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La Palma's Cumbre Vieja eruption
Should we worry about a flank collapse and tsunami?
Some of you may know that my research team was heavily involved in the 1990s and early 2000s in mapping the geology and monitoring ground movement at La Palma’s Cumbre Vieja volcano, which started erupting yesterday afternoon. The work recognised that the steep west flank of the volcano was unstable, and GPS measurements suggested that up to 500 cubic km could be slowly creeping seawards as a single, discrete, block at 1 - 2 cm a year. My colleagues, Simon Day and Steve Ward published a tsunami model, simulating what would happen if the entire block slid rapidly into the sea in a single event, as well as less extreme scenarios. The paper noted that a worst-case collapse would threaten the entire North Atlantic rim with a large and destructive tsunami. This attracted huge media attention, as did the BBC Horizon programme about our work - Megatsunami: Wave of Destruction - broadcast in 2000.
In academic circles, our findings also caused a flurry of interest that continues today. Arguing against the possibility of a major tsunami, some researchers proposed that the collapse would be slow and/or piecemeal, so that a major tsunami would not be generated. Others suggested that even if there was a major collapse, the tsunami would only be destructive locally or just within the Canary Islands archipelago, and would not be destructive at oceanic distances - e.g. the eastern seaboard of North America or Europe.
Other research and observations have, however, pointed to the fact that collapses big enough and fast enough to generate major tsunamis were not only possible, but quite common. Tsunami desposits arising from ancient volcanic collapses in the Canary Islands, Cape Verdes, Hawaii and elsewhere, provide evidence of waves that could be hundreds of metres high. Furthermore, giant boulder and other deposits preserved on Bermuda and in the Bahamas have been interpreted as having been emplaced by tsunamis arising from ancient collapses in the Canary Islands, arguing that destructive ocean-crossing tsunamis from future collapses are possible.
Meanwhile, sea-floor images from the Canary Islands and elsewhere reveal that blocks of rock as large as the west flank of the Cumbre Vieja, or even larger, can slide into the sea in one go, thereby maximising the likelihood of major tsunami generation.
We stopped monitoring movement at the Cumbre Vieja in the early 2000s, so have no idea what has been happening since then, and especially in the build-up to the current eruption. Nonetheless, it is important to recognises that the west flank of the volcano remains an actively unstable rock mass that will fail at some point.
We continue to be confident that a major, tsunami-generating, collapse is perfectly possible - even probable - at this volcano, whether or not any tsunami impact is localised or extended. WE DO NOT, HOWEVER, PROPOSE THAT IT WILL HAPPEN IN THIS ERUPTION. It is likely that when the west flank fails, it will do so during an eruption, but this could be 10, 30, or 50 eruptions down the line. The chances that collapse will occur during the current eruption are very small, but we cannot say they are zero.