A giant underwater avalanche off the coast of West Africa stayed in motion for two days, scientists reveal.
The colossal event occurred underwater in the Congo Canyon, a deep submarine canyon that leads away from the mouth of the Congo River on the west coast of Africa.
Underwater sensors scattered on the ocean floor showed that it spread almost 1,100 km above the floor of the Atlantic Ocean.
The avalanche broke two telecommunications cables on the ocean floor that underpin traffic to West Africa, slowing the Internet from Nigeria to South Africa.
It wasn’t triggered by an earthquake, but rather a combination of two factors – severe floods and unusually large spring tides.
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The event occurred underwater in Congo Canyon, a deep canyon that leads away from the mouth of the Congo River (pictured here in NASA images).
WHAT IS A CLOUDY STREAM?
A turbid flow is a fast, downhill flowing water flow caused by increased density due to high levels of sediment.
Turbid currents can be set in motion when mud and sand on the continental shelf are loosened by earthquakes, collapsing slopes, and other geological disturbances.
The cloudy water then falls down like an avalanche, picks up sediments and speeds up.
It took place on January 14th last year, although a team of scientists including experts from Durham University and the University of Hull have only just fully analyzed the resulting data.
The Congo Canyon is one of the largest submarine canyons in the world. Submarine canyons are steep-walled, winding valleys with V-shaped cross-sections cut into the ocean floor of the continental slope.
“We had a number of oceanographic berths hit by the event that broke them from their anchors on the ocean floor, so they turned up to email us,” Durham University Professor Peter Talling told the BBC.
“This thing was getting faster and faster. As it erodes the ocean floor in the process, it absorbs sand and mud, making the current denser and faster.
“So it has this positive feedback where it can build and build and build.”
The current continuously accelerated itself, increasing from a speed of 16 feet (five meters) per second to 26 feet (eight meters) per second.
“This is the longest outlet turbidity stream that has been monitored in action to date, and the only monitored river that is continuously accelerating itself for over a thousand kilometers,” say the researchers.
The event is officially known as the turbidity current – a fast, downhill flowing water flow caused by increased density due to high levels of sediment.
Turbid currents can be set in motion when mud and sand on the continental shelf are loosened by earthquakes, collapsing slopes, and other geological disturbances – although this event was not due to an earthquake.
The event was triggered by an extreme flood along the Congo River observed in December 2019 that brought sand and mud to the head of the Congo Canyon, as well as some unusually large spring tides two weeks later.
The Congo Canyon is an undersea canyon at the end of the Congo River in Africa. Submarine canyons are steep-walled, winding valleys with V-shaped cross-sections that are cut into the seabed of the continental slope
Buried deep under the world’s oceans and seas is a network of underwater cables that silently connect even the most remote parts of the world to the Internet. The avalanche broke two telecommunications cables on the ocean floor that underpin traffic to West Africa, slowing the Internet from Nigeria to South Africa (stock image)
So far, there has only been one previously directly measured turbidity current of the same magnitude – the Grand Banks earthquake in 1929.
Grand Banks in Newfoundland, Canada broke all 20 submarine cables across the North Atlantic.
The event ran for more than 500 miles (800 km) but was delayed from 62 feet (19 meters) per second to 10 feet (3 meters) per second instead of accelerating continuously like the January 2020 event.
A turbid flow is a fast, downhill flowing water flow caused by increased density due to high levels of sediment
Researchers say it is crucial to determine how the frequency of submarine currents will be affected by future changes in climate and hydrology in the Congo Basin.
The event also has implications for the upcoming construction of submarine communications cables that will provide vital Internet to parts of Africa.
“It is important to understand how such strong and very long outlet turbidity currents are triggered, particularly for threats to strategic seabed cables, including cable routes planned for 2020-21 off West Africa,” they say.
In addition to the University of Hull, scientists from the GEOMAR Helmholtz Center for Ocean Research in Germany and the Institut Français de Recherche pour l’Exploitation de la MER in France were also involved in the analysis.
The team has described the event in more detail in a pre-print paper that has yet to be examined by experts.
WHAT ARE UNDERWATER CABLES?
A submarine communication cable is a cable that is laid on the sea floor between land-based stations.
It is laid by specially designed ships that can transport thousands of kilometers of coiled cables in their holds and lay them on their journey across the ocean.
The first commercial cables were laid to send telegraph traffic in 1850. Since then, telephone traffic and, most recently, data traffic have been transmitted over the cables.
Many of the modern cables are made of fiber optic.
Test cables were laid in New York Harbor in 1842 and insulated with tarred hemp and rubber. Nowadays cables are protected with polyethylene.
Traditionally, the cables have been owned by service providers, but websites have also started buying underwater cables to control their networks including Google and Facebook.