Analysis of a massive boulder in Tonga has unveiled rare evidence of ancient tsunamis and their capacity to reshape Pacific coastlines. Researchers from the University of Queensland identified the Maka Lahi boulder-measuring 14 by 12 by 6.7 meters and weighing nearly 1,200 tonnes-as one of the largest rocks ever transported by ocean waves.
PhD candidate Martin Kohler said the boulder had been shifted over 200 meters inland by a tsunami approximately 7,000 years ago. "We had been surveying the southern side of the island of Tongatapu looking along the coastal cliffs at evidence of past tsunamis," he said. "It was late in the day, and we were talking to some farmers when they directed us to this boulder."
"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."
After creating a 3D model, the team traced the rock's origin to a cliff more than 30 meters above sea level. This confirms the Maka Lahi as the world's largest known cliff-top boulder.
Through numerical simulations, the team determined that waves reaching 50 meters in height and lasting around 90 seconds were needed to dislodge and move the rock.
Dr Annie Lau, co-author and coastal geomorphologist, highlighted Tonga's seismic vulnerability due to its position along the Tofua Ridge and Tonga Trench. "Tonga's most recent tsunami in 2022 killed 6 people and caused a lot of damage," she said. "Understanding past extreme events is critical for hazard preparation and risk assessment now and in the future."
The study reinforces how geological records of ancient tsunamis, like that of Maka Lahi, are essential in refining predictive models for coastal hazards across the globe.
