| Abstrak/Abstract |
Plinian eruptions located near the sea or a lake are likely to generate pyroclastic density currents (PDCs) that trigger tsunamis and propagate on the water surface, thereby widening the range of hazards and possible threats to populations. In this study, we reinvestigate the iconic eruption of Tambora (Sumbawa Island, Indonesia) in 1815 with the aim of determining the source of the tsunami observed at the beginning of the eruptive paroxysm and constraining the propagation of PDCs over water. To this end, we analyzed stratigraphic sections of the 1815 deposits on the coasts of the Gulf of Saleh, 40–55 km southwest of Tambora volcano. Based on grain size distribution and componentry, we distinguished four types of deposits: tephra fall, PDC, co-PDC, and tsunami deposits. Pumice-rich PDCs (so-called PDC-1 phase) propagated over 15–20 km of land and 15–30 km of sea before reaching the opposite (western) shore of the gulf. Except for a slight (~ 10%) reduction of the lithic content, the influence of water on PDC-1 propagation seems minor, possibly due to the formation of vapor at the base of the PDC and/or the presence of pumice rafts at the water surface. Scoria-rich PDCs (PDC-2 phase) apparently did not reach the opposite shore of the gulf. There is only one tsunami recorded in the coastal stratigraphy, above the F4 Plinian fall deposits and below the PDC-1 deposits. The altitude of the tsunami deposits ranges between 1 and 5 m a.s.l. (corresponding to a wave height ~ 1–2 m at the shore). This tsunami was most probably generated by the entrance of the concentrated lower part of PDCs (PDC-1) into the sea on the western coast of the volcano. Tsunami waves travelled faster (~ 50 m/s) than the upper dilute part of PDCs that decoupled upon impact with water and then travelled over the sea, as indicated by the position of tsunami deposits below PDC-1 deposits in stratigraphy. Considering the probable high mass flux of the PDCs produced during the 1815 eruption, one might have expected higher tsunami waves, but the low angle of incidence between the PDCs and the water surface certainly limited their tsunamigenic potential, as confirmed by laboratory experiments. |
