New research from the University of Liverpool has uncovered the sheer scale and destructive power of an ancient underwater avalanche that occurred nearly 60,000 years ago off the Northwest coast of Africa. In the journal Science Advances, it is a detailed analysis of the event, which started as a small landslide but expanded to over 100 times its original size as it traveled across the Atlantic Ocean sea floor.
It is more technically referred to as a turbidity current, and it originated from the large Agadir Canyon, another submarine canyon off the coast of Morocco. First to happen was a fairly small landslide on the sea floor, of about 1.5 cubic kilometers in volume, which initiated the event. Perfect, in the-best-of-all-possible-worlds manner, as the avalanche moved through the canyon it picked up boulders, gravel, sand, and mud. Due to this rather significant accumulation of material, the avalanche increased exponentially to flow 1,600 kilometers across the Atlantic seabed.
This was a force so great that the entire length of the canyon—some 400 kilometers—was eroded, cutting a swath several hundred meters up the sides of the canyon and affecting about 4,500 square kilometers. It is further estimated that this avalanche carried debris along the sides of the canyon to heights of as much as 130 meters, with the whole mass moving at about 15 meters per second—approximately 54 kilometers per hour.
This is the first time that researchers have ever mapped an underwater avalanche of this scale from start to finish. The study was led by Dr Chris Stevenson, a sedimentologist from the University of Liverpool’s School of Environmental Sciences. He explained the event, comparing it to “an avalanche the size of a skyscraper, moving at more than 40 miles per hour,” dug out a trench 30 meters deep and 15 kilometers wide, destroying everything in its path.
The findings by the team highlight the role that underwater avalanches play in moving materials about on the Earth’s surface. Although the event is hidden under the ocean, it is crucial for moving sediments, nutrients, and even pollutants across very vast areas. However, they also pose a substantial geohazard to sea floor infrastructure like undersea cables that carry the majority of global internet traffic.
The research team reconstructed the extent of the avalanche by analyzing more than 300 core samples collected during research cruises over the past 40 years. They pieced this information together with seismic and bathymetry data to reconstruct the event in detail. The study was carried out by Dr. Christoph Bottner, a Marie-Curie research fellow at Aarhus University in Denmark, who explained that the growth factor of the avalanche was at least 100, much higher than the average value of 4-8 times growth common for snow avalanches or debris flows.”.
The research also involved contributions from Professor Sebastian Krastel, head of Marine Geophysics at Kiel University, and other scientists from the Leibniz Institute for Baltic Sea Research and GEOMAR Helmholtz Centre for Ocean Research in Germany. Data was also supported by core samples from the British Ocean Sediment Core Repository at NOCS Southampton, collected aboard NERC ships.
These new insights destabilize the former view of the sources of large underwater avalanches, thought to arise only from significant slope failures. The study demonstrates that small events can escalate into very large and highly destructive phenomena; it thus underlines how an improved understanding of these events can help in mitigating their potential risks to critical undersea infrastructures.