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World’s largest tsunami debris

House-sized boulders were ripped from a reef, then washed 100 meters or more inland

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11:06am, October 7, 2008
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Seven immense boulders of coral have been found far inland on a Tongan island and may be the world’s largest tsunami debris, a new study suggests.

All evidence hints that the boulders, which lie inland of a three-kilometer stretch of coastline, are out of place, says Matthew J. Hornbach, a geologist at the University of Texas at Austin. First, the island is flat, so the immense boulders — each of them weighing more than 46 metric tons — didn’t roll to their current positions more than 100 meters from the beach. Second, the boulders’ composition differs significantly from the island’s volcanic soil, but matches that of the coral reef found just offshore, he notes.

Third, the corals in the boulders were likely alive about 122,000 years ago, but average sea level hasn’t reached the boulders present location at any time since then. Finally, some of the coral masses are oriented sideways or upside-down instead of upward, facing the sun, another testament to an exotic origin, Hornbach reported Sunday in Houston at the annual meeting of the Geological Society of America.

Surveys of the offshore shallows near the largest boulder — a three-story-tall, 1,200-metric-ton whopper — reveal a large gap in the reef where the boulder may have originated. Hornbach and his colleagues also discovered signs of a submarine landslide, but their analyses hint that the wave generated by the slumping material couldn’t have created a tsunami large enough to toss the coral pieces to their current positions. A large earthquake at the nearest subduction zone, the same type of fault that triggered the Indian Ocean tsunami in December 2004, couldn’t have spawned waves large enough to do the job, either, the researchers speculate.

However, oceanographic surveys farther out to sea have identified a possible source of a wave capable of lofting the boulders, says Hornbach. About 35 kilometers offshore sits a submarine volcano that rises to within 100 meters of the water’s surface. That peak is about twice the width of Mount St. Helens and, like that volcano, has a large, crescent-shaped portion of its flank missing — a volume of material that could have suddenly slumped during an eruption and is large enough to have caused a megatsunami that could have carried the boulders inland.

Previously, the largest known tsunami debris was a 600-metric-ton coral boulder. It was flung onshore by a 35-meter-tall wave that was triggered by the 1883 eruption and collapse of the volcano Krakatau. Hornbach says that he and his colleagues don’t yet have enough information to estimate the size of the wave that tossed the Tonga boulders ashore.

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