VOICES Voices Icon Ideas and Insight From Explorers


Massive Tsunami Debris Discovered on Pacific Island


Photo courtesy of M. Hornbach

On the Pacific island Tongatapu a line of enormous coral boulders hundreds of feet from the sea is said by local legend to have been flung ashore by the god Maui in an attempt to kill a giant man-eating fowl.

But now scientists think that the seven boulders, which are up to 30 feet high and weigh as much as 3.5 million pounds, were hurled 300 to 1,300 feet inland by a giant tsunami triggered by a powerful underwater volcano.

“The house-sized boulders were likely flung ashore by a wave rivaling the 1883 Krakatau tsunami, which is estimated to have towered 35 meters (115 feet) high,” University of Texas researchers said today.

tonga-4.jpg“These could be the largest boulders displaced by a tsunami, worldwide,” said Matthew Hornbach of the university’s Institute for Geophysics. “Krakatau’s tsunami was probably not a one-off event.”

Photo courtesy University of Texas

Hornbach and colleagues will discuss these findings on Sunday, 5 October 2008, at the Joint Annual Meeting of the Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, and the Gulf Coast Association of Geological Societies, in Houston, Texas.

Called erratic boulders, the giant coral rocks did not form at their present location on Tongatapu, main island of the country Tonga, the researchers said.

Tonga-3.jpg“Because the island is flat, the boulders could not have rolled downhill from elsewhere. The boulders are made of the same reef material found just offshore, which is quite distinct from the island’s volcanic soil.

“In fact, satellite photos show a clear break in the reef opposite one of the biggest boulders. And some of the boulders’ coral animals are oriented upside down or sideways instead of toward the sun, as they are on the reef.”

Hornbach says the Tongatapu boulders may have reached dry land within the past few thousand years. Though their corals formed roughly 122,000 years ago, they are capped by a sparse layer of soil. And the thick volcanic soils that cover most of western Tongatapu are quite thin near the boulders. This suggests the area was scoured clean by waves in the recent past.

 Photo courtesy University of Texas

Also, there is no limestone pedestal at the base of the boulders, which should have formed as rain dissolved the coral if the boulders were much older, Hornbach added.

krakatau.jpgHis analyses of adjacent seafloor topography point to a volcanic flank collapse as the most probable source of a tsunami that would have swept the boulders into their current location.  Sunken volcanoes lie 20 miles west of Tongatapu.


Photo of boulder flung ashore from the 1883 Krakatau tsunami (left) courtsey M. Hornack

An explosion or the collapse of the side of a volcano such as that seen at the famous Krakatau eruption in 1883 could trigger a tremendous tsunami, Hornbach said.

“We think studying erratic boulders is one way of getting better statistics on mega-tsunamis,” the scientist said. “There are a lot of places that have similar underwater volcanoes and people haven’t paid much attention to the threat.”

The researchers have already received reports of more erratic boulders from islands around the Pacific. Future study could indicate how frequently these monster waves occur and which areas are at risk for future tsunamis.


Photo courtesy University of Texas

More from National Geographic News:

Tsunamis: Facts About Killer Waves

Tsunami Swamped England 400 Years Ago, Study Says

Ancient Mediterranean Tsunami May Strike Again


  1. pooo
    1223 Nowhere Street
    January 28, 2013, 9:15 am