Megatsunami threats

Volcanic islands (such as Réunion and the Hawaiian Islands) can cause megatsunamis to hit other nearby islands in the same chain because often they are structurally little more than large, unstable piles of loosely aggregated material heaped up by successive eruptions. Evidence for large landslides has been found in the form of extensive underwater debris aprons around them composed of the material which has slipped into the ocean. In recent years five such debris aprons have been found in the Hawaiian Islands alone.Some geologists speculate that the most likely candidate for the source of the next large-scale megatsunami is the island of La Palma, in the Canary Islands. During the 1949 eruption the western half of the Cumbre Vieja ridge slipped four metres downwards into the Atlantic Ocean. It is believed that this process was driven by the pressure caused by the rising magma heating and vaporising water trapped within the structure of the island, causing the island's structure to be pushed apart. The island is still considered active, though quiescent at present, but it is expected to erupt again some time in the next few thousand years. Were this to happen it is speculated that a megatsunami would be created as the western half of the island, weighing perhaps 500 billion tonnes, catastrophically slides into the ocean in a single event, causing local wave heights of hundreds of metres and a likely height of around 10–25 m at the Caribbean and the Eastern North American seaboard coast several hours later. However, this is speculative since there is disagreement whether it would in fact happen, when, or how likely it is.There is also disagreement amongst scientists as to if an eruption of Cumbre Vieja would cause a single large landslide (or a series of smaller landslides) and even if such a landslide would generate a tsunami capable of crossing the Atlantic. The Tsunami Society issued a statement in 2003 that such collapses are rare and occur at intervals of thousands or millions of years, that the risk of La Palma collapsing was over-dramatized, and that although the catastrophic collapse of the islands of Krakatoa and Santorini produced megatsunamis in the local region, huge waves did not propagate across oceans to cause similar devastation on more distant coasts, adding that evidence (including computer simulations and experiments with models) suggests this type of wave does not travel great distances in the same way that normal tsunamis do. Besides fjords in Alaska, many locations face threats of localized, but still potentially dangerous, megatsunami-type waves. Some geologists speculate that an unstable rock face at Mount Breakenridge above the north end of the giant fresh-water fjord of Harrison Lake in the Fraser Valley in southwestern British Columbia could collapse into the lake, generating a large wave that might destroy the town of Harrison Hot Springs at the south end