Tsunamis and floods
Invariably tsunamis and floods are localized problems; quite unpredictable and locally devastating, but unlikely to alter the course of the world in general. Tsunamis are not only caused by undersea earthquakes, but also by underwater landslides. Therefore they need not be confined to tectonic plate boundary regions. For example, a Norwegian fjord landslide a few thousand years ago sent a wave hurtling across the North Sea that carried sand and debris some 50 miles across the flat fenlands of England. An analogy of such events is water sloshing along a bath when something is dropped in at one end. More major examples are underwater landslides on the volcanoes of Hawaii sending a surge that rose up mountainsides in North Australia.
Several TV programmes have capitalized on our interest in these natural events by searching for truly mega events that might possibly occur. We watch them—we are impressed and enjoy the programmes—but deep down, we assume that they will never happen. Nevertheless, it should be noted that there is a genuine possibility (indeed a certainty at some future date) that there will be a landslide in the Cape Verde islands that will send a mega tsunami across the Atlantic, destroying the cities on the east coast of the USA. The scale of the event is certainly ‘mega’ as the wave will be several hundred feet high on arrival and persist for hours.
The energy involved, and the prolonged duration of the tsunami wave, will totally destroy the eastern seaboard of the USA. It will kill a vast number of people in the cities, and economically will bring chaos to the countries that it strikes. Tsunamis may not look impressive as they race across a deep ocean—their destructive effects emerge only as they rise up when they hit shallow water. They can travel at speeds up to 600 miles per hour, so advance warnings of a few hours would be possible, but still inadequate in terms of allowing many people to evacuate.
Eastern USA is in line with the predicted main wave front, but the sideways effects will send lesser waves up and down the Atlantic. Furthermore, the shape of the English Channel will cause the wave to be forced ever higher as it reaches the narrow strait between Dover and Calais. For the coastal towns on both sides of this waterway, there will be immense damage and flooding. (Before panicking and relocating, relax: it will happen, but it may be many millennia before it does.)