Mega-tsunami: Wave of Destruction
BBC2 9:30pm Thursday 12th October 2000

NARRATOR (EMMA FIELDING): 40 million people live and work along the east coast of the United States, yet this entire population unknowingly lives under threat of a sudden catastrophe.

DR. SIMON DAY (Benfield Greig Hazard Research Centre, UCL): The east coast of America is, is the worst place this could happen. It’s not some remote, deserted coastline, it’s one of the most densely populated places in the world.

NARRATOR: Scientists have now found evidence that a colossal wave will one day devastate the coast of America. It will be far bigger than any normal tidal waves, or tsunami. It is what scientists call a mega-tsunami.

PROF. BILL McGUIRE (Benfield Greig Hazard Research Centre, UCL): It’s almost inconceivable how much damage this event will cause and yet the general public knows absolutely nothing about it.

DR. GEORGE PLAFKER (US Geological Survey): These forces have almost unlimited power to cause utter destruction and there’s nothing that we can do to stop them.

NARRATOR: Geologist George Plafker first came to this remote bay in southern Alaska 50 years ago. What he discovered were signs that a strange force of nature had once struck here.

GEORGE PLAFKER: Lituya Bay is one of my favourite places in the world. It’s a magnificent landscape. There’re mountains roaring up behind it, glaciers come off those peaks right down to sea-level and you have a bay that is unique along this coast.

NARRATOR: Plafker came to Lituya Bay with his colleague, Don Miller, in 1953. They came to survey for oil.

GEORGE PLAFKER: We didn’t find any petroleum in Lituya Bay but what we did find was something that happened on a cataclysmic scale and we spent a large part of our time trying to understand it.

NARRATOR: What these scientists discovered in the 50s was the evidence for a previously unknown force of destruction. The evidence lay not in the rocks but in the trees.

GEORGE PLAFKER: One thing that was very peculiar is the fact that mature forest did not extend all the way down to the shoreline as was true almost everywhere else in this general area. Instead there were bands of younger trees below mature forest and the line at which the trees of different ages joined we call the trim line.

NARRATOR: Nothing like this had been recorded before. They suspected the surviving trees from just above the trim line might contain evidence of what had happened many years earlier.

GEORGE PLAFKER: We cut selected trees, took samples from those slices and sent them to the Juneau Forestry Research Lab and asked them to count the rings and give us an analysis of just what happened to those trees.

ELLEN ANDERSON (Juneau Forestry Sciences Lab, Alaska): This is a photo of a section of a spruce tree that was taken by Don Miller and George Plafker in 1953 in Lituya Bay. The tree was right above a trim line in the bay and it tells a very interesting story. The early growth in this tree was very good, nice wide rings. All of a sudden it changes. Something happened to it, something must have hit it very hard on this side over here. There’s a large scar.

NARRATOR: The tree ring analysis showed a violent force had struck the entire shoreline of the bay. The forest looked like it had been hit by a giant wave. For the scientists it seemed impossible. No wave in history had ever reached anything like this high.

GEORGE PLAFKER: There was some powerful force at work here that created a wave possibly 150 metres high. That’s the equivalent of say the height of a 50 storey building and we just didn’t have any idea of what could do that.

NARRATOR: Even the most powerful waves, tidal waves, known to science by their Japanese name tsunami, could not have created such destruction. This wave had been caused by something very different.

GEORGE PLAFKER: Well we didn’t know what caused the wave. We considered earthquakes and earthquakes cause tsunami but those tsunamis are usually less than 10-15 metres and here we have something that’s 10 times as high as that.

NARRATOR: In comparison one of the most destructive tsunami this century hit the islands of Hawaii in 1946 destroying the town of Hilo yet it was little more than 10 metres high. Waves surged far inland causing death and destruction before petering out in the town. Like almost all tsunami the wave that struck Hilo was created by an earthquake on the ocean floor far away.

PROF. GARY McMURTRY (University of Hawaii): Well the Hilo tsunami in 1946, the classic earthquake generated tsunami, came from thousands of kilometres away. There was no warning, very little warning at all and destroyed large portions of the city and over 100 people were killed, including quite a few children.

NARRATOR: It is known that for all their destructiveness tsunami formed by earthquakes are inherently limited in size. That is because they are caused by shifts in the seabed. When an earth-quake cracks the ocean floor one side of the fracture rises up. When this happens the water above the fracture is lifted up by the same amount. This movement pushes the water upwards and outwards creating a wave on the surface of the sea which becomes a tsunami, but even the biggest earthquakes can only lift the sea floor by about 10 metres which creates a wave of the same height. That’s about as big as a normal tsunami gets.

BILL McGUIRE: Well the point about earthquake generated tsunami is that their size is inherently limited by what causes them. Now the biggest submarine earthquakes shift the ocean bed up or down by around 10 metres and that produces tsunami on that sort of scale, but not very much bigger.

NARRATOR: What happened in Lituya Bay seemed inexplicable. It had produced a wave not 10 metres, but 150 metres high. Whatever had caused it, it certainly wasn’t an earthquake.

GEORGE PLAFKER: We really did not have any idea of just what happened to create the wave evidence that we saw in Lituya Bay. We knew that, you know, Mother Nature doesn’t give up her secrets readily, but we still were frustrated about the whole thing and we did not expect to ever learn what the answer to our problem was.

NARRATOR: In 1953 the scientists left Lituya Bay baffled, but 5 years later this rare phenomenon was to strike again and this time there were witnesses.

HOWARD ULRICH: The date was July 9th 1958. We came into Lituya Bay about 8 o’clock in the evening. My son was with me.

SONNY ULRICH: I was 8 years old at the time and being a child like I was halfway asleep as well.

HOWARD: Approximately 10.15 there was a large rumbling noise from up at the head of the bay.

SONNY: It was like a big loud noise from over in this direction towards the mountains over there.

HOWARD: There was a slight pause. I thought that everything was over with, but some movement up there caught my attention out of the corner of my eye and so I looked directly up there and what I observed was a, like an atomic explosion. After this big flash came a huge wave. It looked like just a big wall of water.

SONNY: He threw me a life preserver and he said son, start praying.

HOWARD: You’re looking at death and this is exactly my first thought.

SONNY: When the wave hit us I did feel the boat all of a sudden start shooting upwards skywards.

HOWARD: I had 40 fathoms of anchor chain and it started running out off the boat. Came to the end of the 40 fathoms just snapped it like a string and then we were free and, but we were still on the front of the wave. We were swept up over the land and up above the trees. That’s where I assumed that we were going to end up.

NARRATOR: The Ulriches were lucky. They rode the wave as it swept them above the trees and washed them back into the bay. Two other boats weren’t as fortunate. They were carried by the wave into the open sea where they were wrecked.

COASTGUARD RADIO: Has there been a first-hand report from any of the boats up there, over? If there’s any other boats in here, I don’t think they made it. I don’t see ‘em and I don’t hear ‘em. God what an awful sight. You ought to see it in there. Something like the end of the world.

HOWARD ULRICH: I had never heard or seen of anything like this. It was unbelievable. I couldn’t imagine what could have caused anything. I kept wondering just what mechanism could cause something like that.

NARRATOR: Don Miller flew over Lituya Bay the following day. This is the previously unseen film he took. It shows the utter devastation the wave had created.

GEORGE PLAFKER: Well in 1958 I was out of the country. Don Miller wrote to me and told me about this incredible wave that had occurred in Lituya Bay that stripped the timber and soil off to a height of 520 metres above sea-level. That’s a half a kilometre high and this is unbelievable, incredible thing, far greater than any wave ever heard of in history.

NARRATOR: This time Miller could see the cause of the wave: a huge section of rock had fallen off a mountain and hit the water at the head of the bay. The impact of this created a wave which washed over the nearby headland and surged towards the open sea. For the first time scientists realised that giant waves could be caused not by earthquakes but by a different destructive force: landslides.

GEORGE PLAFKER: Don said why haven’t we thought of this before? It was a gigantic landslide that fell into the head of the bay, fell from a height of as much as 1100 metres above sea-level and this created a huge splash and wave that then propagated toward the mouth of the bay and creating havoc in its way.

NARRATOR: This is by far the highest wave ever recorded. Normal tsunami are tiny in comparison. The largest tsunami it is possible for earthquakes to generate are around 10 metres high when they are formed. Compared to a skyscraper, that barely reaches the 5th floor, but when it was formed the wave at Lituya Bay reached half a kilometre high, over 50 times the height of an ordinary tsunami. This is higher than any skyscraper on earth.

GEORGE PLAFKER: The mean lesson of Lituya Bay is that a body of rock that’s large enough and falls from high enough into the water can generate extremely large and violent waves. GARY McMURTRY: Many people start to think about the sheer size, magnitude of landslide generated tsunamis. They’re practically unlimited.

NARRATOR: Scientists now began the task of trying to understand this new phenomenon. Tsunami created not by earthquakes but by massive landslides. They called these waves mega-tsunami. Even though landslides happen all the time, we know that mega-tsunami are extremely rare. That is because the vast majority of landslides are too small to generate such a wave. For a mega-tsunami to be created a large amount of rock must be falling fast enough so that when it hits the water it releases a single huge pulse of energy in the form of a wave. Crucially the size of the wave will be directly related to the size of the landslide.

GARY McMURTRY: Landslide generated tsunamis have the potential to be almost unlimited in size. They can basically be as big as the edifice they come from and that means that when something gives way the displacement of the water can also be enormous.

NARRATOR: Scientists in Switzerland have built the most advanced model in the world to study landslide-generated mega-tsunami.

HERMANN FRITZ (Swiss Federal Institute of Technology): What we have here is a very sophisticated laboratory experiment which allows us to physically model landslide impact into water and what we have up here is a very powerful pneumatic landslide generator with the red box where the gravel is filled in to mimic the landslide. The red box is accelerated down the ramp at high speed, the flap opens and the landslide is released to impact the water.

NARRATOR: The scientists here made a detailed analysis of what happened at Lituya Bay where 90 million tons of rock hit the water. They wanted to see how this impact had released enough energy to create a giant wave able to reach half a kilometre high.

HERMANN FRITZ: We adapted our model to mimic a cross-section of Lituya Bay. Up here we have the landslide generator representing the mountain where the rock fell off. At the other end we have a ramp representing the headland where the wave ran up to and what we found when we ran the experiment was fascinating. What we found explains how a wave could reach the extraordinary height of 520 metres. A high speed landslide impacted the water, the water moved away so fast that it couldn’t flow behind the landslide creating a large air cavity displacing far more water than the landslide volume itself and this explains how a wave can have enough energy to run up half a kilometre and clear out everything in its way.

NARRATOR: The speed and size of the Lituya Bay landslide explains how the wave there could be so big when it was generated. This wave did not travel far. It struck land almost immediately, but mega-tsunami are also able to cross whole oceans. As they radiate outwards their height will drop considerably, yet because they are a powerful single pulse of energy they can travel thousands of kilometres and still cause destruction on distant shorelines.

BILL McGUIRE: The energy that a mega-tsunami unleashes when it hits a coastline is far greater than that of any storm wave imaginable and the reason for this is the huge wave length – that’s the distance from the beginning of the wave to the end of the wave.

NARRATOR: Storm waves, however high they are, have a very short wave length, usually no more than 100 metres from the front of the wave to the back. This is because they are created merely by the effect of the wind on the surface of the sea. Mega-tsunami, on the other hand, move the entire body of the ocean, right down to the seabed several kilometres below. Because of the enormous volume of water that is shifted, mega-tsunami have very long wave lengths, often hundreds of kilometres from the front of the wave to the back. This makes them particularly devastating when they approach a shoreline. As the ocean shallows the front of the wave slows down, but the back of the wave is still travelling fast and pushes against the front making it rise up. This creates the first destructive characteristic of mega-tsunami when they hit the shore. They rear up to become a wall of water. The long wave length also creates a second frightening characteristic. Instead of breaking on the shore the whole length of the wave sweeps onto land engulfing everything before it.

BILL McGUIRE: With storm waves the wave length is just a few tens of metres, but with mega-tsunami it’s hundreds of kilometres and what this means is that when storm waves break onto a coastline they break almost immediately, but mega-tsunami just keep on coming through the whole length of the wave and this makes them incredibly powerful and incredibly destructive.

NARRATOR: Once scientists understood that landslides into water could produce mega-tsunami they began wondering where the next massive landslide would occur. They looked for where the biggest landslides had happened in the past in the belief it could tell them where they might happen in the future. Geologists knew that one type of location was particularly vulnerable to landslides: large volcanic islands. There are dozens of such islands scattered throughout the world’s oceans and the reason they are prone to landslides is because of the way they’re built. These islands began life millions of years ago when lava erupted onto the ocean floor. As this cooled and hardened layer built upon layer until it formed a land platform of volcanic rubble above sea-level. Scientists realised that every few thousand years one of these piles of volcanic rubble had fallen into the sea.

GARY McMURTRY: Volcanic islands grow by one lava flow after another over tremendous amounts of time. As they do this they become over-steepened. At the same time you have the ocean eroding away at the base and eventually all these things together end up weakening the edifice to the point at where it collapses.

NARRATOR: This is Hawaii, the largest chain of volcanic islands on earth. These scarred valleys are where vast sections of Hawaiian coastline fell into the sea hundreds of thousands of years ago. The mega-tsunami they created would have been thousands of times more powerful than Lituya Bay. Underwater lies the debris of ancient Hawaiian collapses. Whole chunks of the islands have fallen onto the sea floor. The biggest section is the Tuscaloosa sea mount, a giant block which fell off the island of Oahu 2 million years ago. This single rock is almost 10 times the volume of Mount Everest. When it hit the water it would have created an unimaginable mega-tsunami. It would have taken 5 hours to travel across the Pacific and strike the west coast of America, but this event was not unique.

GARY McMURTRY: What we notice when we start looking at the volcanic islands is that every archipelago has collapses. The evidence is insurmountable, everywhere you go you find evidence for debris avalanches, slumps.

NARRATOR: The Cape Verde islands in the Atlantic also had a massive landslide. This was 80,000 years ago. The mega-tsunami this created would have taken just one hour to strike the west coast of Africa. These volcanic island collapses seem to be rare, only happening every several thousand years. The most recent one is thought to have occurred in the Indian Ocean when part of the island of Réunion collapsed just 4,000 years ago. Seven hours later the wave it generated would have struck Australia. Although this is believed to be the last volcanic island to collapse it seems inevitable that somewhere it will happen again.

GARY McMURTRY: You can’t build islands over thousands of years flow after flow without having collapse. It just is part of their nature, it’s part of their history and it’s part of their future.

NARRATOR: The big question is where and when the next large volcanic island collapse will occur. Because these landslides all happened in the ancient past no-one has ever witnessed one. They are so rare scientists cannot be sure what the precursors will be, but of all the large volcanic islands around the world one in particular shows disturbing signs of instability. If this island collapses it would create a mega-tsunami that would race across the Atlantic and hit the east coast of the United States. Every city on the shoreline would be destroyed. From New York in the north to Miami in the south. The wave would wreak havoc for as much as 20 kilometres inland. The origin of this wave would be thousands of miles away. This mega-tsunami would come from a volcanic island off the coast of North Africa, from one of the Canary Islands. It would come from the island of La Palma. La Palma is one of the western-most islands in the Canaries. 80,000 people live here making their living from farming and tourism. There are also two volcanoes on the island, one extinct, one active. In the early 1990s a British geologist travelled to the island to study the active volcano called the Cumbre Vieja.

SIMON DAY: I came here to work on the Cumbre Vieja volcano with the aim of understanding the history of the volcano and the volcanic hazards that it produced.

NARRATOR: Simon Day was to discover evidence that the next volcanic island collapse is likely to occur here. La Palma consists entirely of two volcanoes. The extinct volcano makes up the northern part, but the southern half of the island is the active volcano, the Cumbre Vieja. This is shaped like a ridge rather than a cone and for thousands of years volcanic vents have regularly erupted all along the summit of this ridge. In 1949 it suddenly erupted again and this eruption was to change everything. Although the eruption itself posed little danger to islanders, shortly after it started an unusual event occurred.

SIMON DAY: About a week after the start of the 1949 eruption something extraordinary happened. There was a series of very strong earthquakes and the west side of the volcano slid downwards and towards the sea by about 4 metres and for about 2 kilometres along the summit of the volcano this fissure opened up. It’s not an ordinary volcanic vent that just opens horizontally and that magma then erupts out as lava. This side has gone down relative to this side by about 4 metres as this area moved off towards the sea and this was something quite unusual, quite unique that you don’t normally see on volcanoes.

NARRATOR: A section of La Palma had started to slide and then abruptly stopped. Simon Day couldn’t be sure what was happening to the volcano and whether it would slide again or even collapse into the sea.

SIMON DAY: The problem was the Cumbre Vieja had erupted many times in its history, but there’d been no previous history of faulting, so what was so special about the 1949 eruption, why had the faulting occurred then?

NARRATOR: To find out Day had to discover what was happening to the rock structure inside the volcano. This might tell him not just why the fault had appeared, but also whether it was the precursor to a giant collapse. The Cumbre Vieja is still active making it hard to know exactly what’s happening inside, but the other volcano on La Palma is extinct and its rock structure is the same as the Cumbre Vieja’s. Here there is a place where geologists can look inside a volcano.

SIMON DAY: These tunnels are an amazing opportunity for geologists because they allow us to look into the interior of the volcanoes, going in several kilometres horizontally and up to 2 kilometres below the surface, so what we can see in here uniquely almost in the world is the inside of a volcano.

NARRATOR: Deep underground lay a clue which began to explain not only what had caused the fault above, but also whether one day the Cumbre Vieja would collapse. For what lies in the heart of these volcanoes is surprising: water. The volcanoes on islands like La Palma are unusual. They are full of water. Much of the rain which has fallen on La Palma for thousands of years has been trapped inside the volcanoes because of their particular rock structure. Deep within the volcanoes on La Palma are two types of rock. One is permeable rubble which allows rain to soak into it, but standing vertically upright within this rubble is the other type of rock, vast walls of cooled lava which form hardened dykes. These are impermeable and act as dams trapping columns of rainwater in the heart of the volcano.

SIMON DAY: What we’re looking at here is one of the two types of rock that we find within the heart of volcanoes like the Cumbre Vieja. It’s very loose, very permeable material, the water can soak straight down through it from the surface, down to this level within the volcano and the second type of rock is this. This is the lava dyke and it’s very hard, it’s formed by volcanic lava forcing its way up through the volcano. Once the eruption’s ended the lava in the dyke solidifies and produces this very hard rock and the important point about this rock is that it’s very impermeable. The water can’t penetrate through it, it’s trapped behind it and these dykes within the middle of each volcano act as a series of dams.

NARRATOR: Simon Day began to wonder if it was the effect of this water on the volcano which could lead to its eventual collapse. He contacted a geologist who studies the forces that build up inside volcanoes, Derek Elsworth. In particular, Elsworth was interested in the effect of water pressure on volcanic collapses.

PROF. DEREK ELSWORTH (Pennsylvania State University): This is a very simple model of how water pressures can cause instability in landslides and we have two inclined bricks. When the bricks are dry with no water pressures acting between the join in these two bricks then just like in a stable volcanic slope there’s no movement.

NARRATOR: If water is added, however, it begins to force the two bricks apart. The pressure of the water actually lifts the top brick off the bottom.

DEREK ELSWORTH: When we add water between these two bricks if the water pressure’s high enough to reduce the strength sufficiently, then the upper brick will slide off.

NARRATOR: Although water pressure is enough to push bricks apart, Elsworth realised that water on its own couldn’t collapse a volcanic island. He discovered that another element needed to be acting on the water: heat. For when this type of volcano erupts heat from the magma has a crucial effect.

DEREK ELSWORTH: This represents what happens to ground water in the Cumbre Vieja trapped between the dykes and magma rises within the upper regions of the volcano. This heated bath represents the magma within the volcano. The red water within the flask, like water within the volcano, expands as it’s heated and as the pressure within the flask increases the only outlet for the water is in this thin tube. This represents what happened within the Cumbre Vieja. As water trapped between the dykes is heated, expands, pressures increase drastically, ultimately causing the flanks of the volcano to collapse.

SIMON DAY: It was tremendously exciting because here was a mechanism that we could use to explain how it was that these enormous masses of rock could be pushed off the side of the islands.

NARRATOR: The heat from an eruption was the final part of the puzzle which explained the forces that would be working in the volcano. When a new column of magma rises and starts erupting water trapped between the dykes will be heated up. This will make it expand, creating enormous pressures within the heart of the Cumbre Vieja. These scientists believe this will trigger the collapse of the western flank of the volcano into the sea.

DEREK ELSWORTH: We found that the relatively small rising temperature in the core of the volcano due to the injection of magma could result in very large changes in water pressures. These water pressures are large enough to produce strength in the flank and result in collapse of the volcano. What this of course means is that the next collapse will ultimately be tied to a future eruption.

NARRATOR: All the conditions for a giant landslide on the Cumbre Vieja are present. It is an active volcano that is full of water. Simon Day needed to find out how big the potential collapse would be and to do that he had to discover the size of the fault within the volcano. He began a detailed survey of all the volcanic vents along the summit to see how extensive the fault was, something no-one had done before.

SIMON DAY: We mapped the volcano from the south to the north and from the south we followed this line of volcanic vents running north up the volcano to here on the summit region. What we found was that the line of vents continued straight off to the north and this told us about how the magma was coming up from deep within the earth up underneath the volcano and erupting at the surface.

NARRATOR: When Day plotted these volcanic vents on a geological map he realised that inside the volcano the fault was far more extensive that it appeared on the surface. It could be as much as 20 kilometres long, dissecting the entire length of the volcano. Potentially one side of the Cumbre Vieja, half a trillion tons of rock, would fall into the sea.

SIMON DAY: When we started analysing the information that we gained by mapping the Cumbre Vieja we found that a change had taken place in the vents of the volcano. The north/south line of vents had extended further to the north with new vents appearing each younger further to the north. This meant that the western side of the volcano was becoming deeply unstable. The whole of this flank was moving towards the sea as a single block.

NARRATOR: The Swiss scientists began to calculate what would then happen. They had to build a new model to estimate the size of the mega-tsunami this event would create. The Cumbre Vieja landslide would be thousands of times larger than any that the scientists had studied before. Because of this, their experiments could only give an approximate figure for the dimensions of the wave.

HERMANN FRITZ: Of course there’s a huge difference in scale between the Cumbre Vieja collapse and its physical model in the laboratory. We’ve tried to err on the side of the caution when we made our calculations using conservative assumptions, but nevertheless what we’ve found when we ran the model was very disturbing.

NARRATOR: The Swiss scientists’ final calculations produced an extraordinary wave.

HERMANN FRITZ: If the Cumbre Vieja were to collapse as one single block it would create a giant mega-tsunami with an initial wave height of 650 metres and a wavelength of 30-40 kilometres travelling westwards across the Atlantic with speeds up to 720 kilometres an hour towards America.

SIMON DAY: The scale of this produced a feeling of unreality as one realised what could happen. This event was so huge that it will affect not only the people on the island but people way, way on the other side of the Atlantic Ocean, people who’ve never even heard of La Palma will be affected by this event.

NARRATOR: There is evidence that seems to show collapses like La Palma create mega-tsunami that really can cross whole oceans and devastate distant continents. Scientists know that one of the last volcanic landslides in the Canaries happened here on a neighbouring island to La Palma. When a section of the island collapsed around 120,000 years ago it launched a mega-tsunami which would have swept across the Atlantic towards the Americas. Simon Day believes that evidence for its destructive power can be seen thousands of miles away in the Bahamas. He believes the huge wave reshaped some of these islands, blasting these shaped chevron ridges up to 10 kilometres long across parts of the Bahamian coastline. The wave also ripped vast boulders from off the ocean floor, some over 1,000 tons in weight and dumped them high above sea-level.

SIMON DAY: This was astonishing. Here was evidence that an event so extraordinary, that it could really only be produced by something as catastrophic as an ocean island collapse.

NARRATOR: So when is the next catastrophic event going to happen? The geologists had now discovered the Cumbre Vieja could collapse during some future volcanic eruption. The difficulty, however, is in knowing when this will happen.

SIMON DAY: We have no idea when the next eruption will occur on the summit of the Cumbre Vieja. In recorded history there have been eruptions in 1949, in 1712, in 1646 so it looks as though there is an eruption up there once every 2 centuries or so, on average. The last eruption was 50 years ago so it is likely that sometime during the next century there’ll be another eruption up there.

NARRATOR: Tourists in America and the Canaries shouldn’t cancel their holidays. The next summit eruption is unlikely to happen for decades and it may take many more eruptions before the flank of the volcano is pushed into the Atlantic. The problem is scientists cannot tell.

BILL McGUIRE: There could be 5 more summit eruptions of the Cumbre Vieja before the western flank collapses, there could be 10, there could be 20. On the other hand, the west flank could collapse during the next eruption. We simply don’t know, but put it this way, if I was living in Miami or New York and I heard that the Cumbre Vieja was erupting I’d be keeping a very close eye on the news.

NARRATOR: The geological evidence now shows that La Palma may well be the next volcanic island to collapse and when it does so it will create a devastating natural disaster.

BILL McGUIRE: The first thing that you’d feel actually would be seismic activity, earthquakes, because the collapse is going to be related to an eruption.

SIMON DAY: As the forces within the volcano built up to, to the point where they would begin to overcome the friction forces holding the flank in place the flank would begin to move towards the sea.

BILL McGUIRE: And then at some point the rock would fail on a major scale and this huge chunk of rock, maybe 20 kilometres long or more, would start to slide into the sea.

SIMON DAY: The waves initially here would be many hundreds of metres high and those waves would all be moving out into the ocean spreading out laterally, but with a lot of the energy heading across the Atlantic towards the coast of the Americas.

BILL McGUIRE: Looking down on it, it’ll look unbelievable, it’ll look as if the island is falling apart generating these huge waves which are fanning outwards to reach the eastern coast of the United States.

SIMON DAY: The waves will take about 8 hours to travel between here and the coast of America just enough time to get the message out to warn people that this event was happening, but unless evacuation plans were incredibly efficient it would not be enough time to get everybody out of the affected areas. The areas at risk include cities like Miami, parts of Boston, the coastal areas and suburbs of New York.

GARY McMURTRY: If you were standing on a beach in what is presently Miami, the very first effects you’d probably see is what we call drawback. The ocean would suddenly just pull away. You’d see a tide, a low tide like you’ve never seen before in your life. It would be actually spellbinding but in the background you’d be seeing this wall and it’d keep coming at you.

BILL McGUIRE: This would be the biggest natural catastrophe in history. There’s a problem with all major natural catastrophes. Because we’ve never experienced these things we don’t think that they’re going to happen to us. We just ignore them, but these sorts of events have occurred throughout geological history. They’re not going to stop happening just because we’re around. La Palma is going to collapse into the North Atlantic. It’s not a question of if, it’s just a question of when.

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Further information

Mega-tsunami: Questions and Answers