The most active period in the volcanic history of the Chiricahua Mountains began near the end of the Mesozoic. Volcanism, as well as the formation of compressional structures, were due to the presence of a subduction zone beneath the southwest United States associated with the Laramide orogeny. This period began around 70 Ma with the subduction of the Farallon and Kula plates beneath the North American Plate, and ended around 20 Ma when the Pacific Plate's spreading ridge ran into the subduction zone. The subduction of these slabs led to a period of intense volcanic activity in the western United States, and specifically in the Chiricahua Mountains around 35-25 Ma. Small, scattered volcanoes formed in the area, producing dark andesitic and basaltic lava flows, along with more viscous rhyolite flows. Around 26.9 Ma, a massive magma chamber formed beneath the crust just south of the present-day Chiricahua National Monument. The overlying rock eventually ruptured, expelling around 100 cubic miles of magma. Pyroclastic flows of hot silicious ash and pumice were laid down over a 1,200-square-mile region that eventually cooled and lithified into rhyolitic tuff. The volume of material ejected was sufficiently massive to cause the magma chamber to collapse and form what is now known as the Turkey Creek Caldera. It is estimated that the eruption was one thousand times greater than the 1980 eruption of Mount St. Helens. Large folds and faults associated with compressional tectonics can also be observed in Paleozoic and Mesozoic sedimentary rocks.