Besides its beauty, biological diversity and value as a tourist attraction, the Great Barrier Reef does one more thing for Australians; it guards against tsunamis. Strangely, it seems it didn't perform the same role thousands of years ago when the reef was closer to the shore. Now evidence has been found of an underwater landslide that could have been disastrous for the inhabitants of coastal areas at the time.
Using multibeam sonar, Dr. Jody Webster of the University of Sydney examined the deeper parts of the continental shelf beyond the world's largest system of coral reefs. He found evidence of seven ancient landslides, the largest a seven-kilometer stretch of an underwater cliff that collapsed between 14,000 and 20,000 years ago. “You can see a bite out of the continental margin, and also can see where the deposits left behind downslope,” Webster told IFLScience. The finding has been published in Marine Geology.
The event, named the Viper Slide after a nearby reef, would have thrown up a tsunami two to three meters high, sending a wall of water towards the land. “The discovery of the Viper Slide is the first solid evidence that submarine landslides existed on the Great Barrier Reef,” said James Cook University's Dr. Robin Beaman in a statement. Modeling by Webster's team suggests that back then, when ice age sea levels brought the shore nearly to the reef, the protective effect would have been limited and the shoreline could have been inundated.
However, any impact would have been long since drowned under seas that have risen 70 meters since the peak of the last ice age. Webster says such an event could certainly not account for what some have claimed is evidence for prehistorical tsunamis in the area.
While one tsunami-generating event every two or three thousand years didn't arouse too much alarm, Webster wanted to know what would happen if a similar chunk of underwater terrain gave way today. His team found that, contrary to speculation that gaps between reefs might channel tsunamis to make them even larger threats, the reef would now protect the shore.
Webster told IFLScience that the causes of these landslides remain unknown. There is no historical evidence in the area for the large earthquakes that would appear the most obvious culprit. He thinks it is probably relevant that all the landslides detected lie in a small area that once hosted a giant river delta from an ancient version of the Burdekin River. More disturbingly, he says there is an uncomfirmed theory that rapid rises in sea levels could produce sedimentation that destabilizes underwater cliffs, triggering events like this.
He added that rising sea levels, combined with coral deaths from heat waves and ocean acidification, might leave a gap between reefs and the sea surface large enough to let tsunamis through, but his team had not investigated this possibility.
