We present crystallization depth vs. temperature estimates for clinopyroxene phenocrysts from the Springerville volcanic field, Arizona. These calculations reveal several intriguing patterns that have considerable implications for magma transport and genesis. First, partial crystallization occurs over a wide depth range (0–60 km), but most partial crystallization occurs between 0 and 30 km. Second, low crystallization temperatures, low-density magmas, and evolved liquid compositions derive exclusively from two depth intervals, 0–12 and 23–30 km. These intervals coincide with a density contrast in the upper crust and a rheology contrast at the base of the middle crust. They also coincide with two highly seismically reflective depth intervals. These relationships indicate that (1) the Moho is not a staging area for volcanic eruptions; (2) density contrasts in the upper crust, and a rheology contrast in the middle crust, control magma transport and liquid evolution; (3) magma conduits are probably magma mush columns, with a preponderance of sills within the 0–12 and 23–30 km intervals; and (4) seismically reflective layers are sills related to Tertiary–Holocene volcanic activity. Moreover, these sills appear to represent the principal sites of magma evolution.