Well, You Were Warned...

It even made the local news, with that lovely video [I'm sure its on youtube somewhere..] of the Yealm setch [I think that's the spelling, only ever heard it spoken, you see]. No, they don't call it a bore, cause they're not boring [[Boom, boom. Thankyou, I'll be here all week...]]

Time for some geology! 'Underwater Landslide' is probably the best term to put on the news, but 'Slump Leading To Turbidity Current' is more like the proper terminology. Being unable to inflict diagrams upon you [and geology is much less fun without diagrams!] I'll be brief...

Ish.

Ok, I'll be horribly long-winded, but you were expecting nothing less, am I right?

:)

Cast your mind to the sea floor to the south west of the South West... The continental shelf slopes fairly gently away, until it reaches the shelf edge, when the continental slope down to the abyssal plain begins. This is shown usually as a huge steep step, like a massive cliff three kilometres or so high. If you were to go there, you'd need a clinometer to notice it, but its a steeper slope [those diagrams, at least the decent ones, are to scale - its just such a large scale involved..]. There is a constant gentle rain of sediment - at least on geological terms - which has been carried out by rivers and then tides and currents. The deeper water is calmer, encouraging more deposition, and over time the sediment builds up as mud and silt layers, full of water [you'd sink right into it]. The sea bed isn't flat, though, its cut by gullies and canyons, some on ridiculous scales and many, in shallower waters like the Channel, relics of glacial periods when the sea level was lower - submerged river valleys.

Anything built up on a slope is eventually going to have an issue with gravity. Sometimes the sediment slumps due to the shockwaves from an earthquake [the most famous example being the Grand Banks in 1929 - the first record of a turbidity current, when transatlantic telegraph cables were cut in sequence by something that moved at up to 55 knots across hundreds of miles of seabed], or even from wave action [There are some interesting studies into the effects of hurricane-generated waves]. Sometimes they just fail - when the weight of the sediment passes the threshold of its internal cohesion. Being a great mass of gooey liquid mud, there's no spectacular effects, - the word slump is very appropriate. But these slumps can be very big - one off west Africa covers 30,000 square km. [We had a fairly small one, this time.] Once its started moving, the sediment rapidly becomes what is called a turbidity current - a mix of sediment and water which flows down slope with very little friction*. Its the same process as a powder snow avalanche and would look very similar. The current would race away and could cover an amazing distance, all the time becoming more and more diluted and containing finer and finer particles, first following the channels left by previous currents before spreading out in a fan over the abyssal plain.

The initial slump is what generated the tsunami - the descending mass pulls the water column above it down while displacing water below it, generating the waves. Though the volumes involved could have been very large, because this is mud sliding down a fairly gentle slope [as opposed to, say, a chunk of a volcanic island falling off] it lacked the kinetic 'punch' needed to cause a major event - fortunately for us. Also, as the slump was traveling away from us, the main force will have gone that way, and will have hit South America after several hours travel time. I doubt anyone there will have noticed, to be honest, as the waves gradually lose force over distance and they weren't apocalyptic to begin with.

If you look at a map [for example, the satellite version of the bird map on a Certain Internet Forum To Do With Birds], you'll see that the shelf edge is well away from the South West - indeed the likely area that the slump occurred in is due west of Brittany** - another reason why the waves didn't get higher than 3' [and that funneled in river valleys]. While I'm on the subject, if you look just SW of Ireland, you'll see an oval chunk 'missing' from the shelf. The reasons for it being there I won't bore you with, but that structure is one of the reasons why the Bridges of Ross is the best seawatching spot in Europe, as it brings the upwelling [caused by marine currents hitting the slope and pushing nutrients to the surface] much closer to shore and so also the animals that feed on it and the seabirds that feed on them.

[[*The process is known as a density current - if you introduce a liquid into a less dense liquid, it will flow downhill under gravity along the bottom of that liquid, ingesting the less dense liquid and gradually diluting until it either runs out of room or becomes so dilute there is not enough difference in density to drive its movement. Because they ingest the liquid they pass through, they suffer very little loss of energy through friction and so can move very quickly for great distances. Turbidity currents, powder avalanches, pyroclastic flows {more properly called pyroclastic density currents, and capable of some really interesting stuff due to being hot}and sturtzstroms {a rare form of landslide where rocks quite literally flow like water} are the main natural density currents.]]

[[**So the tsunami will have hit there too, probably more strongly {though they can be tricky}. I can just imagine the locals' reaction to the tide going back and forth - a gallic shrug, no doubt. ;) ]]

If anyone who attempted to read this is still awake [well done, btw], I maybe ought to apologise for taking 6 odd hours to write this. [Blogger helpfully uses the time a post starts being written, rather than the time it is posted - I have no idea why!] I was distracted by Swifts, you see. More to follow in a new post which might not drive your brain to crawl out of your ear and make a desperate leap for your computer's OFF switch....

;)