Mudslide

As part of our ongoing exploration of things that are big and scary and powerful and can kill you, today we have: underwater avalanches.

It's a topic I looked at a while back apropos of the Storegga Slide, which sounds like a bizarre mashup of Swedish folk music and a country line dance but isn't.  This was an undersea avalanche that occurred a bit over eight thousand years ago, a catastrophic slope failure between Iceland and Norway that displaced over three thousand cubic kilometers of debris and triggered a methane clathrate explosion -- resulting in a tsunami estimated at thirty meters in height which went on to inundate large parts of coastal northern and western Europe.

Underwater avalanches are a vastly understudied -- and therefore underestimated -- danger.  The reason it comes up today is a paper this week in Science Advances about a newly-discovered one that was on the same scale as Storegga, but in a different location.  This avalanche occurred an estimated sixty thousand years ago in Agadir Canyon, off the coast of Morocco.

The Agadir Canyon avalanche seems to have started small, possibly triggered by an earthquake.  But like snow avalanches in mountainous regions, once a bit of material starts to move, it causes other parts of the slope to fail, and pretty soon what you have is a monster.  From seafloor analysis of the sediment layers, what appears to have occurred is that the initial slide involved about 1.5 cubic kilometers of debris (itself not an inconsiderable amount), but by the time it peaked, the sediment flow was a hundred times that volume.

"What is so interesting is how the event grew from a relatively small start into a huge and devastating submarine avalanche reaching heights of two hundred meters as it moved at a speed of about 15 m/s, ripping out the sea floor and tearing everything out in its way," said Chris Stevenson, a sedimentary geologist from the University of Liverpool, who co-led the research, in an interview with Cosmos.  "To put it in perspective: that’s an avalanche the size of a skyscraper, moving at more than 64 km/h from Liverpool to London, which digs out a trench thirty meters deep and fifteen kilometers wide, destroying everything in its path.  Then it spreads across an area larger than the UK burying it under about a meter of sand and mud."

Yeah, that puts it in perspective, all right.

The path of the Agadir Canyon avalanche [Image credit: Christoph Bottner, Aarhus University]

The Agadir Canyon avalanche undoubtedly caused a massive tsunami, but given how long ago it occurred, it'd be hard to find evidence at this point.  Let's just say that it would have been a very bad time to live along the west coast of Africa or east coast of the Americas.

"We calculate the growth factor to be at least a hundred, which is much larger compared to snow avalanches or debris flows which only grow by about four to eight times," said Christoph Bottner of Aarhus University in Denmark, who also co-led the team.  "We have also seen this extreme growth in smaller submarine avalanches measured elsewhere, so we think this might be a specific behavior associated with underwater avalanches and is something we plan to investigate further."

The problem is, just about every continent is surrounded by a region of relatively shallow water (the continental shelf) with the abyssal regions just beyond its edge; at the boundary between the two is a very steep region called the continental slope, where the depth increases drastically over a relatively short horizontal distance.  These areas are prone to failure, and while most events are minor -- comparable to a small mudslide on a mountainside -- some of them, like Storegga and Agadir Canyon, can grow to colossal proportions.

And at the present, we don't know which areas are likely to be safe, and which are at significant risk.

So that's our unsettling science story of the day.  This kind of thing is why I always get a grim chuckle out of people who say how benevolent the Earth is, some even going so far as to describe the universe as "fine-tuned for our existence."  This ignores the inconvenient fact of how much of it is actively hostile -- and some of the most hostile bits are right below the seemingly tranquil surface of the ocean.

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