Contemporaneous convective and collapsing eruptive dynamics: The transitional regime of explosive eruptions

Two contrasting eruptive regimes have been classically postulated to describe the behavior of explosive eruptions: convective and collapsing. Early studies have recognized that many eruptions evolve from the first to the second regime and have assumed the existence of a sharp boundary between them. Consequently, the dynamics of such transition has never been investigated in detail. Here, we present results of integrated deposit analyses and numerical simulations which demonstrate univocally that both dynamics can, and often do, coexist for hours in a new intermediate transitional regime of explosive eruptions. The study elucidates the features of the new regime in terms of mass partitioning, pyroclastic current dynamics and interplay among convective and collapsing styles.