Crystal-rich (40~50 vol.%) intermediate lava has been the primary eruptive product of several recent hazardous eruptions: Mt. Pinatubo, Philippines (1991), Soufriere Hills, Montserrat (1995-present), and Unzen, Japan (1990-1995). Despite this association with such devastating eruptions, the formation, timing, and evacuation of such magma is not well understood: do such eruptions tap a long-lived, multi-cycle crystal mush, or, is it generated in a single magmatic cycle prior to eruption? This thesis explores this question through research at the Unzen Volcanic Complex (UVC), southwestern Japan, where a 500 ka history of crystal-rich dacitic dome eruptions has built the Unzen Volcanic Complex. Zircon geochronology has revealed the protracted history and evolution of the crystal mush zone at the Unzen Volcanic Complex. Individual zircon surface-interior age pairs, together with zircon age spectra, suggest that portions of this crystal mush have been present in the crust since at least the late Older Unzen period (≥200 ka). Significant zircon growth appears to occur throughout the eruptive hiatus (200-100 ka), suggesting that magmatic activity continued uninterrupted through this period. Increasing hafnium contents and decreasing titanium contents with time suggest that the system has evolved towards an overall more evolved system since the late Older Unzen Period. Additionally, Older Unzen eruption samples show restricted chemistry, suggesting that crystallization may have occurred in a more homogenous, less evolved and possibly warmer crystal mush, compared to zircon of Younger Unzen (100 ka – present) that record variable storage conditions skewed towards a more evolved, poorly-mixed crystal mush. These results lead to the conclusion that eruptions at Unzen are tapping a mature and long-lived, multi-cycle mush of significant longevity.