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Sep 24 / Dave Schumaker

Boulders in Tonga Evidence of Largest Tsunami Debris Ever Found


Source: Tongatapu Boulder, Matthew Hornbach, University of Texas Institute for Geophysics

Large boulders found upwards of 400 meters from the shoreline on the island of Tongatapu may be deposits that show evidence for the largest volcano-triggered tsunami ever found. The boulders are up to 9 meters in height and consist of coral that formed about 122,000 years ago.

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. 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. 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. A chain of sunken volcanoes lies just 30 kilometers (20 miles) 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.


Source: Tongatapu Boulder, Matthew Hornbach, University of Texas Institute for Geophysics

They will be discussing this topic at the GSA conference in Houston on October 5th.

[Via Science Daily]

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