13 December 2013
Killing off the Canary Islands landslide megatsunami scare
Posted by Dave Petley
For those of us working on and researching natural hazards, one topic repeatedly comes up at social events. The conversation usually goes like this:
New Acquaintance: “So what do you do?”
Me: “I research natural hazards, and in particular landslides”
NA: “Oh – is it true that one of the Canary Islands volcanoes is going to collapse in a giant landslide? I was told that it’ll cause a tsunami that’ll devastate the coasts of Europe and America”
Me: “Excuse me – I’m just going to bang my head on the table…” [thump!]
The question of course refers to a scare a few years ago, based on a peer-reviewed paper, that a flank collapse of Cumbre Vieja volcano on the island of La Palma in the Canary Islands could collapse, generating a megatsunami that would initially be about 1,000 metres high, and would still be about 50 metres high when it reached for example the eastern seaboard of the USA (Wikipedia has quite a nice article on megatsunamis if you are interested). It is fair to say that in my experience there is agreement that such a flank collapse could occur (and there is lots of evidence that such events have happened in the past), but the level of support for the megatsunami hypothesis is, shall we say, somewhat lacking within the scientific community. There is a nice evaluation of the work by Dr George Pararas-Carayannis, which concludes that:
“The threat of mega tsunami generation from collapses of oceanic island stratovolcanoes has been greatly overstated. No mega tsunamis can be expected”
A key issue here is the mechanics of the landslide. To generate a very large tsunami, this slide would have to happen very fast and as an essentially coherent block. Remember that this is a landslide of 500 cubic kilometres – we do not think that very, very large landslides usually behave like this. The chances are that a collapse would occur in stages over a longer time period, which would generate a much smaller wave. Most scientists recognise that the single, intact block collapsing very fast idea is theoretically possible, but that it is the extreme end-member of a wide range of scenarios, and thus is highly unlikely. There are other issues too (like where are the tsunami deposits from other megatsunamis given that we know that previous collapses have occurred? A tsunami on this scale should leave deposits that would be very easy to map). Unfortunately, although most landslide scientists view the likelihood of a single coherent landslide as being very low,the actual evidence to support that view in the case of these types of landslides has not been strong..
Unfortunately, despite these rebuttals the story has had remarkable traction. Take this page on Virgin Media for example, which has this wonderful (?) image:
http://www.virginmedia.com/science-nature/natural-world/natural-disasters-waiting-to-happen.php
And text that says:
“A colossal wave caused by a chunk of rock the size of the Isle of Man breaking off La Palma in the Canary Islands, will one day devastate Southern Europe and the entire East coast of America. Scientists believe this disaster is already way overdue. When it does come, it will cause a mega tsunami that will hit with the power of an atomic bomb travelling at the speed of a jumbo jet. Starting off at 2000ft, the giant wave will still be 150ft high by the time it reaches the other side of the Atlantic, destroying everything in its wake and affecting up to 100 million people.”
Given the ridiculousness of this sort of statement (and there are worse examples out there), it is good to see a new paper that erodes the case for the megatsunami still further. This paper, Hunt et al. (2013) has just been published in the journal Geochemistry, Geophysics, Geosystems (sadly the article is behind a paywall). The paper presents a very detailed analysis of the deposits left on the sea floor by Canary Island flank collapses. The research is meticulous and comprehensive. The authors note that the sea floor deposits record eight volcanic flank collapse events, the largest of which was about 350 cubic kilometres. However, the key element is that each deposit is formed from a series of subunits, each of which can be clearly differentiated from other subunits based on the geochemistry of the materials that they contain. So, the interpretation by the authors, which sounds very sensible to me, is that each subunit represents a different phase of the collapse event. In other words, each of these major collapses did not occur as a single, coherent block, but as a series of sections one after the other. If you want an analogy, then what better example than the famous 1993 Pantai Remis landslide in Malaysia:
The implications are clear. Previous flank collapses have occurred as a series of distinct events rather than as a su=ingle coherent block. Each of these could have been able to generate a very large wave, and even a local tsunami. However, they would not have generated a megatsunami. There is no reason to believe that a future event will behave differently, so this scare should be consigned to the garbage can once and for all.
Reference
J.E. Hunt, R.B. Wynn, P.J. Talling, & D.G. Masson (2013). Multistage collapse of eight western Canary Island landslides in the last 1.5 Ma: Sedimentological and geochemical evidence from subunits in submarine flow deposits Geochemistry, Geophysics, Geosystems, 14 (7), 2159-2181 DOI: 10.1002/ggge.20138
Dave Petley is the Pro-Vice-Chancellor (Research and Innovation) at the University of Sheffield in the United Kingdom. His blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Thank you Dr. Dave! I have at last found one sin I am not guilty of – I have never asked anyone, let alone a researcher in natural hazards, about the potential for a Canary Island volcano causing a collapse and tsunami.
WHEW.
I particularly love that the new article image is an uncredited screen capture from Deep Impact, where an asteroid generates a tsunami that devastates New York. (http://spacemika.com/blog/2011/03/23/the-tsunami-of-deep-impact/)
Im confused how you can claim to debunk the Canary Islands Mega-Tsunami theory without looking at ANY of the data they used to come up with the theory? You didnt touch the 1980’s eruption of Mt St Helens that proves that landslides in this magnitude do happen, or the 1958 Lituya Bay event that proved that mega tsunamis from landslides are a fact.. all you did is find a landslide that didn’t happen all at once and said that giant landslides aren’t common, does that make it not possible? The Malaysian land slide doesn’t seem to be on the slope of a active volcano that already has a Two mile long crack long the ridge of it. Im sorry but this just proves you’re bad at contrasting and comparing two separate geological events..
[Nonsense. Mt St Helens was more than an order of magnitude smaller than the event being hypthesised here. The 1958 Lituya Bay landslide tsunami was definitely not a mega-tsunami – it was large simply because it was in a very confined fjord. If these types of mega-tsunamis occur in the Atlantic basin then we should find deposits from them all the way along the coasts. We don’t! D]
I still have to say inconclusive, not enough to dis-prove anything.. we are talking about a 2 mile crack at the top of this mountain, if not a geological event perhaps a mega slide due to explosives from one of the MANY groups that hate the US? But I do feel the information that it might not be the ticking time bomb it was originally thought is very plauasble and should be known.. i have no issue with your data.. i just think either scenario is still likely.. I hope for all of our sakes that you are right on this one..
AS far as finding the deposits, that just means the last mega tsunami was a REALLY REALLY long time ago, still dont disprove the possibility..
[No. If scientists are to propose a hypothesis like this then it is up to them to show that it is credible. They have failed to do so in this case, in my view. D]
Nice job of explaining the issues with the ‘mega-tsunami.’ Just a couple of thoughts, while having no geological evidence doesn’t mean that an ocean spanning mega-tsunami isn’t possible, It does imply that the conditions needed to have one are extremely rare. I also wonder why (I assume it’s the Day, et al. 1999 paper) the authors decided to only model a single slab collapse. Of course it was 1999, so there likely limitations on the model capabilities.
Finally, totally spot on about mega-tsunami’s only happening in a confined body of water. You can’t extrapolate out to an ocean of water where wave energy can dissipate.
maybe just a rename of the post, cuz for me and perhaps many other readers, this information didn’t kill the mega-tsunami theory, just added more data that deserves to be added to the equation..
Science can only prove so much because our observations are so limited.. remember that..
[I have no intention of renaming the post. D]
Thanks for “the other side” of the story. I just learned about La Palma from a 2009 BBC show called “The World’s Worst Disasters,” which left the impression that the western side of the island was almost certain to fall “en masse” during the next volcanic eruption, which they suggested was now overdue after the last event in 1971. I’m glad to read that many scientists don’t believe in the threat. Still, as a layperson who lives on the east coast of America, I am not completely reassured when scientists speak of a “very low” chance of such an event happening. Science has a long and embarrassing history of being “certain” about things that turned out to be far less certain than scientists had imagined.
I’m not saying that science should therefore take into account any lamebrain theory, but if the probability of a truly cataclysmic event is merely small (and moreover if there are at least some reputable scientists who believe in the threat) then surely it’s prudent to consider what we can do to make even that disputed and small possibility of disaster disappear.
And to do that (to remove even the outside chance of what, after all, would be “the mother of all catastrophes”), I would suggest a methodical, step by step, dismantling of the western wall of the mountain. Surely this is doable. Coal companies routinely tear down entire mountains in the States. Shouldn’t we consider slicing layers off the top of the western slope until there is not even the outside chance that the entire East Coast will be obliterated by a sudden collapse?
There’s another reason to move ahead with such a project. As Anthony mentioned in his post, certain anti-American groups might eventually have their own ideas about testing the “collapse hypothesis” in a very practical and potentially devastating way. Shouldn’t the U.S. intervene now with the Army Corps of Engineers to beat the terrorists to the punch by “slicing the western wall down to size,” thereby removing even a small chance of cataclysm?
Indeed, there’s a third reason to move ahead: Canary Islands tourism. Who wants to vacation on ground zero of a potential volcanic Armageddon?
PS I don’t want to add to the sense of panic, but the 2009 BBC documentary failed to mention (or “exploit,” if you prefer) one of the most horrifying aspects of the potential catastrophe that they describe: A tsunami of the size that they envision would turn every single east coast nuclear reactor into another Fukushima!
[This would mean the removal of several cubic kilometres of rock, which is not practicable. This landslide is so large that it is beyond the scope of any human to trigger it short of using a large nuclear weapon. A few tonnes of TNT would not have any effect, so terrorists pose no threat. There are far more serious things to worry about, such as human induced climate change. D]
You seemed to be a reputable scientist, until you started with the AGW nonsense. Credibility shot to hell. Smart move.
[Yawn. D.]
It’s happening right now, there are little seismic quakes occurring. What say you now?
[The seismic events are at a depth of 25+ km, so definitely not associated with the alleged landslide. This is an active volcano, so seismic events are to be expected. D.]
Not going to happen, God promises to never destroy humanity again by water.
[Hmmmm. D.]
I’m taking my family to Tenerife this week and wanted to check up on this story. Your blog post has reassured me and you seem to be a man who knows his onions from his carrots so I’m now a lot happier about the holiday. Thank you for taking the time to put this ‘event’ into perspective.
I was planning on taking my snorkel out with me at night-time but now it seems like this won’t be needed …. 🙂
[Good, in terms of your holiday there is absolutely no reason to believe that there is any significant risk from this type of event. D.]
I usually holiday in Florida or Tenerife. So given the recent tremors in Tenerife I was thinking going to Florida for winter rather than Tenerife. Then I remembered the Tenerife generated tsunami stories and how flat Florida is! Hadn’t been aware of the Las Palmas tsunami “risk” . Anyway I found your article interesting. Though I guess 1500 ft up Tenerife we’d be safe even if a La Palma mega Tsunami occurred! Though knowing my luck Teide would erupt at the same time….
[ 🙂 I think that you are more likely to be injured riding a donkey on the beach! D.]
Just look at Mt. St. Helens. Of course they’re unlikely, but they still happen. Mt. St. Helens is all the proof we need to see this does happen – and if it happened near water it’d be catastrophic (exactly like La Palma).
[This is a straw man argument. No-one is denying that volcanic flanks can collapse. Or that large collapses can generate a tsunami. The question is whether there is any likelihood that this particular volcanic flank can collapse in the specific way that this model implies. It is not impossible, but it is very, very unlikely. I might get hit by an asteroid tomorrow. That happens. But it is very, very unlikely. D.]
In the case Fogo Tsunami they found evidence for mega-tsunami on the neigbouring irland, deposits of rocks…
https://www.youtube.com/watch?v=i7AwUgU84cw
[Very localised, very large tsunami events are undoubtedly associated with large landslides. That is not in doubt. D.]