GEOLOGY OF A RHYOLITE PLUG, PINAL COUNTY, ARIZONA
Southwest of Superior, Pinal County, Arizona, a shallow-depth, rhyolite plug intrudes Precambrian Pinal Schist and associated Late Cenozoic volcanic rocks. Intrusion took place in a single sequence of episodes reflecting progression from an explosive, gaseous beginning to a more quiescent end.

The plug consists of porphyritic rhyolite, brecciated in the core and bounded by a nearly continuous chilled border of fresh glass. The attitude of planar flow structure at the surface defines a steep, downward- converging cone beneath. Flow features show that the magma was highly viscous. Concentric arcuate portions of the peripheral glass shell suggest that intrusion was controlled by concentric fractures, resulting in the circular outline of the plug.

Around half its periphery the plug invades an intrusive breccia containing triturated schist and rhyolite fragments. This breccia sharply crosscuts an outer zone of brecciated Pinal Schist and is interpreted as resulting from a low-viscosity, fluidized, gaseous magma, in which fragments were comminuted by abrasion.