Tony Butt’s A – Z of Surfing. M – Megatsunamis

Tony Butt

A Megatsunami is a large wave generated by the falling of immense amounts of rock into the water, typically from the collapse of volcanic islands. The word ‘megatsunami’ is a popular term for what is scientifically known as the ocean-island collapse landslide tsunami. It is potentially more destructive than earthquake-generated tsunamis, but much less common.

Since a famous article in 1924 describing volcanic material found on the sea bed around Hawaii, geologists have known that volcanic islands are extremely unstable and are prone to occasional catastrophic failure. This failure is usually connected to the eruption of the volcano that comprises part of the island in question.

One particular volcano in the Atlantic Ocean has been extensively studied for its potential to create a landslide tsunami. It is the Cumbre Vieja, on the island of La Palma in the Canary Islands. The Cumbre Vieja is one of the most active volcanoes in the world, and has erupted several times in the last few centuries. Since the last eruption in 1949, a large fault has opened up, causing the entire western half of the island to become unstable. According to geologists, the next eruption might be the one that causes that half of the island to fail, releasing about five hundred billion tonnes of rock into the sea.

When this happens, the experts say, a series of waves will be generated with an initial height of about 600 metres, which, after causing immense coastal destruction in the Canary Islands themselves, will propagate north and eastwards across the Atlantic. The waves will travel at speeds of several hundred kilometres per hour. Around six to nine hours after impact, the waves are predicted reach the coasts of North and South America, with heights of around 20 to 25 metres.

It is important to realize that these waves are thousands of times more powerful than normal wind-generated waves of the same height. Even a one-metre landslide tsunami can cause considerable destruction. That is why, when these waves reach the coast, they won’t stop until they have flooded everything up to tens or even hundreds of kilometres inland.

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There have been several notable cases of real landslide tsunamis. One occurred in 1888 on the island of Ritter, off the northwest coast of New Guinea, when a chunk of rock weighing about five billion tonnes crashed into the sea. It generated a tsunami which caused significant damage on neighbouring islands, killing about 3,000 people. In this case, there were direct eyewitness accounts from German settlers on New Guinea. The witnesses had stopwatches, so their accounts of the incident include not only estimates of wave height but also of wave period and arrival time of the tsunami. Scientists were able to use these accounts to compare what happened in real life with a simulated event from a mathematical model. This was highly valuable for testing simulation models which can then be used to predict future events, like the Cumbre Vieja.

More recently, in 2002, an ocean-island collapse was reported on the island of Stromboli, near Sicily. It was a ‘relatively small’ event, in which 17 million tonnes of rock fell into the sea, generating a tsunami of around 15 to 20 metres high. On neighbouring islands, many boats were damaged and at least three people were injured.

Scientists have considered various different hypotheses to explain ocean-island collapses, and the one that seems to have triumphed is the forced-dyke injection mechanism. This theory is based on the premise that there are two different types of rock inside the mountain: permeable, loose rock; and impermeable, solid rock. The impermeable rock forms a number of walls or ‘dykes’ between which lies the permeable rock. The permeable rock is saturated with water, amassed from years of intensive rainfalls on the mountain. Under normal circumstances, the water is contained in this permeable rock within the dykes and cannot escape. However, as soon as the volcano starts to erupt, hot magma rises up through the middle, which heats up the entire mountain including the water trapped between the dykes. Now, as the temperature of the water rises, enormous pressures begin to build up inside the mountain. The water, now in the form of steam, will try to escape through small cracks in the rock. This, of course, reduces the stability of the mountain until a point is reached where it can no longer hold itself together. In the case of the Cumbre Vieja, the most important implication of the forced-dyke injection theory is that it unquestionably links the eventual collapse with some future eruption of the volcano.

 

Sequence of events at various stages after the Cumbre Vieja Collapse, according a simulation by Steven Ward of the University of California: The red and blue lines are the peaks and troughs of the waves, and the numbers indicate the sea-surface displacement in metres from the average sea level. For example, 30 minutes after impact, there are 188-m peaks and 175-m troughs; when the waves reach the east coast of North America, there are 13-m peaks and 10-m troughs.

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