How a Monster Wave Hauled a 2.6-Million-Pound Rock Inland: The Incredible Power of Nature

Deep in southern Tongatapu, there’s a giant limestone rock called Maka Lahi, which means “Big Rock” in Tongan. This massive stone has witnessed a significant event from thousands of years ago—a prehistoric tsunami.

Martin Köhler, a PhD researcher from the University of Queensland, stumbled upon this rock while surveying the coast. Local farmers guided him to this surprising find, far from the shore and hidden by thick plant life. “I couldn’t believe my eyes; it’s so far inland and must have come from a tremendous tsunami,” he said. The boulder weighs about 1,180 tons and lies over 200 meters away from the sea.

Researchers have confirmed that Maka Lahi is likely evidence of a powerful tsunami that struck around 6,900 years ago. This stone is not just big; it’s about 14 meters long, 12 meters wide, and nearly 7 meters high—heavier than three fully-loaded Boeing 747 planes. Its chemical makeup ties it to a limestone formation 30 meters above sea level, indicating it was once part of a cliff before being swept inland.

To understand how such a large rock could end up so far from the coast, researchers used wave modeling. They ruled out regular storms and cyclones. A Category 5 cyclone would lack the force needed to move something of this size. Instead, they believe a landslide-triggered tsunami created a gigantic wave, likely over 50 meters high, which lasted about 90 seconds. This wave would have rushed at over 20 meters per second, pushing the boulder high above the current sea level.

Co-author Annie Lau, a coastal geomorphologist, highlights the importance of studying past tsunamis for present-day safety. “The last tsunami in Tonga in 2022 was deadly and damaging. Learning about past extreme events is vital for future risk assessment,” she explained. This rock reveals that extraordinary tsunamis have struck this region before, possibly waves more than double the height of the 20-meter tsunami from last year.

Experts suggest that the ancient tsunami could have been caused by a massive underwater landslide or a volcanic collapse near the Tofua arc, a tectonically active area. This is similar to the events surrounding the recent Hunga Tonga eruption, which sent tremors around the globe and created huge waves across the Pacific.

Such insights are crucial for Tonga, where oceanic events can have devastating consequences. By understanding the history of tsunamis like the one that moved Maka Lahi, researchers can better predict and prepare for future risks.

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