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Tsunami finding rocks research

The first time it occurred to Phillip Playford that tsunamis could have hit the WA coast was when he saw giant blocks of limestone stranded above cliffs on Legendre Island as he sailed past the Pilbara landmark in 1977.

Then in 2009 the geologist was sailing past Koks Island, near Shark Bay, on the Leeuwin when he noticed similar limestone chunks on the flat surface of the island.

But it was not until 2010, while surveying at Shark Bay, that he was able to find evidence the blocks were the result of huge tsunamis that pummelled the WA coast before white settlement, eroding cliffs and throwing the giant boulders up to hundreds of metres inland.

The waves would probably have killed thousands of Aboriginal people when they slammed into the coast from Shark Bay to beyond the Northern Territory border and are probably responsible for the unique sculptured coastline of the Kimberley.

Dr Playford said the tsunami deposits in WA were some of the biggest anywhere in the world and were probably from tsunamis that reached about 20m high with a run-up height of up to 35m.

He said the biggest block he knew of, on Dirk Hartog Island, weighed about 700 tonnes.

"That's been moved about 250m inland and 15m above sea level and the only way it could have been moved there is by a huge tsunami," he said.

Dr Playford said the most recent tsunami deposits had been dated as from 2900 years ago and the oldest from more than 5000 years ago.

"But there's one date that suggests as recently as 600 years ago," he said. "There clearly have been several tsunamis."

The finding is the latest discovery in an extraordinary life for Dr Playford.

He made headlines in 1954 when as a fresh-faced 22-year-old he rediscovered the wreck of the Zuytdorp, a Dutch East India company ship that sank north of Kalbarri in 1712.

Then in the 1960s, Dr Playford travelled to the Great Sandy Desert and met Aboriginal communities who had never come across white people before, taking many photographs. Now, when he is 81, his tsunami research marks yet another incredible contribution to the State's history from the former director of the Geological Survey of WA.

"Most tsunami deposits are subtle but these ones along here sure aren't," Dr Playford said.

"And the amazing thing is they've gone almost unreported, left to a fellow like me in the closing phases of my career to have found them."

It is not known what caused the tsunamis but Dr Playford has three theories about what could be responsible - movement along local faults, the impact of one or several asteroids in the Indian Ocean or landslides on the slope of the Continental Shelf.

He said there was no telling when another tsunami of the same magnitude might hit WA.

"If it hit today it would have catastrophic consequences . . . but it might be tomorrow or it might be 10,000 years, no one knows," he said.

Dr Playford's research is to be published in a Geological Survey of WA bulletin on Shark Bay and he has given talks about the findings to the Royal Society of WA and the Kimberley Society.

A tsunami pushed a 700 tonne rock on Dirk Hartog Island 250m inland and 15m above sea level. Picture: Dr Phillip Playford