Researchers surveying the island nation of Tonga in the South Pacific have made a surprising discovery: a massive 1,200-ton out-of-place boulder that mysteriously made its way to an unlikely location thousands of years ago.
Martin Köhler, a PhD candidate with the University of Queensland’s School of the Environment, said the odd discovery was made while he and other researchers were exploring the southern side of the island of Tongatapu.
“It was late in the day, and we were talking to some farmers when they directed us to this boulder,” Köhler said of the discovery, which appears to be very out of place compared to its surroundings.
“It was quite unbelievable to see this big piece of rock sitting there covered in and surrounded by vegetation,” he said. “I was so surprised.”
Köhler says that the massive boulder’s location much further inland beyond the team’s field work area means that it must have been transported there by some tremendous force. But how could this have occurred?
How Nature Moves Massive Stones
In geology, materials that have been moved from their original location and deposited in a new area are known as erratics. Apart from being curiously out of place, geologists often easily spot these errant stones because their composition differs from the surrounding bedrock in the immediate area where they are located.
When the first erratics were identified in the 18th century, they were initially considered a major paradox. However, it was eventually determined that glacial movement in Earth’s past had been a possible mechanism for the transport of these peculiar stones, a revelation that helped reveal evidence of an ice age prior to the Holocene.
Several other mechanisms exist for transporting materials away from their native areas. These include stones that become ensnared by the roots of trees that eventually become driftwood, seaweed like kelp, which can sometimes capture and move rocks, and even certain animals that may consume small rocks and then deposit them after traveling great distances.
However, geologists are aware of another way that nature can relocate stones, even particularly large ones.
Solving the Riddle of Tonga’s Mystery Monolith
According to Köhler and his colleagues, the mystery behind the boulder they discovered on Tongatapu points to a massive tsunami, which evidence suggests must have carried the gargantuan stone to its current resting place close to 7000 years ago.
“We made a 3D model and then went back to the coast and found the spot the boulder could have come from, on a cliff over 30 metres above the sea level,” Köhler explained in a press release announcing the discovery.
Further studies determined the weight of the massive boulder, indicating that it is one of the largest known wave-transported rocks in the world, if not the largest ever discovered. The remarkable new findings offer researchers significant new insights into the history of the Pacific region and the ever-present threat that future tsunamis could represent.
In the case of the stone Köhler located, which has been dubbed the Maka Lahi boulder, the tsunami in question must have moved the stone more than 200 meters inland when it occurred, a remarkable distance for an object of this boulder’s size.
Calculating the Force of an Ancient Tsunami
Based on the stone’s size and position, Köhler and his colleagues have determined that moving the boulder to its current position would have required 50-meter-high waves, which lasted close to a minute and a half.
Such waves would have offered the force required to dislodge the stone from its point of origin near the cliff’s edge and transport it to its current resting place. The discovery aligns with Tonga’s history of tsunami activity, often resulting from earthquakes and volcanic activity around the Tofua Ridge and Tonga Trench.
The most recent of these tsunamis, which occurred in 2022, killed six individuals and caused widespread damage.
Dr Annie Lau, a coastal geomorphologist and the co-author of the new paper with Köhler, said that understanding past tsunamis and other potentially catastrophic events “is critical for hazard preparation and risk assessment now and in the future.”
“The findings we have reported on the Maka Lahi boulder are the evidence of a tsunami in the Pacific region in the Holocene epoch which began around 11,700 years ago,” Lau said in a statement, adding that the team’s analysis “strengthens our understanding of wave transportation of rocks to improve coastal-hazard assessments in tsunami-prone regions around the world.”
Köhler, Lau, and their colleagues’ 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 Marine Geology.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.