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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.
![megamud valcano map megamud valcano map](https://www.mdpi.com/water/water-13-00155/article_deploy/html/images/water-13-00155-g001.png)
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.
![megamud valcano map megamud valcano map](https://pbs.twimg.com/media/D-6ISv8VUAAGloJ.jpg)
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.