“It was late in the day,” says Martin Köhler, PhD student at the School of the Environment from the University of Queensland. “We were still talking to some farmers on the island of Tongatapu, when they suddenly referred to this rock. It clearly did not belong there. It was incredible to see such a large piece of rock lying there, covered and surrounded by vegetation. We were so surprised.”
It was surprising in particular how the rock had ended up so far inland. “It had to be transported there with an enormous force,” said Köhler. He and his team decided to further investigate the origin of the ‘stray stone’.
‘Stray stones’ is the geological name for stones that have been moved from their original location and were placed in a new area. They usually easily recognize experts, because their composition differs from that of the surrounding rock in the immediate vicinity. When the first copies were identified in the eighteenth century, they were considered a great mystery, but it was eventually established that in many cases glacier movements from the past had been responsible for the ‘relocations’.
Rock of tonga
For the enormous rock of Tonga, which was named Maka Lahi, there is another explanation. According to Köhler and his team, everything points in the direction of a huge tsunami. Evidence shows that the gigantic stone was taken to its current resting place about 7,000 years ago. “We first made a 3D model and then went back to the coast,” says Köhler. “There we found the place where the rock probably comes from: a cliff more than 30 meters above sea level.”
Thanks to further research, the weight of the rock could be set at 1,180 tons, or almost 1.2 million kilos. That means that it is one of the greatest known, waves transported in the world. Possibly even the biggest ever discovered.
A tsunami must have moved the colossus more than 200 meters inland, a remarkable distance for an object of such a size. Taking into account the size and location of the stone, Köhler and his team found that waves of 50 meters high were needed to move the Maka Lahi to his current location. The waves would have lasted almost a minute and a half.
The findings, published in trade journal Marine Geology, offer researchers new insights into the history of the region and the ubiquitous threat that future tsunamis can pose. The most recent tsunami took place in 2022, six people died and a lot of damage was caused.