The nature and timing of caldera collapse as indicated by accidental lithic fragments from the AD ∼1000 eruption of Volcán Ceboruco, Mexico

One way to determine the mechanics of caldera formation is through quantitative component analysis of accidental lithic fragments in pyroclastic deposits erupted before and during caldera collapse. All previous studies of this sort, however, are based on pyroclastic deposits from large-volume (>10 km3) caldera eruptions. In this study, we use quantitative component analysis of lithic fragments to determine the mechanics of the AD ∼1000 small-volume (3–4 km3 DRE) caldera eruption of Volcán Ceboruco, located in western Mexico. During this eruption, caldera collapse occurred in such a way that lithic fragments of decreasing depths were preferentially erupted with time. Prior to caldera collapse, deep-origin lithics (∼6 km depth) and vent-derived lithics were erupted. Deposits emplaced during collapse, however, contain a distinctly different population of lithic fragments, such as lithics from mid-depth origin (∼1 km depth) and from the vent. By the end of the eruption, the total amount of lithic material in the deposits had increased from <15 wt% before collapse to as much as 90 wt%, yet this material contains essentially no lithics of deep origin. This suggests that collapse resulted in the obstruction of both deep-origin lithics and magma from reaching the surface. Based on these data, we suggest that collapse occurred as a result of the fracturing and subsequent failure of the overlying magma reservoir roof that occurred along inward dipping faults. The collapse forced the fragmentation depth and progressive erosion of lithic material from deep to shallow depths, which subsequently sealed the conduit, stopping the eruption.