Tsunami Erratics

Why do the islands of Tonga have huge boulders of coral sitting there on land? The first thing you might think is that they must have arisen when sea level was higher, like examples in the Caribbean. But Matthew Hornbach (Univ. Texas Inst. Geophysics) looked more closely at them and determined that they must have washed up there.

For one thing, the corals in these boulders are tipped, even overturned. Nor did the boulders have pedestals of limestone underneath them. This example, on Tongatapu island, rests on volcanic soil. Something carried them ashore; in geologic terms, they are erratics. No ordinary wave could have lifted this house-sized stone. But the area has unusually thin soil, as if it had been washed away recently, and the reef offshore appears to have chunks ripped out of it. And just 30 kilometers away is a chain of undersea volcanoes.

Hornbach noted that the great Krakatoa eruption of 1883 threw large coral builders ashore, like this example. The simplest explanation for the Tonga boulders, then, is that eruptions pushed large tsunamis toward Tonga which left these blocks on land.

Tsunamis don't arise only from earthquakes; landslides, eruptions and cosmic impacts create them too. Studying tsunami erratics, here and elsewhere, could yield important data on tsunami occurrence and hazards from this underappreciated source. This research is being presented next month in Houston at the Geological Society of America annual meeting, just one of many fascinating presentations.

Tongatapu and Krakatoa erratics — Photos courtesy Matthew Hornbach