Emplacement of a Debris Avalanche during the 1883 eruption of Krakatau (Sunda Straits,Indonesia)

Conclusion The data collected during the Mentawai cruise help to clarify understanding of the 1883 eruption of Krakatau. We have previously discussed the weaknesses of the interpretation of Williams (1941) and others (Self and Rampino 1981) and emphasized that only a Mount St. Helens-type collapse during the course of the eruption could account for all the characteristics of the eruption and of the related deposits.The discovery on land of deposits attributable to a debris-avalanche, in the stratigraphic position where they were expected, is a strong argument for the validity of our scenario.Marine surveys confirm that the sea bottom around Krakatau is covered by a thick ignimbritic deposit. But the presence of this deposit does not invalidate the presence of a debris-avalanche deposit under the ignimbrites. The hummocky morphology favours this hypothesis.Flank-failure of volcanoes is generally considered as a very efficient mechanism for triggering tsunamis (Kienle et al. 1987; Siebert et al. 1987). However, the majority of the volcanoes where flank-failure has been described are tall and bulky and the collapse of a broad edifice like Krakatau may be surprising. However the geological evidence shows that such a mechanism can act at various scales; for example the flank collapse of Mayu Yama volcano (height 700 m, volume 0,3 km³), a parasitic cone of Unzen volcano (Japan), triggered a debris-avalanche into the sea that was 1 km long, with a characteristic hummocky surface; the resulting tsunami killed 9528 people (Katayama 1974). In the same way, a partial collapse of Iliwerung volcano, Indonesia (50 × 10⁶ m³) in July 1979, triggered a tsunami which killed several hundred people (McClelland et al. 1989). At Krakatau, the main summit was 822 m asl; the collapse took place along the edge of the prehistoric caldera and this structural unconformity probably facilitated the triggering of the process.