A giant boulder in Tonga rode a wave from an ancient tsunami 7,000 years ago. The 1,300-ton rock traveled a distance twice the size of a football field, swept away by the sheer force of the wave, according to a new study.
But this was no ordinary giant rock. It was also sitting on a 100-foot (30-meter) tall cliff before surfing to its new location—hinting at a past mega-tsunami that swept the region.
In July of 2024, Martin Kohler, a PhD candidate from the University of Queensland, and his team were doing fieldwork on Tonga’s cliffs, searching for evidence of past tsunamis. In particular, they were looking for large boulders that could have been carried inland, which can tell researchers a lot about the history of tsunamis in earthquake-prone regions, like Tonga.
On one of their last field days, some local farmers told the researchers about a boulder they might be interested in. The rock, familiar to locals as Maka Lahi, or literally “large rock,” had never been studied by scientists. It had likely eluded previous satellite searches for tsunami-swept rocks because of its thick coat of vegetation.
The boulder was made of coral reef limestone breccia, hinting that it had come from somewhere near the coast. “I was so surprised; it is located far inland outside of our field work area and must have been carried by a very big tsunami,” Kohler said in a statement. The researchers’ findings were published in the journal Marine Geology.
The researchers later found a huge gash in a 100-foot (30-meter) tall cliff near the ocean, around 650 feet (200 meters) away from the boulder’s current position. They also determined the rock was deposited 6,891 years ago, before humans settled on Tonga.
The researchers measured the boulder at 45 by 40 by 20 feet (14 by 12 by 7 meters) high, roughly the size of a two-story house. It now has the honor of being the third largest tsunami-swept rock in the world. And it’s the largest boulder known to have surfed a tsunami from a cliff.
The team then used computer modeling to figure out roughly how tall the ancient tsunami that carried Maka Lahi must have been. They concluded that the tsunami was, at minimum, 164 feet (50 meters) tall and would have taken about a minute and a half to pass. The tsunami likely reached speeds of over 70 miles per hour (113 kilometers per hour). It was huge and very powerful, and the researchers think a landslide, rather than an earthquake, initially triggered the wave.
Tonga sees a lot of geologic activity that can trigger tsunamis, like the devastating tsunami in 2022. Understanding more about the mechanics of past tsunamis can help researchers better prepare for future events, co-author Annie Lau, a coastal geomorphologist at the University of Queensland, said in a statement.
“The analysis strengthens our understanding of wave transportation of rocks to improve coastal-hazard assessments in tsunami-prone regions around the world,” Lau said.
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