Sunday, February 2, 2014

Will the Canaries Collapse?


If you’ve ever watched geology-related television shows, one favorite story to tell is the idea of a mega-tsunami in the Atlantic Ocean caused when of one of the Canary Islands collapses.

The Canary Islands are islands off the coast of northwest Africa built by volcanoes. These volcanoes grow when magma, generated in the Earth’s mantle, forces its way upwards through the above rocks, fracturing them and pushing them apart.

Volcanoes like these literally break themselves in half. On several of these islands, most notably the island of La Palma, there are a series of faults (A good chance one of them is seen in the foreground here, the sharp break in the slope) where parts of the island are sliding into the sea. Every time magma comes up it pushes on faults like this, forcing part of the island outwards.

Surrounding all of the Canary Islands (and others in places like Hawaii) there are large debris fields made of up rocks that broke off the islands and slid into the sea. If you total up the size of these debris fields, they would have been massive – so massive that if all that material fell into the ocean at once, it would generate an enormous tsunami wave, one that could still be 50 meters high when it reached New York.

It’s a great story for TV; it’s plausible, you can go to the place where such a breakaway would happen and put your finger on it, and then you can animate that wave hitting New York, Boston, and Washington D.C.

The good news for most people is that the threat of such a mega-landslide is almost certainly overstated for one reason; the islands usually don’t collapse like that.

A paper published last year by Dr. J.E. Hunt and colleagues at the University of Southampton, UK, performed a detailed mapping study of the landslides over the last 1.5 million years. When they looked carefully at the largest landslide deposits, they found that instead of being one large landslide, they were each composed of several smaller units with large time breaks in-between.

In other words, these big landslides didn’t come down all at once, they came down slowly. The largest slide they found was about ¼ the volume of the full debris area on the side of an island.

Tsunami heights don’t scale directly with landslide volume, but it takes a really enormous landslide in the Canaries to do major damage to the U.S. East Coast. The largest slide they found would still cause a tsunami that would damage the islands themselves and possibly the coasts of Africa and Spain, but it wouldn’t be a major threat to far-flung population centers.

This research, of course, does not mean that such a giant collapse is impossible. Unique things do happen, and just because the previous collapses were small doesn’t 100% guarantee that any future ones will be also.

But, if you hear someone say “this island will collapse in the next few thousand years and cause a tsunami” or any version of that…that’s not how things have happened for the last 1.5 million years, and if we’re planning for the future, it’s not something we need to fear happening any time soon.


Image credit: Wikimedia commons
http://commons.wikimedia.org/wiki/File:Plantation_banana_La_Palma.jpg

Original paper (Subscription):
http://onlinelibrary.wiley.com/doi/10.1002/ggge.20138/abstract

Article:
http://blogs.agu.org/landslideblog/2013/12/13/canary-islands-tsunami/

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