The Salton Buttes include: Mullet Island, Obsidian Butte, Red Island and Rock Hill. Red Island is composed of two domes separated by some 200 m of Quaternary gravels. They consist of rhyolite and the latest volcanic activity may have occurred in 6450 BC.
A new study appearing in the journal Geology in 2012 indicates the buttes may have last erupted more recently, between 940 and 0 BC.
Within the geothermal area older Pleistocene age buried domes have been encountered in drilling operations.
^ abRobinson, Paul T., Wilfred A. Elders and L.J.P. MufflerR, Quaternary volcanism in the Salton Sea geothermal field, Imperial Valley, California, Geological Society of America Bulletin 1976;87;347-360
^Schmitt, Axel K., et al., (U-Th)/He zircon and archaeological ages for a late prehistoric eruption in the Salton Trough (California, USA), Geology, published online on 15 October 2012 as doi:10.1130/G33634.1 Abstract