House-sized boulders were ripped from a reef, then washed 100 meters or more inland
All evidence hints that the boulders, which lie inland of a
three-kilometer stretch of coastline, are out of place, says Matthew J.
Hornbach, a geologist at the
Third, the corals in the boulders were likely alive about
122,000 years ago, but average sea level hasn’t reached the boulders present
location at any time since then. Finally, some of the coral masses are oriented
sideways or upside-down instead of upward, facing the sun, another testament to
an exotic origin, Hornbach reported Sunday in
Surveys of the offshore shallows near the largest boulder —
a three-story-tall, 1,200-metric-ton whopper — reveal a large gap in the reef
where the boulder may have originated. Hornbach and his colleagues also
discovered signs of a submarine landslide, but their analyses hint that the
wave generated by the slumping material couldn’t have created a tsunami large
enough to toss the coral pieces to their current positions. A large earthquake
at the nearest subduction zone, the same type of fault that triggered the
However, oceanographic surveys farther out to sea have identified a possible source of a wave capable of lofting the boulders, says Hornbach. About 35 kilometers offshore sits a submarine volcano that rises to within 100 meters of the water’s surface. That peak is about twice the width of Mount St. Helens and, like that volcano, has a large, crescent-shaped portion of its flank missing — a volume of material that could have suddenly slumped during an eruption and is large enough to have caused a megatsunami that could have carried the boulders inland.
Previously, the largest known tsunami debris was a
600-metric-ton coral boulder. It was flung onshore by a 35-meter-tall wave that
was triggered by the 1883 eruption and collapse of the volcano
Hornbach, M.J., C. Frolich, and F.W. Taylor. 2008. Unraveling the Source of Large Erratic Boluders on Tonga: Implications for Geohazards and Mega-Tsunamis. Presentation 149-8 at the annual meeting of the Geological Society of America. Oct. 5-9. Houston, TX.