North-westerly view of the Gloria Knolls Slide and Gloria Knolls off Innisfail, Australia. Depths are coloured red (shallow) to blue (deep), over a depth range of about 5,500 feet (1,700 metres). (Image: www.deepreef.org)

Around 300,000 years ago, a ridge near Australia’s Great Barrier Reef collapsed, unleashing a massive undersea avalanche. The ensuing landslide scattered debris for miles and triggered a sizeable tsunami, according to new research.

The remains of the ancient slip, known as the Gloria Knolls Slide, were discovered 46 miles (75 km) off the north Queensland coast, and at the bottom of the Queensland Trough—a vast basin that runs adjacent to the Great Barrier Reef. A team led by geologist Robin Beaman from James Cook University made the discovery while conducting 3D mapping of ancient reefs in the giant gully.

Advertisement

The research, published in the journal Marine Geology, identifies a debris field of large blocks, known as knolls, and numerous other blocks scattered over 18 miles (30 km) from the source of the collapse. When the cliff wall failed, it released a whopping eight cubic miles (32 cubic km) of rock, spilling to a depth of 4,500 feet (1,350 meters). That’s about 30 times the volume of sediment contained in Australia’s famed Uluru sandstone monolith, also known as Ayers Rock.

After discovering the knolls, the researchers used 3D mapping to uncover the source of the landslide.

“We were amazed to discover this cluster of knolls while 3D multibeam mapping the deep GBR seafloor,” said Beaman in a statement. “In an area of the Queensland Trough that was supposed to be relatively flat were eight knolls, appearing like hills with some over 100 meters [328 feet] high and 3 km [1.8 miles] long.”

Advertisement

The sudden collapse of the wall contained enough potential energy to produce an 88-foot-high (27 meters) tsunami wave, but the giant wave, as it moved towards the ancient Australian coast, was likely dampened by the surrounding corals. “The Great Barrier Reef acts like a giant porous breakwater to reduce the energy [of ocean swell],” said Beaman in an interview with the BBC. “If it was in existence at the time of this landslide, it would have done a similar job.” Looking ahead, the researchers would like to know if the tsunami risk posed by similar landslides threaten the Queensland coast.

As part of the study, the researchers took sediment samples from a knoll at a depth of 3,840 feet (1,170 meters). Analysis of the samples confirmed the presence of both living and fossilized coral species, gorgonian sea whips (a fan-shaped soft coral), bamboo corals, molluscs, and stalked barnacles. The oldest fossilized coral was dated to 302,000 years ago, so the landslide couldn’t have happened any earlier than that.

Prehistoric undersea landslides have been recorded before, including the Agulhas slide—an event that unleashed 4,800 cubic miles (20,000 cubic km) of debris off South Africa about 2.6 million years ago. If that sounds like a lot, it is; that’s about 625 times bigger than the Gloria Knolls Slide, and it released nearly enough mass to fill all of the Great Lakes.

[Marine Geology]