Mud and debris avalanches are not only found on land – massive landslides also occur underwater. Geologists have now identified one of the largest events of this kind off the coast of Morocco. Around 60,000 years ago, around 162 cubic kilometers of material rushed down the underwater Agadir Canyon, leaving a path of erosion and destruction that stretches around 2,000 kilometers out into the Atlantic. Using core samples, the team was also able to determine for the first time where and how this mega-landslide began. According to the team, it originally started as a slide of only around 1.5 cubic kilometers in size in the southern upper reaches of the underwater canyon, but then increased in volume by more than 100 times as it progressed through the canyon – more than any other landslide known on earth.
Whether landslides, debris avalanches, rock falls or snow avalanches: Landslides caused by gravity and unstable slope deposits occur again and again on Earth. These avalanches often start small and then get bigger and bigger because they erode the ground on their way and pick up more material. “This can dramatically increase their size, speed and range,” explain Christoph Böttner from the University of Kiel and his colleagues. Snow avalanches can grow four to ten times from their point of origin, while debris avalanches can even increase by 50 times their original volume. Several places are also known in the ocean, mostly on continental slopes and other steeper coastal sections, where massive landslides have occurred, including the Storegga landslide off the coast of Norway at the end of the last ice age and an underwater landslide that extended particularly far into the Atlantic off the mouth of the Congo on the west coast of Africa.
Landslide in a sea gorge
Until now, however, it was unclear to what extent such underwater landslides can grow as they progress. Böttner and his team have now investigated this at the Agadir underwater canyon off the coast of Morocco. At 450 kilometers long, up to 30 kilometers wide and 1.2 kilometers deep, the Agadir Canyon is considered one of the largest underwater canyons in the world. Previous geological studies have shown that large landslides have repeatedly broken loose in this canyon and raced down the enormous underwater gorge. One of the last and largest events of this kind was the so-called “Bed 5” landslide almost 60,000 years ago. “This flow contained around 162 cubic kilometers of sediment and covered the extraordinary distance of more than 2,000 kilometers,” report the researchers. Until now, however, it was not known where this enormous landslide originated and how much material formed its beginning, because obvious landslide scars or other traces are missing on the areas of the near-shore seabed above the canyon.
To answer this question, Böttner and his colleagues analyzed more than 300 sediment cores that had been taken from various locations in the Agadir Canyon and the slopes above it over the past 40 years. They also mapped the entire submarine canyon using high-resolution sonar surveys. These revealed, among other things, that there are two deeply incised, branched tributaries above the gorge that join together around 80 kilometers down the slope to form the Agadir Canyon. The upper part of the canyon is relatively steep, with a slope of four to five percent, and then flattens out to a gradient of 0.3 percent in the main part. On its side edges, the sonar images revealed clearly pronounced erosion edges caused by the Bed 5 slide. “Mapping these edges reveals really extensive, strong erosion of around 4,473 square kilometers along the entire length of the canyon,” the researchers report. They determined exactly how the landslide progressed and where it began by combining these bathymetric mappings and the core sample data.
Volume increase by a hundredfold
“This is the first time that we have managed to map an entire submarine landslide of this size and determine its growth factor,” says co-author Christopher Stevenson from the University of Liverpool. The analyses showed that the region of origin of the landslide must be in the southern inflow of the canyon. There, the entire seabed could have come loose in an almost closed layer up to 30 meters thick. “Our scenario assumes that no scars from this event are visible anymore because the landslide occurred as a blanket remobilization of the sediment at the bottom of this southern inflow,” explain the scientists. According to their calculations, the volume of this triggering landslide was around 1.5 cubic kilometers. This means that this submarine avalanche must have increased dramatically in volume from its beginning to its end. “The small initial slope failure grew by more than a hundred times its initial volume and developed into a catastrophic giant flow of around 162 cubic kilometers,” write Böttner and his colleagues. The massive slide carried mud, rocks and sediment from the floor of the Agadir Canyon, eroding the seafloor along the entire length of the canyon and hundreds of meters along the canyon walls.
“After a relatively small start, the event grew into a massive and destructive underwater avalanche that reached heights of 200 meters and swept the sea floor with it at speeds of around 15 meters per second,” says Stevenson, describing the scenario. “For comparison: this slide was the height of a skyscraper and raced down the slope at more than 65 kilometers per hour. It dug a trench more than 30 meters deep and 15 kilometers wide and ultimately covered an area larger than Great Britain under a good meter of sand and mud.” The researchers suspect that the combination of high speed and a sediment consisting primarily of fine mud gave the slide its great reach and force. This is because the free particles remain suspended for a long time, but at the same time promote the cohesion of the mudslide. “Ultimately, the Bed 5 event was only limited by the cross-section of the canyon,” explain Böttner and his team. “Because it is exceptionally large, the landslide could become a catastrophic, massive event.”
According to the scientists, the Bed 5 landslide and its drastic increase in volume are not an isolated case: “We assume that this is a specific behavior of submarine avalanches,” says Böttner. His colleague Sebastian Krastel, senior author of the study, adds: “Our new findings fundamentally change our view of such events. Before this study, we thought that large submarine avalanches only start from correspondingly large slope failures. Now we know that they can start small and then grow into extremely strong and extensive events.” This is also of great importance for assessing the risk posed by such submarine landslides.
Source: Christoph Böttner (University of Kiel) et al., Science Advances, doi: 10.1126/sciadv.adp2584