Obsidian Buttes Area

Rising from the thick alluvium and lacustrine sediments on the southeastern edge of the Salton Sea are four volcanic domes that lie on an axis trending roughly northeast-southwest. From north to south they are Mullet Island, Pumice Buttes (Red Island), Salton Dome (Rock Hill) and the misnamed Obsidian Buttes. Though all are the result of late Pleistocene volcanic activity, they differ from one another in form and composition. Obsidian Buttes is composed primarily of pumice and has several small obsidian outcrops on the periphery. The Salton Dome is of a dense rhyolitic material and differs from the others in structure, which borings have shown that the volcanic rock has a mushroom-like shape, in that the form is plug-like as shown in the old quarry. Pumice Buttes is primarily pumice in composition, while Mullet Island consists of a metamorphosed rhyolitic material.

Between the two northerly domes there is further evidence of the recent volcanism. Here are several clusters of hot springs, mud pots (mud "volcanoes") and small geysers which vent steam, carbon dioxide and hydrogen sulphide. The axis of the mud-pot clusters is at an angle to that of the volcanic domes and more nearly parallels the main trend of the San Andreas fault zone which here runs in a northwesterly direction .

The entire area is of considerable geologic interest, for the volcanism appears to be an on-land manifestation of the East Pacific Rise which has opened the Gulf of California and is now being overridden by the North American continent. Traces of wave-cut terraces may be seen on several of the domes. These were formed during the Lake Cahuila era (which may have been extant as late as 400 years ago) when the water level fluctuated, depending on whether the Colorado River was flowing into the Salton Trough or the Gulf of California. During periods of high water these domes were below the surface. Since the Salton Sea was formed in 1905-1907, the domes have at various times been islands. (See also Salton Sea Margin Areas and Salton Sea Wetlands).

Integrity: Quarrying operations have taken place on several of the domes. In the mud-pot area a number of carbon dioxide wells were drilled, with some of the gas used for making dry ice. As this area is one of the most promising geothermal fields in the State, it has been extensively test drilled. Interest is in both the generation of power, for which there is a tremendous potential, and the recovery of the minerals from the brine. However, the highly corrosive action of the brine and the waste disposal problem has, to date, prevented significant development of this resource.

Use: Educational, research, present. It is to be hoped that some of the mud pots will be protected when the area is commercially developed.

Ref: Kelley, V. C. and J. L. Soske, 1936. Origin of the Salton Sea Volcanic Domes, Salton Sea, California. Jour. Geology Vol. 44, No. 4, pp. 496-509.

December 1975