Underwater avalanches are powerful natural phenomena that occur constantly beneath the ocean’s surface. Because avalanches are invisible and extremely difficult to measure, little is known about how they work.
But these phenomena pose dangers to our global communications networks: the rise of the Internet has necessitated an ever-expanding network of fiber-optic undersea cables that carry virtually all of the world’s Internet traffic.
My new research into ancient underwater avalanches calls into question our understanding of how underwater avalanches occur and could change the way geologists assess the potential risk of avalanches.
There are currently over 550 submarine cables in operation around the world, totalling an estimated 1.4 million km of cable – enough to circle the Earth 35 times.
If an undersea avalanche cuts off undersea cables, the consequences can be widespread and costly: the 2006 Pingtung earthquake in Taiwan triggered an undersea avalanche that cut many of the undersea cables connecting Southeast Asia to the rest of the world.
China’s largest internet operator reported a 90% drop in traffic to the United States at the peak of the incident, while Taiwan saw internet traffic to its neighboring islands fall by 74-100%.
This caused a huge drop in the volume of financial transactions, damaging global markets, and it took 39 days and millions of dollars of shipping time to restore the network to full operation.
The underwater avalanches that broke these cables were fast-moving, up to 72 kilometers per hour, but they were relatively small compared to the giant underwater avalanches I have studied in the Atlantic.
Fortunately, there are plenty of undersea cables, so the chances of an undersea avalanche shutting down the internet around the world are extremely low. The Pingtung earthquake is an example of how at least some traffic can travel on alternative routes even if the main route is cut off.
In a new research paper, my colleagues and I map the damage caused by a massive underwater avalanche that occurred 60,000 years ago in a source area off the coast of Morocco.
The avalanche traveled 400 km through the world’s largest undersea canyon and 1,600 km across the Atlantic Ocean floor, making it the second largest submarine avalanche ever recorded.
“We created this map of the avalanche by combining detailed topographical maps of the ocean floor with hundreds of sediment cores that penetrated the avalanche deposits over vast areas. By analysing the fossils in the sediments in all the cores, we were able to date this event to 60,000 years ago, and correlate the individual avalanche layers over thousands of kilometres.
The avalanche contained debris equivalent to 140,000 Wembley Stadiums (162 km³), was as tall as a skyscraper (over 200 metres), travelled at over 54 kilometres per hour, and dug a 30 metre deep, 15 kilometre wide trench over 400 kilometres (the distance from London to Liverpool), destroying everything in its path.
The ash was then spread over an area the size of Germany, burying it under about a 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 triggered small underwater avalanches.
By comparison, a massive earthquake off the coast of Newfoundland, Canada in 1929 triggered the largest underwater avalanche ever recorded.
Using detailed seafloor surveys and sediment cores, my colleagues and I reconstructed the characteristics of this earthquake, which carried a mixture of rock, sand and mud at high speed, severing 11 undersea cables on its way down.
The avalanche was so large that it generated a tsunami that killed 28 people along the local coast. This is the first and only large underwater avalanche to be directly measured by a cable break.
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.
Christopher Stevenson, Senior Lecturer in Quantitative Sedimentology, University of Liverpool
This article is republished from The Conversation under a Creative Commons license. Read the original article.
