Sept. 16, 2008 -- On a remote tropical island in the South Pacific Ocean, geologists have uncovered signs of ancient devastation.
Sitting on the west coast of the flat, unassuming island of Tongatapu are seven giant boulders made of coral. Identical in makeup to the reefs that line the coast, they may represent the largest rocks ever deposited by a mega-tsunami, a furious wall of water some 130 feet high that stormed ashore thousands of years ago.
When Matthew Hornbach of the University of Texas, Austin, and a team of researchers discovered the boulders last year, he was baffled by how they might've gotten there.
"They just looked so out of place," Hornbach said. "Tongatapu is flat as a pancake, and here are seven boulders from bus-sized to house-sized sitting hundreds of meters inland and 10 to 20 meters [33 to 66 feet] above sea level. I mean, you can see these things on Google Earth, from space."
Back in the lab, Hornbach and colleagues ran several computer simulations of the type of wave necessary to deposit such massive boulders. Underwater landslides sometimes produce tsunamis, as do powerful earthquakes that form subduction zones.
"But the estimated wave energy generated by the landslides we modeled was too low by an order of magnitude," Hornbach said. And earthquake models generated widespread tsunamis that would be evident across the region, not just on western Tongatapu.
Hornbach needed a more powerful, focused source.
Luckily, one was just discovered last year -- an unnamed underwater volcano 22 miles west of Tongatapu, the main island in the country of Tonga. A survey of the sides of the volcano reveals a three-mile-long section of its flank may have collapsed, sending a wave up to 130 feet high crashing into the island.
"Our guess is that it was comparable to Krakatau's wave," Hornbach said, referring to the famous eruption in 1883 that inundated the islands of Java and Sumatra with a tsunami 100 to 130 feet high.
But the boulders strewn along Tongatapu are bigger than deposits made by Krakatau's tsunami. The team estimates that the largest, easily the size of a house, weighs some 1,600 metric tons. By comparison, the largest Krakatau boulder weighed 600 metric tons.
"Look, we can't discount that something else may be going on here. A hypercane or some extremely powerful storm could have done it," he said. "But to my knowledge no storm has ever deposited anything bigger than car-sized boulders."
"The giant-ness of the wave proposed here hasn't been seen in human time," Jose Borrerros of ASR Research, a marine consulting firm in Raglan, New Zealand, said. "I wouldn't dispute that it's possible, though you can never say for sure what caused it. It's really hard to nail this stuff down."
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