Lots of attention has focused on impacts by Earth-crossing asteroids and comets as potential causes of economic and biological catastrophe, as too on hazards from climate change induced by major volcanic activity. To these fears can be added the effects of tsunamis, but not those caused by even the largest conceivable eathquake. Oceanic islands can fall apart by a process that is identical to, though vastly bigger than a landslip, thereby displacing their equivalent volume of seawater.
Britain has experienced the effects of tsunamis driven by collapse of part of the Norwegian continental slope, triggered by massive methane release from gas hydrates in sea-floor sediments. The last of these was when its shores were colonised by Bronze Age people, and left its mark in the form of high-level sand beds on the flanks of eastern Scotland’s firths. The north-east part of the Isle of Skye preserves spectacular results of landslips of volcanic rocks, that represent the largest mass movement known in Europe. However both examples are dwarfed by features off the Hawaiian islands, that sonar has revealed. There are some 70 debris fields that date back to 20 Ma, some of which contain up to 5 000 cubic kilometres of rock from collapses of the flanks of the growing volcanic islands. Surveys around other large oceanic islands of volcanic origin suggest that such flank collapses occur around every 10 000 years. Movement of masses so large involves energy equivalent to the world’s arsenal of nuclear weapons, so flank collapses are comparable in magnitude with moderately sized impacts. They would generate tsunamis waves as high as 30 metres, that would devastate coastal areas around large ocean basins.
One area on Hawaii is indeed liable to collapse, and in November 2000 it moved, only to stop short of a full collapse. Geoscientists from the US Geological Survey and Stanford University used GPS receivers to monitor movement on the southern flank of Kilauea, and after a series of barely detectable earthquakes they recorded slips of up to 6 centimetres per day (Cervelli, P. et al. 2002. Sudden aseismic slip on the south flank of Kilauea volcano. Nature, v. 415, p. 1014-1018). Careful analysis of many kinds of motion sensors suggests that the moving block sits on top of a low-angle fault or detachment, that may eventually carry the block seawards to unleash tsunamis. It is uncertain how much warning there would be of a fully fledged collapse, but is does seem sensible to establish such monitoring on active volcanic islands in the world’s oceans. Since expansion of humanity following the retreat of the last continental ice sheets would have largely been along coasts, with their easy terrain and abundant food supplies, tsunamis would have been an ever present, but never suspected risk. Britain’s example is minor in comparison to those that would stem from flank collapses, and perhaps the near-miss of November 2000 may encourage searches for the scars that huge tsunamis generate in relation to maritime archaeological records.