Subplinian eruption mechanisms inferred from volatile and clast dispersal data

Subplinian tephras erupted during the North Mono eruption of 1350 A.D. contain abundant obsidian clasts. The weight percent of water in the clasts from each tephra bed can be used to infer the depths from which the clasts originated within the magmatic system. A comparison of the depths of origin with inferred eruption dynamics and local bedrock stratigraphy suggests that the clasts were eroded from conduit walls during repeated withdrawal of magma from a replenished shallow magmatic system. Furthermore, the obsidian was preferentially formed at and eroded from sections in the conduit where increased cooling and magma fragmentation took place due to proximity to groundwater or to the surface. Finally, the data are compatible with magma withdrawal during separate explosive phases resulting from the movement of a disruption surface downward into magma columns that became larger in cross-sectional area with each explosive phase.