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.

Volcanic Disaster

Tsunamis are usually caused by earthquakes, underwater landslides, or volcanoes. They can also be triggered by meteor strikes, but there was no evidence that a meteor landed near Tonga, Frohlich said.

Initially, earthquakes were the most obvious candidate. But the wave struck Tongatapu's western shore, and an earthquake-triggered tsunami would have come from the east, where the fault line lies.

(Related: Explore a map of Earth's tectonic plates.)

"It just didn't smell like an earthquake," Frohlich said.

The scientists used computer models to determine the power that nearby underwater landslides could have had, but "we couldn't get waves high enough," he said.

The researchers believe the wave was most likely triggered by one in a string of underwater volcanoes about 21 miles (35 kilometers) away from the island.

A 2007 mapping survey of the seafloor near Tonga revealed flank collapses and calderas, geological features generated by volcanic instability or extraordinarily large eruptions.

Eyes Opened to Tsunamis

Team member Allan Morton, a retired geology professor from Central Arizona College, first identified the massive boulders in a 2003 paper on evidence of tsunamis in Tonga.

"There's a general feeling in Tonga they don't have tsunamis," Morton said, noting that the most recent recorded event was in 1919.

The study could help raise awareness of the dangerous waves in Tonga, helping its people to be better prepared, he said.

(See aerial views of tsunamis)

Dale Dominey-Howes, director of the Australian Tsunami Research Center, was not involved in the research.

"What they have found is interesting and, if proved, is profound in terms of improving our understanding of potential volcano-triggered tsunami in the Southwest Pacific," he said.

The finding could also have implications for Australia's eastern seaboard. An extension of the Tonga volcano line runs north-south about 1,600 miles (2,500 kilometers) east of Australia.

"Work is underway to try and evaluate which marine-based volcanoes in the Southwest Pacific might have the capacity to generate eruption tsunami," Dominey-Howes said in an e-mail. "We'll have a better idea in a year or two."