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Rolling in the deep

in Featured/Old Lost Sea by

In November 1929, as Jenny Higgins explained here recently, an earthquake-generated tsunami overwhelmed and devastated communities on the south coast of the Burin Peninsula. It was a shocking and sudden demonstration of the ocean’s power.

Though the Burin bore the brunt, the quake’s impact was felt well beyond Newfoundland.  By rupturing a series of submarine cables connecting North America and Europe, it played havoc with the burgeoning global communications network. Yet amidst all the death and destruction, there was a positive story to be found, thanks to a brilliant piece of scientific detective work.

Surprisingly little is known of the deep sea, even now, but in the early 20th century the science of oceanography wasn’t so much in its infancy as lolling about in its cot, sleeping almost every hour of the day.

One of the great puzzles was how sediment was moved about in the deep sea.  Beyond the reach of wind, waves and storms, the ocean floor was commonly thought to be a fairly flat, boring place where little happened. That misconception was shattered by the Grand Banks earthquake.

With a magnitude of 7.2, the quake is the largest ever known to have hit Atlantic Canada.  Its epicentre was off St Pierre Bank, on the southwestern edge of the Grand Banks, and was caused by the abrupt slippage of two fractures (faults) separating different parts of the Atlantic crust.

‘Something very odd had happened’

This just happened to be the oceanic area containing the world’s densest quantity of submarine telegraph cables, laid down to link the eastern seaboard of North America with western Europe. When a swath of these communication lines failed suddenly and unexpectedly, it indicated something very odd had happened at the seabed, but in the subsequent confusion, it wasn’t immediately obvious what.

Eventually, after gathering the data on when different cables had broken, and where, a clear pattern emerged. Within 100 kilometres of the epicentre, there had been an instantaneous rupturing of half a dozen lines.  This was then followed, over a period of more than 13 hours, by the progressive failure of cables that lay south/south-east of the epicentre.

To some scientists, the explanation was simple: the earthquake had broken the lines.

American geologists Bruce Heezen and Maurice Ewing weren’t quite so convinced.  If all the cables had failed at the time of the earthquake, the hypothesis would have been acceptable, but why did only some lines snap?  And why was there then a systematic breakage of cables, one after the other, at distances increasingly further from the epicentre?

An enormous underwater avalanche

One of the key aspects was the region’s seabed topography.  The earthquake had occurred right on the edge of the Grand Banks, where the continental shelf drops off to the deep ocean floor. Cutting into that submarine slope is the Laurentian Channel, a huge underwater canyon representing the route of the St Lawrence River during the last Ice Age, when sea levels were much lower.

The instantaneous cable breaks had occurred along the slope just below the point where it is intersected by the Laurentian Channel.  Each subsequent rupture had taken place downslope from this area.

Heezen and Ewing argued that the earthquake itself hadn’t broken the cables, but that it had instead triggered an enormous underwater avalanche. This submarine super-slump had surged down the continental slope, snapping through each of the telegraph cables that crossed its path.

The time intervals separating each cable break could be used to calculate the speed of the avalanche, and this further supported their theory. The gaps between breaks got greater as you moved away from the epicentre, just as would be expected when the mass of sand, mud and water flowed out onto the flatter, more distant parts of the seabed, and lost speed and energy.

Subsequent studies have refined the story slightly, and shown the truly astonishing scale of the avalanche.

It was the first detailed proof that vast quantities of sediment could be moved into deep water, by a process known as a turbidity current. Laboratory reconstructions give an idea of what the current would have looked like, whilst a video of Bruce Heezen explaining his theory can be viewed here.

Subsequent studies have refined the story slightly, and shown the truly astonishing scale of the avalanche. At a speed of up to 80 kilometres an hour, faster than a galloping horse, around 200 cubic kilometres of sediment thundered their way into the abyss.  Enough sand and mud was moved to bury the entire Avalon Peninsula to the height of a three-storey house.

Perhaps most extraordinarily, it is now known that it wasn’t the earthquake itself that caused the tsunami, but the turbidity current. So much material was thrown up into suspension by the avalanche that the water had nowhere to go but up, and out, generating the giant wave that battered the Burin.

Forewarned is forearmed

It is of little consolation to those families who suffered so much, but the 1929 turbidity current wasn’t even that big. It pales into insignificance when compared with turbidites and debrites from the geological record. In the cliffs around Cow Head, in western Newfoundland, for example, there are rocks that preserve submarine slumps of unimaginable power. Some of the grains of “sand” are the size of cars!

If it all sounds frankly terrifying, don’t panic.  These events are very rare, and so much Ice Age sediment was shifted by the Grand Banks earthquake that it will take an extraordinarily long time for it to build up again.  So even if another quake did hit the area, the submarine landslide (and therefore the tsunami) would be considerably smaller.

And, thanks to Heezen and Ewing’s ground-breaking work, we are forearmed by being forewarned.

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