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Thanks to Lenode for supporting this SciShow video! That link gives you a $100, 60-day credit on a new Lenode account. Here on land, steep slopes can give way to enormous landslides, but even the biggest ones are nothing compared to the landslides that happen under the ocean. Underwater landslides can be the size of entire cities and trigger gigantic tsunamis, but weirdly enough, it doesn’t take a dramatic event like an earthquake or a volcano to set one off. The things causing many of these enormous slides might actually be so tiny that you’d need a microscope to see them.
Most massive landslides happen at the edges of continental shelves, where the shallow ocean floor surrounding the continents drops off into deep ocean basins. These transition areas are called continental slopes, but it’s not like the sharp drop off into the deep end of a pool. These slopes are mostly gradual, often not even half as steep as a standard wheelchair ramp on land. That kind of gradient normally wouldn’t send land sliding, but all over the world, massive chunks of the ocean floor hurdle down these continental slopes in so-called mega slides.
As catastrophic as that might sound, events like these are actually really important for ocean ecosystems. The continental shelves are home to the vast majority of the ocean’s life, so they’re full of organic matter. Mega slides transfer this nutrient-rich sediment from the coastal areas into the deep ocean, which is more barren most of the time.
We never even notice all these big chunks of Earth rearranging themselves deep underwater, but sometimes they can affect people. Over 8,000 years ago, an epic mega slide in the North Sea sent a wave up to 20 meters high crashing to the shore. We don’t know the death toll, but some scientists think this wave dealt the final blow to an ancient human settlement known as Doggerland.
Thankfully, this kind of thing doesn’t happen a lot, but it’s not so rare either. In 1999, an underwater landslide created a tsunami off the coast of Papua New Guinea that killed over 2,000 people. These days, mega slides can also damage critical infrastructure. For instance, they’ve been known to break cables running along the sea floor, which we rely on for the internet and other global communications, and they could damage other structures we build offshore, like oil platforms and wind turbines.
So when it comes to mega slides, it’s not just “out of sight, out of mind.” There’s a lot at stake here, which gives scientists plenty of reasons for wanting to know what causes mega slides in the first place. Unfortunately, finding an answer is complicated because the sea floor is a super complex place. There are a bunch of things happening at once: you’ve got currents, sediments, and tectonic plates all moving around. Plus, it’s not easy to explore the ocean floor. We can’t just send scientists to scope things out the way they do on land. And without knowing when and where the next mega slide will happen, it’s hard to detect these things in real time.
So far, scientists’ best bet has been to piece together clues from mega slides that have happened in the past. One important clue came from the fact that these slides happen on shallow slopes. For that to be possible, researchers were pretty sure there had to be a weak layer that was giving way somewhere in the slope, and once that gave way, everything on top of it was basically just sledding downhill. On that weak layer, the question was: what was that layer? Some thought it might be a layer of rock or sediment that was prone to breakage or collapse. Others thought it might be grains that were saturated with water.
In a 2018 study, one team considered another possibility: what if it was a layer of tiny dead organisms? To get to the bottom of this, they studied the remains of a landslide that happened nearly 150,000 years ago off the coast of Mauritania. To find the weak layer, they compared two data sets that gave them a glimpse at the sediments piled underwater: one was a map of the sea floor from a sonar survey, and the other was a core sample from the site of the slide.
They found that the weak layer was made of tiny shells and other organic material way down in the sediment. It seemed that all that organic matter had soaked up water, making it so heavy that it eventually gave way and caused the landslide.