North-westerly view of the Gloria Knolls Slide and Gloria Knolls off Innisfail. Depths are coloured red (shallow) to blue (deep), over a depth range of about 1700m. Image Credit: © www.deepreef.org

Massive undersea landslide on the Great Barrier Reef revealed

  • BY Ellen Rykers |
  • February 08, 2017

Debris displaced in the landslide was 30 times the volume of Uluru, according to Australian scientists.

THE REMNANTS OF A massive undersea landslide on the Great Barrier Reef have been discovered by Australian researchers, 75km off the north Queensland coast near the town of Innisfail.

The landslide occurred more than 300,000 years ago, but debris blocks from the huge event can still be seen today in a debris field scattered 30km across the seafloor. The blocks, collectively known as the Gloria Knolls Slide, are hundreds of metres high and up to 3km long.

“It would have been a spectacular event when it happened,” said Associate Professor Jody Webster, a geologist at the University of Sydney. “Our modelling suggests that the landslide produced a tsunami.” 

The tsunami could have been as high as 27m, according to the team’s research published in Marine Geology.

However, Jody explained that it was unclear what the size of the wave would have been at the coast. “If there were a landslide today, we would see the effects of a tsunami on the coast,” he said. “But the exact impact is hard to quantify because there are unknown variables, such as the blocking effect of the reef.”

VIDEO: 3D flythough over the Gloria Knolls Slide and Gloria Knolls off Innisfail. Depths are coloured red (shallow) to blue (deep), over a depth range of about 1700m. (© www.deepreef.org)

A geologic detective mystery

The discovery came as a surprise to the scientists, who were conducting research from the Marine National Facility’s blue-water research ship Southern Surveyor.

“We were amazed to discover this cluster of knolls while mapping the deep Great Barrier Reef seafloor,” said Dr Robin Beaman, a marine geologist from James Cook University in Cairns. “In an area of the Queensland Trough that was supposed to be relatively flat were eight knolls.”

“We put a dredge down, not knowing what we’d find,” explained Jody. “It was mostly mud, but then we found pieces of deep-water coral. This was the first step towards realising that the blocks had originated from the slopes on the margin of the Great Barrier Reef.

“We did more mapping and identified the ‘scar’ or ‘bite’ where material had collapsed from during the submarine failure,” he added.

The presence of the knolls wasn’t the only surprise: the features harboured a remarkable community of cold-water corals and other organisms, including barnacles and gorgonian sea whips. “This is the first discovery of these sorts of deep-water biota on the Great Barrier Reef,” said Jody. “It’s a whole new habitat.”

“The oldest fossil corals recovered off the top of the knoll were 302,000 years,” said Dr Angel Puga-Bernabéu at the University of Granada and lead author on the study, “which means the landslide event that caused these knolls must be older.”

Tsunami risk “relatively low”

In recent history, undersea landslides have been known to generate tsunamis – with deadly consequences.

In 1998, an earthquake off the coast of Papua New Guinea triggered a submarine collapse, which in turn produced a tsunami that killed more than 2000 people.

However, Jody said that the risk of tsunami here is likely to be comparatively low when compared to other, more tectonically active parts of the world. “That said, there is a 2.4km section of the sea floor called Noggin Block on the upper slope of the Great Barrier Reef margin that may potentially collapse,” he said. “But modelling suggests we’d need quite a large earthquake, of a magnitude not seen in the area for at least the last 150 years, to trigger collapse”.

According to Jody, scientists still have a lot to learn about undersea landslides on the Great Barrier Reef. “We don’t know much about these types of landslide,” he said. “We don’t really know how frequent they are. More work is needed to map them, figure out their size and how far they travelled, and when they occurred”.

But this discovery reveals a far more complex undersea landscape than previously known. “This tells us a lot about the deep seascape of the Great Barrier Reef, and reveals the importance of mapping the sea floor,” said Jody.

“We’ve discovered remarkable things that would have otherwise remained undiscovered,” he said.

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