Giant boulder on clifftop in Tonga was carried by a 50-metre-high wave

A 1200-tonne boulder in Tonga was swept inland when a 50-metre-high wave slammed into a 30-metre-tall cliff.

“This is not just a boulder; it’s the biggest wave-lifted boulder ever found on a cliff and the third largest boulder in the world, so it really needed gigantic forces to move it that far across such a high place,” says Martin Köhler at the University of Queensland in Brisbane, Australia.

While the boulder has long been known to some locals as Maka Lahi, which means large rock, it had never before been studied by scientists.

Köhler and his colleagues were conducting fieldwork in Tonga in July 2024 looking for boulders deposited by tsunamis on cliffs. On their final day in the Pacific nation, villagers told them of a boulder they may wish to see.

“We were definitely not expecting to find such a large boulder basically during the very last minute of our fieldwork and I knew quite quickly that this was a major discovery,” says Köhler.

At 14 metres long, 12 metres wide and nearly 7 metres high, it was a “very striking” boulder, he says, made of coral reef limestone breccia. It had eluded previous searches for possible tsunami boulders in satellite images as it had vegetation growing all over the top of it and forest around it.

After seeing the boulder, the researchers were able to find a massive gash in the clifftop above the ocean, 200 metres away, from which they think the rock was torn.

Next, the team used computer models to determine how such a large boulder, so high above sea level, could be moved so far inland.

Shifting it would have required a wave with a minimum height of 50 metres and a 90-second period, meaning it would have taken a minute and a half to pass and had flow speeds of over 22 metres per second, says Köhler. It is thought that such an enormous tsunami may have been relatively localised and caused by a nearby underwater landslip.

Dating revealed an age of 6891 years, thousands of years before human settlement of the island.

“It was, for me, hard to believe that it was a 50-metre wave because we hadn’t really seen or known of such a large wave before,” says Köhler. “But if you think that this massive boulder is sitting 200 metres inland on a 39-metre-high cliff, then it’s easier to understand.”

Only two tsunami-deposited boulders that have been found on land are bigger: the Obiishi rock on Shimoji-shima, Japan, which weighs 3400 tonnes, and Maui rock, which is also on Tonga, which weighs 1500 tonnes.