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.
“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.”
“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.
His 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
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