Catastrophic giant underwater avalanche hit 60,000 years ago

Scientists have mapped the path of an ancient underwater avalanche which travelled 2,000km across the seafloor off the northwest coast of Africa.

The event started as a small seafloor landslide about 1.5 km³ in volume in one of the largest submarine canyons in the world, the Agadir Canyon, about 60,000 years ago.

It quickly grew more than 100 times in size into a “catastrophic giant event”.

“What is so interesting is how the event grew from a relatively small start into a huge and devastating submarine avalanche reaching heights of 200 meters as it moved at a speed of about 15m/s, ripping out the sea floor and tearing everything out in its way,” says Dr Chris Stevenson, a UK sedimentologist from the University of Liverpool who co-led the research.

“To put it in perspective: that’s an avalanche the size of a skyscraper, moving at more than [64km/h] from Liverpool to London, which digs out a trench 30m deep and 15km wide destroying everything in its path.

“Then it spreads across an area larger than the UK burying it under about a metre of sand and mud.”

Underwater avalanches, like terrestrial landslides and snow avalanches, are what’s known as sediment gravity flows. These are the main mechanisms through which material, such as sediments, nutrients and pollutants, moves across the surface of the Earth.

The researchers analysed more than 300 core samples taken from the area over the past 40 years, along with seismic data and measurements of the depth of the ocean floor.

“We calculate the growth factor to be at least 100, which is much larger compared to snow avalanches or debris flows which only grow by about 4-8 times,” says Dr Christoph Bottner, a research fellow at Aarhus University in Denmark who 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 behaviour associated with underwater avalanches and is something we plan to investigate further.”

Professor Sebastian Krastel, head of marine geophysics at Kiel University in Germany and chief scientist aboard the cruises that mapped the canyon, says the new insight fundamentally challenges how we view these events.

“Before this study, we thought that big avalanches only came from big slope failures. But now, we know that they can start small and grow into extremely powerful and extensive giant events,” he says.

“These findings are of enormous importance for how we try and assess their potential geohazard risk to seafloor infrastructure like internet cables that carry almost all global internet traffic, which are critical to all aspects of our modern societies.”

The new study appears in Science Advances.