By Sid Perkins
Normal 0 false false false MicrosoftInternetExplorer4 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