Marginal stability of atmospheric eruption columns and pyroclastic flow generation

Explosive volcanic eruptions frequently generate fall and flow deposits sim ultaneously, which can be attributed to a marginally stable atmospheric col umn in transitional conditions between the buoyant and collapse regimes. Th is behavior is reproduced by laboratory experiments and numerical simulatio ns. Ten well-documented eruptions are used to test theoretical models of ex plosive eruptions. Three types of deposits, fall, flow, and composite depos its made of intercalated flow and fall units, are observed in these eruptio ns. Estimates of mass discharge rate and initial volatile concentration in the magma are available for each eruptive phase. Using the simple assumptio ns that (1) the mass fraction of gas in the mixture is equal to the initial volatile content of magma and (2) jet expansion outside the vent is uncons trained by crater dimensions, theoretical predictions are not consistent wi th the data. Agreement between data and theory may be achieved by appealing to imperfect degassing of pyroclasts, which lowers the gas content of the erupted mixture. The effective amount of continuous gas phase carrying pyro clasts in suspension depends on the size distribution of pyroclasts. In coa rse pyroclast populations a large amount of magmatic gas remains trapped in bubbles within the pyroclasts and is not involved in the bulk volcanic flo w. A new regime diagram based on estimates of the effective gas content in the erupted mixture allows good agreement with the observations. For given mass flux and initial dissolved volatile content, changes of the size distr ibution of pyroclasts may have a strong effect on atmospheric column behavi or.