Tsunami

They're the Tyrannosaurus Rex of the ocean waves, they can move as fast as a jumbo jet, they're probably behind the legend of the lost civilisation of Atlantis, and they've killed a few hundred thousand people in the last few centuries. They're 'tsunamis', and people have surfed on them - and lived. The word tsunami comes from the Japanese - tsu meaning harbour and nami meaning wave. Tsunamis usually appear as a series of waves, that suddenly pop up in your harbour to create devastation. But tsunamis come in all shapes and sizes. Sometimes they're a wall of water, and sometimes they're a series of crests. Sometimes a tsunami is just a rapidly rising tide, and sometimes it can be foreshadowed by the sea suddenly drawing back to the horizon, leaving fish flopping on the sea bed.

According to the Guinness Book of Records, a 15-metre-high tsunami (back on April 3, 1869) was probably the highest wave ever ridden on a surfboard. The lucky surfie was a Hawaiian called Holua.

But it was Bill and Vivian Swanson who surfed the largest recorded wave in history, back in 1958 - and they used a 12-metre commercial trawling boat. They had anchored for the night just inside the 11-kilometre-long Lituya Bay in south east Alaska. This bay is actually a spectacular fjord, with mountains rising some 1800 metres straight up out of the water. At 10.15 pm, an earthquake began shaking their boat.

There was still enough light for them to see the earthquake trigger a giant avalanche on the sheer rock face, about one kilometre above the water level. As nearly half a cubic kilometre of rock splashed down on one side of the fjord, a giant wave roared up the other side, to a height of half a kilometre. The devastation was incredible, as all the trees and soil were scraped off down to bare bedrock.

Then, the water rolled back down into the fjord, and out of the mist, the Swansons saw a giant tsunami, 50 metres high heading for them at over 160 kilometres per hour. This giant wall of water, covering the 3-kilometre width of the bay (as well as reaching a kilometre or so inland from the shoreline), took 4 terrifying minutes to reach them. When the 50-metre high wave finally reached the trawler, it snapped the anchor chain like an old rubber band and flung the boat upwards to the crest of the wave - and then set them surfing backward out to sea, on the biggest wave in recorded history. But the tsunami began to dissipate as it hit the open sea, and the trawler was tossed stern first into the ocean. Luckily, air was trapped inside her hull, and she floated with the bow pointing up. They made it into the life boat, and were rescued some 2 hours later.

Now even though people call it a tidal wave, a tsunami is definitely not a tidal wave. Tidal waves, otherwise known as tides, are caused by the Sun and the Moon pulling with their gravity on the water. A tsunami is also quite different from your standard wind wave, the one you surf on. A wind wave is caused by air pushing the surface of the water. You might have noticed that, when you dive underneath an incoming wave at the beach, it's quite calm a metre or so beneath the surface. Wind waves have all of their energy very close to the surface. But that's not the case with the third type of wave - the tsunami. In a tsunami, the energy reaches right from the ocean bottom all the way to the surface, so there's a heck of a lot of energy to play with.

A tsunami is usually started by some change in the Earth's crust - a landslide (either above or below the water), or an earthquake or a volcano. If you want make your very own tsunami, just get an area of sea floor about half the size of Tasmania, and move it suddenly upwards or downwards a few metres.

As an example of a landslide, what about the giant tsunami waves that hit the east and north coasts of Scotland some 7,000 years ago, when some 4,000 cubic kilometres of mud on an undersea slope in the Norwegian Sea, suddenly slipped down onto the ocean floor? A volcano? Well, when Krakatoa exploded in 1883, tsunamis some 40 metres high killed 36,000 people on nearby islands. A good example of an off-shore earthquake causing a tsunami was in Lisbon in Portugal in 1755. 60,000 people died as buildings collapsed, and a 12-metre tsunami raged across the city.

Now out on the high seas, even though the wave peaks of a tsunami might be a metre high, the peaks can be several hundred kilometres apart. So you wouldn't even notice a tsunami as it rushed past you at 700 kilometres per hour. But as the tsunami comes onto the shore, it slows down and the waves begin to pile up - and that's when the damage happens. Buildings are demolished, bridges are splintered like matches, and people are crushed to death, or washed out to sea.

Now the earliest tsunami we know of is the one associated with the giant rock that wiped out the dinosaurs some 60 million years ago. It looks as though when it hit the water near the Yucatan Peninsula in Mexico, it sent a sandy tsunami surging over the Texas plains. It left behind a very unusual 70-cm thick bed of sandstone, packed with fragments of shells and fish teeth, wood debris and clumps of mudstone.

Another tsunami happened around 1500 BC, when the volcanic Greek island of Santorini exploded, blowing some 50 cubic kilometres of rock into the air. An island about 18 kilometres in diameter, with a 1.5 kilometre-high volcano in the middle, suddenly turned into a lagoon! Some historians think that the volcanic debris combined with a gigantic tsunami destroyed the Minoan civilisation which was on the nearby island of Crete. The Minoans were instantly wiped out. Even today, along parts of the Mediterranean coast, you can still find an ancient flood contour, about 100 metres above sea level.

Maybe that single tsunami started two major legends - first, the vanishing of Atlantis, and second, the Great Flood that appears in the cultures of the Egyptians, Babylonians and Greeks, and of the Hebrews and the Christians.

We haven't had any major tsunamis for a while, so we're overdue for one. The Hawaiian islands rise almost vertically out of the seafloor. About 100,000 years ago, a giant submarine landslide set off a tsunami that was nearly 400 metres high on the nearby islands, and 40 metres high by the time it reached the east coast of Australia. When the next one happens, it will take about 9 hours to get here. If you happen to be on the east coast when it happens, either head for the Great Dividing Range, or grab your surfboard and go surfing tsunami.

References
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Tidal Wave Changed The History of Peru, New Scientist, No. 1571, 30 July 1987, p 30.
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A Tsunami Tale From Sydney, by Garry Davidson, New Scientist, No. 1843, 17 October 1992, p 17.
Shallow Shock, New Scientist, No. 1852/53, 19/26 December 1992, p 10.
When Nightmare Waves Appear Out of Nowhere to Smash The Land, by Scott McCredie, Smithsonian, March 1994, pp 28 - 39.

Copyright © Karl S. Kruszelnicki