What is an ocean avalanche?


Dr Karl Kruszelnicki


Dr Karl Kruszelnicki

Dr Karl is a prolific broadcaster, author and Julius Sumner Miller fellow in the School of Physics at the University of Sydney. His latest book, Vital Science is published by Pan MacMillan. Follow him on Twitter at @DoctorKarl
By Dr Karl Kruszelnicki 30 November 2023
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Avalanches are terrifying things: they career down snow-covered mountains, smashing everything in their paths.

In 1929, an ‘avalanche’ shifted more than twice the volume of Mt Everest – but nobody saw it! Why? Because it was entirely underwater. When there’s no snow involved, scientists call such an event a “turbidity current flow”. 

Back in 1929, an earthquake triggered one of these massive underwater flows at the Grand Banks, in Newfoundland in eastern Canada. The bottom of the ocean there was quite flat and hardly sloped at all – just one-quarter of a degree. Even so, this slope was enough to allow the sediment already sitting on the ocean floor to slide downhill at nearly 70km/h. 

By the time the underwater avalanche stopped, some 800km out to sea, it had shifted about 200km3 of sand, mud, and organic material – more than twice the volume of Mt Everest. The mass was roughly 14 times the mass of all the sediment dumped by all the rivers on Earth, each year. The flow-on effects were extremely destructive, including a tsunami that killed at least 28 people and severed all the undersea telecommunications cables between the North American and European continents.

Undersea cables carry about 99% of all human telecommunications. The sections of cable that run across undersea canyons are easily disrupted. Image credit: 3D illustration/shutterstock

Scientists have mapped about 9000 very long and very deep undersea canyons on the ocean floors of the world, and it seems that they were carved out by enormous turbidity current flows in the past.

Today about 99 per cent of all human telecommunications are carried by two million kilometres of undersea cables. Unfortunately, about 3 per cent of these cables run across undersea canyons and are easily disrupted. In 2009, off the coast of Taiwan, telecom cables running across the Gaoping Canyon were sliced by a turbidity current – it took 11 ships and some 49 days to repair them.

These underwater avalanches shift huge quantities of material that was originally on the land down to the ocean floor – and it’s not just sand and mud they move; there are also plastics, pharmaceuticals and pesticides – and huge amounts of carbon in various organic chemicals. 

So, we’ve made the murky hidden depths of our oceans even “muddier”.