Researchers Stumble Over World's Largest Known Tsunami-Transported Rock

Forbes

22-05-2025

A 1,200 tons limestone boulder in Tonga is one of the largest known wave-transported rocks in the ... More world.
Analysis has shown a boulder weighing almost 1,200 tons is one of the largest known wave-transported rocks in the world, providing new insights into the Pacific region's history and risk of tsunamis.
During fieldwork on Tongatapu, the main island of the Tonga archipelago, searching for clues of past tsunamis, Ph.D. candidate Martin Köhler from the University of Queensland's School of the Environment was made aware by a farmer of a huge boulder — locally known as Maka Lahi meaning 'Big Rock' — hidden deep in the jungle.
A survey quickly revealed that the boulder consists of fossiliferous limestone, a type of rock commonly found along the coast of the island. So-called erratic rocks, deposited far from their original source, are common in areas once occupied by a glacier. As the glacier melts, he leaves large boulders and debris behind. However, Tonga was never covered by ice. The only remaining force strong enough to move the boulder is a large wave.
'I was so surprised; it is located far inland and outside of our field work area and must have been carried by a very big tsunami. It was quite unbelievable to see this big piece of rock sitting there covered in and surrounded by vegetation,' so Köhler.
In a new study Köhler and other researchers show that the boulder was moved more than 200 meters inland by a tsunami around 7,000 to 10,000 years ago.
Based on computer simulations, the researchers conclude that wave heights of around 50 meters lasting around 90 seconds were needed to dislodge it from its origin and move it to its resting place.
'Tonga has a long history of tsunamis triggered by volcanic eruptions,' explains coauthor Dr. Annie Lau, a coastal geomorphologist at the University of Queensland.
Three years ago the previously unremarkable submarine volcano Hunga Tonga-Hunga Haʻapa erupted with the most powerful explosion since Krakatoa in 1883. The final blast triggered a tsunami as high as 20 meters above sea level.
Most tsunamis are generated by earthquakes, when the seafloor shifts suddenly and displaces a large quantity of water, but they can also be triggered by a landslide, a volcanic eruption, or a meteor impact.
Modern databases list more than 2.000 tsunamis in the last 4.000 years. Most of these records are based on historical documents. Geological evidence for tsunamis is surprisingly hard to find.
'Understanding past extreme events is critical for hazard preparation and risk assessment now and in the future,' so Dr. Lau.
Local legends suggest that the islanders were well aware of this tsunami risk. According to one story, the boulders are there because the god Maui once tried to kill a monstrous chicken by throwing rocks from the sea on the island.
The study,"Discovery of the world's largest cliff-top boulder: Initial insights and numerical simulation of its transport on a 30–40 m high cliff on Tongatapu (Tonga)," was published in the journal Marine Geology.
Additional material and interviews provided by the University of Queensland.