Volcanic eruption columns typically have unsteady source conditions, where mass and heat fluxes from the vent evolve or fluctuate on time scales from seconds to hours. However, integral plume models routinely assume source conditions that are statistically stationary, and the degree to which source unsteadiness influences the mechanics of column rise and air entrainment has not been established with quantitative predictions. We address this knowledge gap by examining eruptions with varying unsteady character at Sabancaya Volcano, Peru. Using a novel tracking algorithm based on spectral clustering, we track the spatiotemporal evolution of coherent turbulent structures in columns using ground-based, thermal infrared imagery. For turbulent structures tracked in time and space, we calculate the power law decay exponent of excess temperature with height. In general, the starting pulses of transient events are characterized by power law exponents matching theoretical predictions for an instantaneous point release of buoyancy (i.e. a thermal), which evolve with sustained emissions to values consistent with steady plumes. Our results support previous findings from field evidence and laboratory experiments that entrainment and gravitational stability in unsteady volcanic columns are inadequately captured by time-averaging or constant entrainment coefficients. We propose a quantitative definition for column source unsteadiness which captures the timing and magnitude of source fluctuations on time scales that influence entrainment mechanics, and which provisionally predicts our observed differences in power law behavior. We argue for systematic experimental and numerical studies of the relationship between source unsteadiness and entrainment to develop unsteady entrainment parameterizations for integral plume models.

Relating the mass eruption rate (MER) of explosive eruptions to column height in the atmosphere is key to reconstructing past eruptions and forecasting volcanic hazards. Using 134 eruptive events from the Independent Volcanic Eruption Source Parameter Archive (IVESPA v1.0), we explore the canonical MER-height relationship for four measures of column height: spreading level, sulfur dioxide height, and top height from both directly observed plumes and those reconstructed from deposits. These relationships show significant differences and should be chosen carefully for operational and research applications. The roles of atmospheric stratification, wind, and humidity remain challenging to assess across the large range of eruptive conditions in this database, ultimately resulting in empirical relationships outperforming analytical models that account for atmospheric conditions. This finding reveals the complexity of the height-MER relation that is difficult to constrain based on available heterogeneous observations, which reinforces the need for improved datasets to develop eruptive column models.

250 Ma, Pangea had just reached an equatorial position of dynamic equilibrium, after a 60° northward migration due to True Polar Wandering. It then began oscillating about itself for the next 150 Myr. The resulting extensional stresses triggered three successive phases of breakup, controlled by the mechanical resistance of a crescent of thick lithosphere, surrounding the Tethyan realm, which had adjusted the supercontinent to its hemispheric shape. The fracturing of the crescent was produced in three successive generations, each new generation corresponding to Coulomb fractures, conjugates of the preceding set. Flood basalts were associated with these deep fractures within the thick lithosphere crescent. We consider unlikely that this highly ordered pattern of fracturing was determined by the locations of the impacts of successive plumes. Between 260 and 180 Ma, thermal isolation was maximal and the asthenosphere of Pangea was about 150°C warmer than below Panthalassa. From 180 to 100 Ma, the breakup elongated Pangea by about 3 000 km in a NNW-SSE direction, producing gaps in the subduction girdle. Lateral mixing began, leading to a continuous rise in global sea level and progressive return to a globally homogeneous upper mantle with sea-level at its maximum 100 Ma. This Cretaceous Revolution marked the end of the Pangea tectonics, radically different from our present plate tectonics. Neither post-Cretaceous plate kinematic inferences, nor mantle dynamic and associated planetary cooling inferences are extendable to Pangea times.