House-sized boulders were ripped from a reef, then washed 100 meters or more inland
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HIGH ON CORALBIG CHUNK OF CORAL. On an expedition to a Tongan island, a scientist investigates the top of a three-story-tall, 1,200-metric-ton coral boulder, which probably was tossed 100 meters inland by an ancient tsunami. Full story.Hornbach et al 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.
Found in: Earth
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