The ~4-ka Rungwe Pumice (South-Western Tanzania): A wind-still Plinian eruption

The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall deposit with an inversely graded base. Empirical models suggest a maximum eruption column height H_T of 30. 5-35 km with an associated peak mass discharge rate of 2. 8-4. 8 × 10⁸ kg/s. Analytical calculations result in H_T values of 33 ± 4 km (inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2. 3 to 6. 0 × 10⁸ kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of thickness data yields volume estimates between 3. 2 and 5. 8 km³ (corresponding to an erupted mass of 1. 1-2. 0 × 10¹² kg), whereas analytical derivation yields an erupted mass of 1. 1 × 10¹² kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption. The plume corner is estimated to have been ca. 11-12 km from the vent. After an opening phase with gradually increasing intensity, a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic density current deposits.