Discovering the Mystery: How a Giant Boulder Came to Rest on a Cliff in Tonga

A massive boulder sits hundreds of feet from a cliff edge in Tonga, likely moved there by an ancient tsunami. Found in 2024 on Tongatapu, this giant rock is 656 feet inland and 128 feet above sea level. It measures about 46 feet long, 39 feet wide, and 22 feet tall, weighing over 1,300 tons—making it the largest cliff-top boulder in the world.

Initially spotted by locals, researchers were surprised by its location. “It’s astonishing to find such a large rock so far inland,” said Martin Köhler, a researcher at the University of Queensland. He learned about the boulder while surveying the area for signs of past tsunamis.

Scientists believe this boulder, named Maka Lahi (Tongan for “big rock”), may have been deposited by a massive tsunami approximately 7,000 years ago. Using advanced modeling, they estimated that the wave that carried the boulder was around 164 feet high and lasted about 90 seconds, comparable to Niagara Falls.

Köhler and his team created a 3D model of the boulder’s original position on the cliff. Their dating techniques suggest it was moved there around 6,891 years ago, coinciding with evidence of a significant tsunami that impacted New Zealand’s North Island around the same time.

Interestingly, the tsunami’s arrival might have coincided with an earthquake, creating a powerful “coseismic” event. “The earthquake could have not only caused a tsunami but also triggered a landslide that produced additional waves,” the researchers noted.

The Tonga islands lie within the South Pacific Ocean, an area prone to tsunamis due to tectonic activity. It’s part of the “Ring of Fire,” where several tectonic plates converge. Powerful underwater earthquakes can lead to these destructive tsunamis, especially near the Tonga Trench, where the Pacific Plate is subducting under the Indo-Australian Plate.

Recent events underline this danger. In 2022, a tsunami struck Tongatapu, triggered by the eruption of the Hunga Tonga–Hunga Ha’apai volcano, reaching 62.3 feet high and sweeping over a mile inland. The disaster resulted in six fatalities and significant damage, a reminder of how vulnerable the region is.

“Understanding past tsunamis is essential for future readiness,” noted Annie Lau, a coastal geomorphologist at the University of Queensland. This discovery of the boulder’s journey can help Tonga and nearby Pacific nations improve their tsunami preparedness and risk assessment. “It’s vital that we learn how massive waves transport rocks, as it strengthens our ability to assess coastal hazards where tsunamis are a constant threat,” Lau added.

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