 10.1 km2 and are made of basalt and dacite. The volcanoes also erupted cinder and volcanic ash that reached eastward.

As of 2012, the Twin Buttes were still monitored by the United States Geological Survey for deformation, an indicator of pre-eruptive activity. However, they are considered to have "low to very low" threat potential for future eruptive activity.

Geography
The Twin Buttes are two cinder cone volcanoes that lie north of Lassen Peak in Shasta County, California. The Twin Buttes reach a summit elevation of about 5351 ft. Nearby towns include Burney, Old Station, and Viola. About 6,700 people live within 30 km of the volcanoes, though the population within 100 km climbs to more than 260,000. The buttes are aligned north–northwest and lie at the center of the Bidwell Spring volcanic chain.

Geology
The Twin Buttes — and other volcanoes near Lassen Peak — are part of the Cascade Volcanic Arc, which was produced by subduction of the oceanic Juan de Fuca tectonic plate under the North American tectonic plate. Volcanic activity in the region is also influenced by the westward expansion of the Basin and Range Province into the Cascades. Volcanism in the region encompasses a wide variety of eruption types, ranging from cinder cones to shield volcanoes. Eruptive activity has for the most part produced overlapping, mafic volcanoes through nonexplosive to weak explosive eruptions. Volcanic activity during the Quaternary has produced basalt, basaltic andesite, and olivine tholeiite. Other major volcanic centers near the Twin Buttes include the Yana, Maidu, Dittmar, and Latour centers, which were long-lived volcanic systems with magma ranging in composition from andesite to silicic rhyolite; these four systems are now eroded with extinct hydrothermal systems.

The Bidwell Spring chain consists of five eruptive units including the Twin Buttes basalt. 40Ar/39Ar has placed two other deposits, basaltic andesite from Black Butte and andesite from Bidwell Spring, at 62,000 ± 10,000 years old and 68,000 ± 6,000 years old, respectively. These results suggest several small eruptions between 65,000 and 45,000 years ago from the Bidwell Spring chain. Two other basaltic andesite deposits from the chain have been described by the United States Geological Survey (USGS), which considers the Bidwell Spring chain as part of the Caribou Volcanic Field, a system of 11 eruptive sequences between 200,000 and 100,000 years ago with vents aligned with faults that focused surface volcanic activity. Basalt from the Twin Buttes overlies eruptive material from the Poison Lake Chain and the Cone Lake Chain, which are also within the Caribou Volcanic field. Twin Buttes basalt is overlain by basaltic andesite erupted during the late Pleistocene from an unnamed volcanic vent, which is thought to be between 45,000 and 25,000 years old.

Subfeatures
Subfeatures of the formation include Red Rock Hill, which has an elevation of 1597 m. Other nearby features include basaltic andesite and a distinct basalt deposit southeast and northeast of the Twin Buttes, respectively, both of which were produced by eruptive activity between 50,000 and 35,000 years ago.

Eruptive history
Located in an area that was highly active during the Quaternary, the Twin Buttes volcanoes formed during the late Pleistocene. The North and South Twin Buttes erupted blocky, partially unvegetated lava flows that moved north toward the southeastern base of Burney Mountain, a lava dome, between 25,000 and 15,000 years ago, covering an area of 10.1 km2. They have an overall volume of 0.296 cumi. Eruptive material consisted of basalt (including picrite basalt) and dacite. Basalt erupted from Twin Buttes forms part of the Bidwell Spring chain with an 40Ar/39Ar age of about 46,000 ± 3,000 years, covering an area of 19.8 km2. The basalt deposit has a total volume of 0.8 km3. At the edges, lava flows are steep, with up to 10 m of relief. The lava flows from the Twin Buttes are porphyritic with about 53% silica content; they have vesicular surfaces and dense interiors. Phenocrysts range from 0.5 to 1 mm; there are also coarse xenoliths of quartz throughout the lava flows. Cinder and volcanic ash erupted by the Twin Buttes extend to the east.

While Twin Buttes last erupted during the late Pleistocene, the area is still monitored by the USGS given its proximity to Lassen Peak. As of 2012, there were three GPS receivers continuously monitoring Twin Buttes for deformation, an indicator of pre-eruptive activity. In 2014, the USGS considered Twin Buttes to have "low to very low" threat potential for a future eruption.