Simulating the dispersal of tephra from the 1991 Pinatubo eruption: Implications for the formation of widespread ash layers

PUFF and HAZMAP, two tephra dispersal models developed for volcanic hazard mitigation, are used to simulate the climatic 1991 eruption of Mt. Pinatubo. PUFF simulations indicate that the majority of ash was advected away from the source at the level of the tropopause (~ 17 km). Several eruptive pulses injected ash and SO₂ gas to higher altitudes (~ 25 km), but these pulses represent only a small fraction (~ 1%) of the total erupted material released during the simulation. Comparison with TOMS images of the SO₂ cloud after 71 and 93 h indicate that the SO₂ gas originated at an altitude of ~ 25 km near the source and descended to an altitude of ~ 22 km as the cloud moved across the Indian Ocean. HAZMAP simulations indicate that the Pinatubo tephra fall deposit in the South China Sea was formed by an eruption cloud with the majority of the ash concentrated at a height of 16–18 km. Results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height (~ 40 km) and at a level below the calculated height of neutral buoyancy (~ 25 km). Simulations showed that distal ash transport was dominated by atmospheric circulation patterns near the regional tropopause. In contrast, the movement of the SO₂ cloud occurred at higher levels, along slightly different trajectories, and may have resulted from gas/particle segregations that took place during intrusion of the Pinatubo umbrella cloud as it moved away from source.     

Contrasting styles of explosive eruption at Sete Cidades,São Miguel,Azores, in the last 5000 years: Hazard implications from modelling

The deposits of three eruptions in the last 5000 years are described in detail in order to constrain eruptive parameters and allow a quantitative assessment of the hazard from a range of explosive eruption types at Sete Cidades volcano, São Miguel, Azores. These deposits include: the Caldeira Seca eruption (P17) which occurred around 600 yr BP, which was the last explosive event from inside the Sete Cidades caldera, the P11 eruption, dated at 2220 ± 70 yr BP, and the undated P8 eruption (< 3000 yr BP). These deposits were chosen to represent the range of likely explosive activity from the caldera.