IntroductionThe rim of the approximately 10-km-wide resurgent caldera is not exposed, but an arcuate line of andesitic-to-rhyolitic lava domes covers its northern margin. Formation of the rhyolitic caldera was associated with the eruption of ignimbrites, K-Ar dated at 0.76 +/- 0.06 million years ago (Ma), most of which were deposited in the Gulf of California.
Rhyolitic lava domes on the northern margin have been dated at 0.5 +/- 0.04 Ma; dacitic lava domes on the southern side of the caldera appear to be older. The regional NNE-SSW-trending Cimarron fault cuts across the center of the caldera. Hot springs are located along the southern caldera margin, and active seismicity was noted during geothermal exploration. (source: Smithsonian / GVP)
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