Why is it said that a volcano in Cumbre Vieja, in the Canary Islands, will cause a megatsunami?

In 2001, Steven N. Ward and Simon Day published a study warning about the danger of a tsunami caused by the collapse of the slope of one of the active volcanoes of the Atlantic Ocean. Specifically, it identified Cumbre Vieja as the most active volcano in the last 125,000 years.

Assuming a worst-case scenario, where 500 km³ of land 25 km long, 15 km wide and 1.4 km thick would be displaced, and assuming that there is little difference in ocean depth, they calculated that a 900 m high wave would be produced, which would be reduced to 500 m at 50 km. At 10 minutes the wave would have moved 250 km away and would still be several hundred meters high. Between 15-60 minutes, they indicated that waves of 50-100 meters would reach the African coast, slowing down, while the European coasts would see waves of 5 to 7 meters in height. In the west, they estimated that the waves would reach Newfoundland at 10 meters high, while on the coasts of North and South America they would reach 15-20 meters.

This prediction was based on the premise that the eruption of the San Juan volcano on June 24, 1949 had created an unstable zone that reached the base of the volcano and could collapse without warning. Although this type of behavior has been part of the island's history, like the Cumbre Nueva collapse, it is unlikely to occur at Cumbre Vieja in the near future. Even if it were to occur, it is estimated that it would not have the same volume. Dikes and basaltic landslides on the west coast, both on land and underwater, would also limit its extent.

A megatsunami is not really expected to occur, either in the Atlantic Ocean or elsewhere. For it to occur, a large landslide would have to occur virtually instantaneously. If it were not swift or collapsed in stages, as usual, the effects would be local. In fact, on the seafloor of La Palma there are deposits of eight landslides of the volcanic slope. The largest has a volume of 350 km³ and it has been observed that they were deposited intermittently. Even with all this, if a landslide of the characteristics described by Ward and Day were to occur, the consequences would not be so catastrophic, as their data did not take into account seafloor features that could affect tsunami displacement over large distances and would only be applicable in a local environment, with shallower water. Nevertheless, the megatsunami threat is regularly and sensationally repeated in the media.

And if you don't know how a landslide behaves, I recommend this recording of the Pantai Remis landslide in a tin mine in Malaysia on October 21, 1993. Although the quality is not the best, it shows the process very well.

Sources

• Ward, S. N., & Day, S. (2001). Cumbre Vieja volcano—potential collapse and tsunami at La Palma, Canary Islands. Geophysical Research Letters, 28(17), 3397-3400.
• Pararas-Carayannis, G. (2002). Evaluation of the threat of mega tsunami generation from postulated massive slope failures of island stratovolcanoes on La Palma, Canary Islands, and on the island of Hawaii. Science of Tsunami Hazards, 20(5), 251-277.
• Hunt, J. E., Wynn, R. B., Talling, P. J., & Masson, D. G. (2013). Multistage collapse of eight western Canary Island landslides in the last 1.5 Ma: Sedimentological and geochemical evidence from subunits in submarine flow deposits. Geochemistry, Geophysics, Geosystems, 14(7), 2159-2181.