Thaindian News

Washington, Sep 26 (IANS) A line of massive boulders on the western shore of an island in the Tonga group stands as a mute testimony to the most powerful volcano-triggered tsunami ever recorded. Nearly 30 feet high and weighing up to 1,600 tonnes, the seven coral boulders are located 100 to 400 metres from the coast. The huge, house-sized boulders were possibly flung ashore by a wave rivalling the 1883 Krakatau tsunami, estimated to be 12 storeys high.

“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.”

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, according to a statement by the Geological Society of America.

A chain of sunken volcanoes lies just 30 km west of Tongatapu. 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.

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.

Hornbach said 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.

Hornbach and his colleagues will present the findings Oct 5 at the joint annual meeting of the Geological Society of America, Soil Science Society of America and American Society of Agronomy, among others.