Formation of Turkey Creek Caldera, SE Arizona

In the Oligocene, about 26.5 Million years ago, southern Arizona was the site of a cataclysmic volcanic eruption that formed the 12-mile-wide circular Turkey Creek Caldera. As shown in this graphic, huge clouds of volcanic ash and gas belched at supersonic speeds from ring dikes along the perimeter of the formative caldera. The resulting volcanic plume rose tens of miles high before collapsing into ground-hugging, pyroclastic flows that covered large parts of southeastern Arizona and western New Mexico. It is estimated that more than 120 cubic miles of molten rock was erupted. Online resource Pallister and others, Guide to the Volcanic Geology of Chiricahua National Monument and Vicinity, Cochise County, Arizona. The report includes a geologic map, field shots and colored illustrations: >http://pubs.er.usgs.gov/publication/i2541