- The risk of a landslide in the Canary Islands causing
a tidal wave (tsunami) able to devastate America's east coast is vastly
overstated.
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- That's according to marine geologists studying ancient
landslides in the area
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- In typical Canary Island landslides, chunks of land break
off in bits, not in one dramatic plunge, they claim.
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- This contradicts previous warnings that an Isle of Man-sized
chunk of land could fall off the island of La Palma into the sea, causing
a mega-tsunami.
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- However, the researchers behind the original claim are
sticking to their guns, pointing to evidence of catastrophic past events
in the region.
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- Back in 1999, scientists at University College London
published a paper about a volcano on the island of La Palma. They predicted
that, if it erupted, the volcano could cause a catastrophic landslide in
which a chunk of land the size of the Isle of Man would fall into the ocean.
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- They then proposed that a landslide this big would generate
a mighty tsunami big enough to cross the Atlantic, devastating the Caribbean
and the eastern seaboard of the US.
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- With talk of a possible wall of water 50m high, their
predictions were jumped on by the world's media.
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- 'Worst-case scenario'
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- But researchers taking part in a three-week research
cruise aboard Southampton Oceanography Centre's research ship the RRS Charles
Darwin say the threat is far lower than previous warnings would suggest.
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- Doug Masson, who has been researching Canary Islands
landslides for 20 years, says the models are a worst-case scenario.
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- Coring equipment is being used to collect samples of
rock sediment deposited by underwater avalanches that were in turn caused
by previous landslides on the island.
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- By looking at layering in the sediments, the scientists
can work out whether the debris landed on the ocean floor in one big lump
or in several smaller stages. And the Southampton researchers claim that
other samples from the Canaries suggest their "bit-by-bit" scenario
is common, if not ubiquitous to these landslides.
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- Russell Wynn, who is leading the research cruise says
it means there's a lot less to worry about if a landslide is triggered:
"If you take a brick and drop it in a bath you're going to generate
quite a big splash.
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- "But if you break the brick up into 10 pieces and
drop them in one by one you're going to get 10 much smaller splashes".
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- Melting rock
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- None of this impresses the team that proposed the original
mega-tsunami theory. Bill McGuire is director of the Benfield Hazard Research
Centre at University College London. He argues that evidence on the surface
of the Canary Islands shows that previous landslides have been catastrophic.
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- On the island of El Hierro a semi-circular escarpment
of rock left behind after a landslide is covered in melted rock. Conclusive
evidence, says Bill McGuire, of a dramatic event.
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- "This thing moved so quickly that it heated the
rock through friction and melted it. That is a catastrophic event,"
he said.
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- Other evidence that Canary Islands landslides have had
a colossal impact has come from the Bahamas themselves where boulders of
rock have been discovered 20m above sea-level.
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- How they got there was a mystery until the timing of
their deposition was linked to a past landslide - in the Canaries. Many
scientists now believe that landslides in the past have triggered deadly
mega-tsunamis.
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- So Bill McGuire is sticking to the predictions his team
have made. Making no apology for backing a worst-case model, he says: "There's
no question of hiding things. If you're planning for any future disaster
you're not going to consider the least disastrous scenario, you're going
to consider the most."
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- When Southampton's marine geologists return they hope
to bring with them evidence that landslides in the Canary Islands are more
gradual events.
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- So will a landslide on the island of La Palma create
the world's biggest wave? Unlikely, they say, but we have absolutely no
way of knowing.
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- © BBC MMIV
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- http://news.bbc.co.uk/1/hi/sci/tech/3963563.stm
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