Ancient Tsunami Carried Giant Boulders to Tonga

Rebecca Carroll
for National Geographic News
September 30, 2008

Huge coral boulders on Tonga's main island of Tongatapu were possibly tossed hundreds of feet inland by one of the largest tsunamis ever triggered by a volcano.

Researchers believe the ancient wave may have hit the island's shore sometime within the past 7,000 years, after the melting of the most recent ice age brought sea levels to roughly where they are today.

The largest of the seven boulders is 50 feet (15 meters) wide and estimated to weigh 1,600 tons. It currently sits more than 300 feet (100 meters) from the sea and 30 feet (10 meters) above sea level, an anomaly on the South Pacific island's flat landscape.

"We suspect that this may be the largest [object] moved uphill by a tsunami," said Cliff Frohlich, a senior scientist at the Institute for Geophysics at the University of Texas at Austin.

"It wasn't like we found these rocks everywhere in Tonga," Frohlich noted. "We found them just in one place on one island."

By comparison, the 130-foot (40-meter) waves triggered by the 1883 eruption of the Indonesian volcano Krakatau—the most powerful explosion in recorded history—are known to have moved a boulder only about a third that size the same distance, Frohlich said.

It's possible there simply were no larger boulders for that tsunami to displace, he added. But he and his colleagues believe the Tonga tsunami may have been as big or larger than Krakatau's.

The team will be presenting some of its findings at a geology conference in Houston on October 5. Their study is currently under peer review.

Striking Distance

The sea was likely close to present-day levels when it carried the boulders, which Frohlich believes were formed in the reefs surrounding the island.

The coral boulders were alive and growing about 122,000 to 130,000 years ago. It's possible that this is when the wave occurred, but the researchers favor a more recent date.

They estimate it struck within the last 7,000 years, because the surrounding area lacks the erosion and other signs of weathering one would expect after more than 100,000 years.

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