page_title: Salton Buttes
section_title: Composition
hierarchical_section_title: Salton Buttes / Geology / Composition
caption_attribution_description: Red Island - dark rock in red rock. One of the five volcanic Salton Buttes at the Salton Sea - in Imperial County, Southern California
context_page_description: The Salton Buttes are a group of volcanoes in California, on the Salton Sea. They consist of a 7 kilometres-long row of five lava domes, named Mullet Island, North Red Hill, Obsidian Butte, Rock Hill and South Red Hill. They are closely associated with a fumarolic field and a geothermal field, and there is evidence of buried volcanoes underground. In pre-modern times Obsidian Butte was an important regional source of obsidian.
  The Salton Buttes lie within the Salton Trough, a tectonic depression formed by the San Andreas Fault and the San Jacinto Faults. The depression forms the northward extension of the Gulf of California, and is separated from it by the Colorado River Delta. A number of geothermal and volcanic features are located in the area, which is a region of active seafloor spreading.
  While the Salton Buttes were formerly considered to be of the late Pleistocene epoch, newer dating efforts have determined that all of them formed more recently, during the current Holocene epoch, mostly through effusive eruptions. Future eruptions are possible and could endanger the surroundings.
context_section_description: The domes are formed by rhyolite, which has an alkaline and calcium-poor composition and defines a potassium-rich suite (see QAPF diagram). Scarce phenocrysts include amphibole, anorthoclase, apatite, clinopyroxene, ilmenite, magnetite, orthopyroxene, quartz and zircon. The isotopic composition of these volcanic rocks differs from that of the Salton Trough sediments. The total volume of the domes is about 0.5 cubic kilometres (0.12 cu mi).
  Obsidian from Obsidian Butte has been identified in distant archeological sites. It has been found in sites in the Colorado Desert, across San Diego County and across northern Mexico and the Southwestern United States. Before Obsidian Butte was emplaced and thus became available for obsidian production, people in the region used obsidian from the Coso volcanic field; this trend away from Coso towards Obsidian Butte was recognized even before the exact timing of the Obsidian Butte eruption was known.
  The way in which the magmas are formed is controversial. Explanations have called on several different processes, including assimilation of hydrothermally altered rocks, fractional crystallization and partial melting. One model presumes the existence of a continuously active deep magmatic reservoir, which generates precursor magmas and episodically delivers them to a shallower magmatic reservoir, where the rhyolites are formed.
  Xenoliths found in the volcanic rocks include granite, granophyre, metasediments and tholeiite, often heavily altered by interactions with the volcanic system. Some of these basalts resemble those from active rift zones in the Gulf of California and the East Pacific Rise. The northernmost segment of the East Pacific Rise appears to be identical to the Brawley Seismic Zone.
caption: A xenolith on Red Island
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