An international team of researchers, including from Germany and the UK, showed how a giant underwater avalanche which took place nearly 60,000 years ago off the coast of Africa caused a massive trail of destruction as it travelled 2000 km over the Atlantic Ocean, according to a study published in the journal Science Advances. The authors provide an unprecedented insight into the scale, force, and impact of one of nature’s mysterious phenomena, underwater avalanches.
The team analysed more than 300 samples from the area taken during research cruises over the last 40 years. These results were combined with seismic and bathymetry data to map out the giant avalanche.
This analysis showed how the event started as a small seafloor landslide about 1.5 km in volume but then grew over 100 times in size as it picked up boulders, gravel, sand, and mud while travelling through one of the largest submarine canyons in the world before going a further 1600km across the Atlantic Sea floor. The avalanche was so strong that it destroyed the entire 400 km length of the canyon and several hundred metres up the sides – about 4500 km in total – carrying stones more than 130m up the side of the canyon.
Unlike snow avalanches, underwater avalanches are impossible to see and extremely difficult to measure. Nevertheless, they’re still the primary mechanism for moving material, including sediments, nutrients, and pollutants, across the earth’s surface. They may not affect humans directly, but they represent an important geohazard to the seafloor infrastructure, such as internet cables.
“This is the first time anyone has managed to map out an entire individual underwater avalanche of this size and calculate its growth factor,” said Dr Chris Stevenson, a sedimentologist from the University of Liverpool’s School of Environmental Sciences.
“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 15 m/s, ripping out the sea floor and tearing everything out in its way. To put it in perspective: that’s an avalanche the size of a skyscraper, moving at more than 40 mph from Liverpool to London, which digs out a trench 30 m deep and 15 km 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.”
“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. 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,” concluded Dr. Christoph Bottner, a Marie-Curie research fellow at Aarhus University in Denmark.
Christoph Böttner et al., Extreme erosion and bulking in a giant submarine gravity flow.Sci. Adv.10,eadp2584(2024).DOI:10.1126/sciadv.adp2584