METEORITES may slam into the ocean and cause tsunami more often than anyone realised. Imprints of tsunami impacts discovered on the coast of Australia suggest these giant waves were so powerful that they could only have been triggered by something as cataclysmic as a meteor impact. And we could be due for another one soon.
Geomorphologist Ted Bryant at the University of Wollongong in New South Wales and his colleagues have catalogued a mass of evidence around the Australian coast for these tsunami (see Map). This includes boulders jammed into crevices far above the high tide mark, car-sized blocks of rock lifted over 100-metre-high cliffs, and seashells deposited up to 35 kilometres inland. 鈥淧eople just couldn鈥檛 explain these things before,鈥� Bryant said last week at a conference in London on environmental catastrophes.
Other researchers say unusually violent storms could be the cause. But Bryant insists this doesn鈥檛 explain all the evidence, particularly why many of the boulders are stacked like dominoes, as if carried by a single wave.
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Huge tsunami are very rare around Australia. The tectonic plate it sits on is quiet, so there are few earthquakes to trigger a giant wave. Another main cause of tsunami is a slump of a massive amount of land under water, but the waves created by this would not be big enough to produce the effects Bryant found.
But his computer models show that they could have been produced by a body 6 kilometres across falling into the middle of the Pacific, or a much smaller rock landing closer to Australia鈥檚 coast.
By carbon dating some of the shells found inland, Bryant and his team say they have found evidence for at least six tsunami events. Two of the largest, one about 4000 BC and another in the 16th century AD, were powerful enough to swamp headlands 130 metres above sea level. The more recent one corresponds with a peak in meteor activity noted by Chinese scholars and a Maori legend called the 鈥淔ires of Tamaatea鈥�, which describes fire falling from the sky and killing many people.
The waves appear to have hit at intervals of roughly 1000 to 1100 years. This could coincide with the Earth passing through the debris of a comet trail. If smaller impacts are included, that figure drops to 500 years. And since the last major event was almost 500 years ago, we could be due for another one. 鈥淚 guess you could say there鈥檚 a worry,鈥� says Bryant.
While no one disputes that a meteorite landing in the ocean would cause a tsunami, the suggestion that so many occurred raises the possibility that we have been underestimating the number of large meteorites hitting the planet. Suitable bodies in space are too rare to account for so many tsunami, says Duncan Steel, a meteor expert from the University of Salford. But in the light of what Bryant found so far away from the sea, he is keeping an open mind: 鈥淪omething had to put those boulders up there.鈥�