Apart for this example, such causal links remain speculative, especially for larger-scale flank failures 22. A unique example of coupled slope instability and explosive activity at an oceanic shield volcano is the relatively small debris avalanche (<200 × 10 6 m 3) triggered during the Helecho explosive eruption on the south-eastern flank of Tenerife 733 kyr ago 21. The stratigraphy of the tsunami deposits and characterization of pumice clasts found in these deposits allows us to distinguish at least two successive tsunamis, identify their possible source, and demonstrate the association between a massive flank failure (Icod collapse) and an explosive ignimbrite-forming eruption (El Abrigo) both about 170 kyr ago. Here we document tsunami deposits (marine gravels with pumices) on the north-western flanks of Tenerife, Canary Islands, at altitudes up to 132 m a.s.l. Their composition reflects a mixing of different sediment provenance and assemblages of bioclasts (planktonic, benthic, littoral and subaerial), which are never found in growth position. These conglomerates are preserved at different elevations, often out of the range of marine highstand deposits. Marine conglomerates and megaclasts found at unusually high elevations in Hawaii, Cape Verde, Mauritius and Canary Islands were interpreted as being the result of tsunami waves generated by massive flank failures of oceanic shield volcanoes 14, 15, 16, 17, 18, 19, 20. However, the failure mechanisms and dynamics of flank failures of oceanic shield volcanoes are still poorly documented, thus resulting in great uncertainties on related tsunami hazards 2, 12, 13. Models of evolution of oceanic shields and their rift-zones focus on the relationships between gravitational spreading, the intrusive system and the formation of shallow magma reservoirs 7, 8, 9, 10, 11. There is an abundant literature on volcanic instability and the possible role of internal versus external factors 1, 4, 5, 6. With a volume of 5 km 3 the collapse of Ritter Island stratovolcano (Papua New Guinea) in 1888 was the largest historical volcano flank failure and it produced a 10–15 m tsunami on the coasts of the Bismarck Sea 3. Massive flank failures of oceanic shield volcanoes are often an order of magnitude larger (tens to hundreds of km 3) than the largest debris avalanche affecting other types of volcanic edifices such as stratovolcanoes, and represent a potential source of megatsunami 1, 2.
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