The ‘Bed-5’ event: Uncovering one of the world’s largest underwater avalanches

Stretching 280 miles long and plunging over 1,000 meters deep, the Agadir Canyon, located off the Morocco, is one of the largest submarine canyons in the world. A source of giant submarine sediment gravity flows to the Agadir Basin and the wider Moroccan Turbidite system, the canyon has been the subject of numerous marine geophysical studies through the years. The latest one, however, has proved crucial, shedding more light into one of the most pivotal events to have occurred in the canyon nearly 60,000 years ago. An underwater avalanche of gigantic proportions that erased everything in its wake as it spread through an area the size of Germany.

Underwater avalanches, also known as turbidity currents, are fast-moving flows of water that travel downhill through more than 9,000 submarine canyons across Earth’s oceans. Once triggered—by earthquakes, collapsing slopes, or other geological disturbances—they cascade downward, collecting sediment and increasing their speed as they flow.

As a result, they have the power to transform the seafloor by transporting vast quantities of sediment into the deep sea, with the largest particles settling at the bottom and the smallest ones remaining on top. Turbidity currents also pose a significant hazard to modern marine infrastructure , such as cables and pipelines.

Now researchers from University of Liverpool in the UK, Aarhus University in Denmark, and the University of Kiel in Germany, have successfully mapped the path and reconstructed a historical underwater avalanche , touted to be one of the world’s largest, from head to toe for the first time in history.

According to the study titled ‘Extreme erosion and bulking in a giant submarine gravity low’, published in the journal Science Advances , the monumental event took place nearly 60,000 years ago in the Agadir Canyon , the world’s largest undersea canyon. The avalanche then traveled more than 1,200 miles across the Atlantic Ocean seafloor, roughly traveling a distance similar to that between Toronto and Miami, and leaving behind a massive trail of destruction.

The ‘Bed-5’ event

The phenomenon, which the scientists call the ‘Bed-5’ event, started as a small seafloor landslide with a volume of approximately 1.5 cubic kilometers. However, according to Christopher Stevenson, a sedimentology expert from the University of Liverpool ‘s School of Environmental Sciences and co-author of the study, it then increased in size by more than 100 times and went on to cover more than 250 miles through the canyon. Along the way, it collected boulders, gravel, sand, and mud before continuing for an additional 1,000 miles across the Atlantic seafloor. The current was potent enough to carry cobbles up to a distance of 130 meters to the side of the canyon.

In an interview with Interesting Engineering , Stevenson explains how the team examined over 300 cores from the region, gathered during various research cruises over the last 40 years. They also evaluated seismic and bathymetric data collected on two research cruises to the Moroccan coast from expeditions with the German research vessel Maria S. Merian in 2013 and 2023.

Stevenson highlights two groundbreaking aspects of the research. “The first thing is that by using a multibeam echosounder to map the seabed, we could see the damage that had been caused by this avalanche event and the area that it had eroded,” he says. “And the second thing is that we managed to trace this avalanche event all the way back to where it came from.”

“We can’t see the flow anymore, it’s long gone, but what we could do is map and analyze the deposit. So we estimated how big the avalanche was, and we compared it to how big we know the deposits are,” Stevenson continues. “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 meters per second ripping out the sea floor and tearing everything out in its way.”

The event unleashed a huge mass of sediment, the size of a modern skyscraper. It moved across the seafloor at speeds of up to 40 miles per hour, destroying everything along the way. “It was catastrophic coming through the canyon, and then it grew a hundred times and spread across an area the size of Germany,” says Stevenson. “But the initial failure wasn’t that big, it wasn’t big at all.”

The Ignition Theory

Stevenson notes that the findings present the first field evidence backing the ‘Ignition Theory,’ a concept acknowledged by the scientific community for more than four decades. This theory provides a mathematical model to explain how underwater avalanches behave.

“A flow will start moving, and that movement means it will pick up sediment, and when it does it becomes bigger and moves faster,” says Stevenson. “If it moves faster, it could pick up more sediment, leading to this ignition process.” However, Stevenson points out that at a certain point, the sediment concentration becomes too high, causing the flow to become saturated. “When that happens, the flow can’t grow anymore and it starts to decelerate and begins depositing.”

According to Stevenson, this positive feedback mechanism ends when the sediment concentration reaches a level sufficient for viscous forces to dominate.
“With our study, we have not only succeeded in completely mapping an entire individual turbidity current of this size, but also in precisely calculating its growth factor.”

“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 Christoph Böttner , a Marie-Curie research fellow at Aarhus University and co-author of the study. “We have also seen this extreme growth in smaller submarine avalanches measured elsewhere, so we think this might be a specific behavior associated with underwater avalanches and is something we plan to investigate further.”

Threat to modern society

Sebastian Krastel , a marine geophysics expert at Kiel University, states that the study fundamentally challenges the perspective on turbidity currents. “Before the study, we thought that big avalanches only came from big slope failures,” he says. “But now, we know that they can start small and grow into extremely powerful and extensive giant events.”

Underwater avalanches, unlike landslides or snow avalanches, are incredibly difficult to measure. Yet, they are the primary force responsible for transporting materials like sediments, nutrients, and pollutants across the planet’s surface.

Predicting potential geohazards and evaluating their effects on marine infrastructure is vital, according to the researchers. Once an underwater avalanche is triggered, it can easily destroy any cables in its path. This could potentially cut off entire continents from the internet, which the modern world heavily depends on.

“The results of the ‘Bed-5 event’ are important for the prediction and assessment of marine natural hazards such as landslides,” says Krastel. “They underline the need to better understand the processes of flow bulking, erosion and distribution of sediments in submarine structures such as canyons in order to better predict future events and assess their potential impacts on maritime infrastructure and ecosystems.”

On whether the ‘Bed-5’ event had an impact on the marine ecosystem in the affected areas, Stevenson admits that it remains entirely unknown. “What we do know is that the destruction basically dug a trench, 30 meters deep and 15 kilometers wide,” he says. “It basically would have destroyed everything, so I imagine anything living on or in the seafloor would have a very hard time.”

“What we don’t know yet is what type of animals can survive these things,” he concludes. “There’s all sorts of life on the seafloor, and we just don’t know what these avalanches did to them, but I believe soft-bodied creatures or crabs would be completely obliterated in an avalanche.”

Recent cruises led by GEOMAR Helmholtz Centre for Ocean Research Kie l and the Leibniz Institute for Baltic Sea Research Warnemünde-IOW in Germany also mapped the Agadir Canyon.