Washington, September 26 : A line of massive boulders on the western shore of Tonga may be evidence of the world's largest tsunami debris, which is up to 9 meters (30 feet) high and weighing up to 1.6 million kilograms (3.5 million pounds).
The seven house-sized coral boulders were likely flung ashore by a wave rivaling the 1883 Krakatau tsunami, which is estimated to have towered 35 meters (115 feet) high.
Currently, they are located 100 to 400 meters (300 to 1,300 feet) from the coast.
"These could be the largest boulders displaced by a tsunami, worldwide," said Matthew Hornbach of the University of Texas Institute for Geophysics. "Krakatau's tsunami was probably not a one-off event," he added.
Called erratic boulders, these giant coral rocks did not form at their present location on Tongatapu, Tonga's main island.
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.
Some of the boulders' coral animals are oriented upside down or sideways instead of toward the sun, as they are on the reef.
According to Hornbach, 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.
Finally, 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.
Many tsunamis, like the one that struck the Indian Ocean in 2004, are caused by earthquakes. But the boulders' location makes an underwater eruption or submarine slide a more likely culprit.
Two more possibilities are that a storm surge could have brought the boulders ashore, or a monster undersea landslide caused the tsunami.
But Hornbach's analyses of adjacent seafloor topography point to a volcanic flank collapse as the most probable source of such a wave.
Future study could indicate how frequently these monster waves occur and which areas are at risk for future tsunamis.