The avalanche contained debris equivalent to 140,000 Wembley Stadiums (162 km³). It was as tall as a skyscraper (over 200 metres), travelled at over 54 kilometres per hour, and dug a 30 metre deep and 15 kilometre wide trench over 400 kilometres (the distance from London to Liverpool), destroying everything in its path. It then spread over an area the size of Germany, burying it under around one meter of sand and mud.
However, we show that the avalanche actually started as a small landslide and then expanded more than 100 times along its path. This extreme expansion is much larger than land-based avalanches, which typically expand by a factor of 4-8 and are very small in comparison. This calls into question scientists’ view that large avalanches start as large slope failures.
Instead, we now know that underwater avalanches can start small and develop into catastrophic events of extraordinary force as they progress. These insights may therefore change the way we assess the geohazard potential of these phenomena, leading to a greater focus on the avalanche’s path rather than the initial landslide zone.
How often this happens varies by location: Submarine valleys relatively close to rain-filled river mouths can experience several small avalanches per year. Other systems farther from river mouths, like the Agadir Valley off the coast of northwest Morocco, experience huge avalanches only once every 10,000 years.
There are many different factors that can trigger underwater avalanches, including earthquakes, tides, typhoons, river floods, and even volcanic eruptions. Climate change will make some of these triggering factors more frequent and intense.
However, a trigger does not guarantee an avalanche, nor is it related to the size of the avalanche. For example, in 1755 a major earthquake occurred off the coast of Portugal, destroying much of Lisbon and killing tens of thousands of people. However, this earthquake only caused small underwater avalanches. By comparison, a major earthquake off the coast of Newfoundland, Canada, in 1929 caused the largest underwater avalanche on record.
Using detailed seafloor surveys and sediment cores, my colleagues and I reconstructed the characteristics of this avalanche, which carried a mixture of rock, sand and mud at 68 kilometers per hour and severed 11 undersea cables on its way downhill. The avalanche was so large that it triggered a tsunami that killed 28 people along the local coast. This is the first and only large undersea avalanche to be directly measured by cable breakage.
While our understanding of underwater avalanches is still in its infancy, research continues to provide new insights into where they happen, how they occur, and how powerful and destructive they can be. These fascinating phenomena serve as a reminder of the many wonders that still lie hidden in the ocean depths.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Christopher Stevenson does not work for, consult, own shares in, or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond his academic appointment.
