Just a stone’s throw from the ocean, indeed.
Small family farms dot the southern coast of Tongatapu, the largest island of Tonga in the South Pacific. But something lies amid the cassava and banana plants that doesn’t belong: a staggeringly large, off-white boulder.
The rock, which features prominently in Tongan mythology, was recently scrutinized by scientists. New results suggest that the object was transported inland thousands of years ago when tsunami waves breached a 120-foot cliff. That event might have been set in motion by an earthquake in the nearby Tonga-Kermadec Trench, the team reported last month in the journal Marine Geology.
Seemingly out-of-place boulders are found in coastal regions around the world. From Japan to the Bahamas, scientists have spotted hulking rocks that simply don’t seem to fit their surroundings. Researchers generally believe that such boulders were transported by moving water, and the powerful waves of tsunamis are often invoked as culprits.
Last July, researchers traveled to Tongatapu to analyze several coastal boulders. On the last day of their planned fieldwork, the team received an unexpected tip from a group of Tongan farmers: A boulder, far larger than the ones the researchers were studying, lay nearby on farmland belonging to the Teisina family.
That boulder turned out to be a behemoth — roughly the size of a two-story house. “It was unreal,” said Martin Köhler, a geoscientist at the University of Queensland in Australia, who led the research. The rock, made of limestone, was also nearly completely camouflaged by vegetation.
“It looked like a hill,” said Mafoa Penisoni, a seismologist at Tonga Geological Services in Nuku’alofa, the country’s capital, who accompanied the researchers.
Mr. Köhler and his collaborators surveyed the rock, which is known locally as Maka Lahi or “Big Rock.” The team calculated that it weighed more than 1,300 tons. It’s surprising to find a tsunami-transported boulder that large, said Ricardo Ramalho, a geologist at Cardiff University in Wales, who was not involved in the study. “Boulders this size are rare,” he said.
The researchers also sampled formations growing on the sides of the boulder. Known as flowstone, they are akin to stalactites and stalagmites, and they build up slowly over time as limestone is dissolved by water.
Maka Lahi is over 650 feet inland and sits atop a 12-story limestone cliff. As there’s nothing taller around, the rock must have been dislodged from the edge of the cliff by one or more enormous waves, the team surmised.
Using computer simulations, the researchers modeled waves of different heights that rolled in anywhere from every 10 to 600 seconds. The team found that waves of at least 160 feet high were required to dislodge the boulder from the cliff edge and send it rolling or sliding to its final resting place.
That’s far larger than the waves recorded for either the Indian Ocean tsunami in 2004 or the Tōhoku tsunami in 2011. Those tsunamis were caused by an earthquake, however, and the wave heights of such events are generally limited to a few tens of feet. So a tsunami caused by an earthquake probably wasn’t the culprit in this case, said Annie Lau, a coastal geomorphologist at the University of Queensland and Mr. Köhler’s thesis adviser. “We’re thinking it’s likely a landslide tsunami.”
When a large amount of material suddenly slides into the water — or something underwater rapidly shifts position — the result is a landslide-triggered tsunami. That’s what happened in 2022, when the Hunga underwater volcano near Tonga erupted, sending waves roughly 50 feet tall into the northern coast of Tongatapu.
Mr. Köhler and his team estimated that the massive waves that transported Maka Lahi struck about 7,000 years ago. That’s based on the age of the flowstone growing on the lower half of the boulder. Because Tonga was settled only around 3,000 years ago, no humans were around to witness this event. “It’s older than the human occupation of Tonga,” Mr. Köhler said.
It’s impossible to know for sure what might have triggered massive waves that long ago, but there’s geological evidence that large waves also inundated the North Island of New Zealand around 7,000 years ago. That event is believed to have been caused by an earthquake in the roughly 1,000-mile-long Tonga-Kermadec Trench that stretches from New Zealand to Tonga. Maybe that ground movement spurred a landslide near Tongatapu that launched the waves that ultimately transported Maka Lahi.
Testing the idea will mean searching for evidence of a landslide. “We need to look for the landslide scar,” Dr. Lau said.
Such follow-up work is important, Dr. Ramalho said, for helping to reveal the triggering mechanisms and frequency of these kinds of hazards. And that’s critical because such events are bound to happen again, he said. “One day perhaps we’ll be faced with an event like this.”
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