Thursday, 11 March 2004

The Last Wave

North HeadThere's something peculiar about the geology around Sydney. The surface geology, that is. There's evidence that not so long ago, North Head (the northernmost part of the entrance to Sydney Harbour - see picture to right) was scoured clean by a monster wave. And there's similar evidence up and down the coast round Sydney. I found some myself after a particularly bad storm, pulverised puddingstone and other volcanic rock that had been transported from a known site at the water's edge, taken inland, and buried for a few hundred years. (In was studying geology at High School at the time).
There are Aboriginal artworks inland from the Sydney region that could be interpreted as recordings of an awesome tsunami, and by comparing dates with known records of the arrivals of European ships, it looks like it happened less than 1,000 years ago. There's also evidence that the Ku-Ring-Gai and other Sydney area Aboriginal tribes were relative latecomers to the scene.
Considering that over 4 million people live in the Cumberland Basin ( the geologiocal area that Sydney's on), a repetition would be a bit of a worry. One of the most likely hypotheses to explain what may have occurred was a sub-surface massive landslip of cubic kilometres of rock from the continental shelf and into the ocean depths. Certainly there's evidence to show that this happens periodically.
But we may now have found the Smoking Gun. From Science News, via A Voyage To Arcturus :
Scientists may have discovered the impact site of one big space rock that smacked into the South Pacific just a few hundred years ago. In eastern Australia, researchers have found jumbled deposits of rocks more than 130 m above sea level that they propose were left by a tsunami. That debris has been dated to about A.D. 1500—a date that matches when the Maori people inexplicably moved away from some areas of New Zealand's coast, says Stephen F. Pekar, a sedimentologist at Queens College in New York. On New Zealand's Stewart Island, two sites sport possible tsunami deposits at elevations of 150 m and 220 m, respectively.

The source locations and heights of waves that could have lofted materials to those elevations steered the search for the impact's ground zero to beneath the sea southwest of New Zealand, says Pekar. Sure enough, he and his colleagues have discovered a crater there that's about 20 km wide and about 150 m deep. Samples of sediment taken from the seafloor southeast of the crater, but not those obtained elsewhere around the crater, contain small mineral globules called tektites, one hallmark of an extraterrestrial impact. That pattern suggests that an object may have struck from the northwest—a path that would have taken the blazing bolide over southeastern Australia, where aboriginal legends mention just such a fireball.

The rock that created tsunamis off New Zealand 500 years ago may have been around 1 km across, the researchers say.
That would fit. A wave 500 ft high, big enough to wash inland till it reached the nearest mountain range, 30 miles inland, round about 1477.

The title of the post comes from a particularly Weird 1977 Peter Weir film, whose final scene shows just such a wave approaching Sydney.

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