When we think of powerful geological forces, we often imagine earthquakes, volcanoes, and hurricanes. We also tend to think of them on the ground. However, hidden beneath the ocean’s surface, there’s another formidable geological phenomenon: submarine landslides. Technically, they’re called gravity flows. These flows can start small but grow to catastrophic proportions as they travel across the ocean floor, entraining sediment and gaining momentum.

A recent study, published in Science Advances, sheds light on the scale, force, and impact of this dramatic phenomenon, mapping a giant underwater avalanche that took place 60,000 years ago.

The Bed 5 Event

The massive avalanche took place on the Agadir Canyon, located off the coast of Northwest Africa. This submarine canyon is one of the largest in the world, stretching over 450 kilometers long and plunging to depths of 1.2 kilometers. The event in question, known as the “Bed 5 event,” started as a relatively small underwater landslide. It grew and grew, gathering a massive volume of around 1.5 cubic kilometers, rapidly escalating into a massive gravity flow that eventually moved 162 cubic kilometers of sediment.

Basically, it reached a volume 100 times greater than what it started with.

The avalanche eroded the entire 400 km length of the canyon and several hundred meters vertically. It carried cobbles more than 130 m down the side of the canyon.

“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 meter of sand and mud,” says Chris Stevenson, a sedimentologist from the University of Liverpool’s School of Environmental Sciences.

However, figuring out what happened wasn’t straightforward at all. Unlike a regular avalanche, you can’t really see this one. So, to research it, researchers analyzed 300 core samples from the area taken during research cruises over the last 40 years.

Mud, stones, and gravity

Mud played a pivotal role in the Bed 5 event’s extreme bulking. Unlike coarser sediments, mud has a very low settling velocity, meaning it stays suspended in the water for long periods, maintaining the flow’s density and speed. This property makes mud an excellent fuel for submarine gravity flows, allowing them to sustain their momentum over great distances.

The research team found that the floor of the Agadir Canyon is covered with thick deposits of remobilized mud, remnants of earlier landslides on the Moroccan Continental Slope. These mud deposits barely change in stability with depth. So, they were easily eroded by the Bed 5 flow, which continued to expand as it descended the canyon. The flow’s ability to entrain mud was so effective that its size was ultimately limited not by its sediment-carrying capacity but by the physical dimensions of the canyon itself.

“This is the first time anyone has managed to map out an entire individual underwater avalanche of this size and calculate its growth factor.”

“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.”

Learning from this underwater avalanche

The findings from this study have profound implications for our understanding of submarine gravity flows and their potential hazards. Submarine gravity flows are capable of reshaping the seafloor which could affect underwater infrastructure like communication cables and oil pipelines. Understanding how these flows grow and evolve is crucial for predicting their behavior and mitigating their impacts.

“Our 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,” says Professor Sebastian Krastel, head of Marine Geophysics at Kiel University and chief scientist aboard the cruises that mapped the canyon.

“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.”

Journal Reference: Christoph Böttner et al, Extreme erosion and bulking in a giant submarine gravity flow, Science Advances (2024). DOI: 10.1126/sciadv.adp2584. www.science.org/doi/10.1126/sciadv.adp2584