There's a humongous boulder on a cliff in Tonga. Now we know how it got there.
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A massive boulder perched hundreds of feet from the edge of a cliff in Tonga appears to have been transported by an ancient tsunami, making it one of the biggest rocks moved by a wave on Earth.
The boulder, which was discovered in 2024 on the southern coast of the Tongan island of Tongatapu, sits 656 feet (200 meters) inland from the cliff edge, at an elevation of 128 feet (39 m) above sea level. And it is enormous, measuring 45.9 x 39.3 x 22 feet (14 x 12 x 6.7 meters) and weighing over 1,300 tons (1,180 metric tons) .
It's the world's largest cliff-top boulder and was first identified by locals. "We had been surveying the southern side of the island of Tongatapu looking along the coastal cliffs at evidence of past tsunamis," lead author Martin Köhler, a researcher at the University of Queensland in Australia, said in a statement. "We were talking to some farmers when they directed us to this boulder."
But exactly how the big rock ended-up on a cliff was unclear. "I was so surprised," Köhler said. "It is located far inland outside of our field work area and must have been carried by a very big tsunami. It was quite unbelievable to see this big piece of rock sitting there covered in and surrounded by vegetation."
According to a new study published online on 21 April in the journal Marine Geology, the boulder — named Maka Lahi, which is Tongan for "big rock" — may have been deposited in its unlikely home by a huge tsunami that struck the island around 7,000 years ago.
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The researchers measured the boulder's properties then modeled how large a wave would have needed to be in order to deposit such a large rock so far inland. They suggested that the boulder originally sat at the cliff's edge, but was washed inland by a tsunami wave that lasted around 90 seconds and was up to 164 feet (50 m) tall – almost the height of Niagara Falls.
"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 said.
Based on dating methods involving isotopes present in the rock, Köhler and colleagues believe that the boulder was likely washed to its current location a minimum of 6,891 years ago, plus or minus 97 years. This date aligns with evidence of a huge tsunami that hit on New Zealand's North Island — around 1,300 miles (2,000 kilometers) south west of Tonga — between 7,240 and 6,940 years ago.
The Maka Lahi boulder may have moved because the wave's arrival coincided with an earthquake – a "coseismic" event. "It is possible that the earthquake not only generated a tsunami that inundated the North Island of New Zealand but also triggered a coseismic landslide, which in turn produced a separate tsunami that deposited Maka Lahi," the researchers wrote in the paper.
The islands of Tonga are located in the South Pacific Ocean, a region that is extremely prone to tsunamis due to being surrounded by tectonic plate boundaries known as the "Ring of Fire."
Subduction zones — where one plate is forced under another — or large underwater volcanic eruptions can generate powerful undersea earthquakes that can trigger tsunamis. The Tongan islands are located near the Tonga Trench, where the Pacific Plate is being subducted beneath the Indo-Australian Plate, making it especially vulnerable to tsunamis.
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In 2022, Tongatapu was hit by a 62.3 feet (19 m) tsunami triggered by the eruption of the Hunga Tonga–Hunga volcano, with water reaching as far as 0.62 miles (1 km) inland.
"Tonga's most recent tsunami in 2022 killed 6 people and caused a lot of damage," Annie Lau, a coastal geomorphologist at the University of Queensland, said in the statement.
The researchers hope that this discovery of how far such a large boulder was moved by a wave may help Tonga and surrounding South Pacific nations prepare for large tsunamis.
"Understanding past extreme events is critical for hazard preparation and risk assessment now and in the future," Lau said. "The analysis strengthens our understanding of wave transportation of rocks to improve coastal-hazard assessments in tsunami-prone regions around the world."
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In the Himalayas, elite climbers (and paying clients) on expeditions aiming for the highest peaks enter what is known as the Death Zone, an extremely dangerous zone above 26,247ft (8,000m), where besides having to deal with temperatures potentially tumbling to -31°F (-35°C) and highly technical terrain, the oxygen in the air is so low it's impossible for humans to survive for long. "You have to go to the real mountain for at least a few days before you travel," advises Millet, an expert in acclimatization technology. He advises climbers to use the tents in combination with traditional acclimatization methods, like sleeping atop smaller mountains, before tackling any major peaks. "It's better to use the tents, and then before you go to the Himalayas, it's recommended that you go for at least a few days to the Alps," he says. He says your body acclimatizes differently at real altitude and, despite their many benefits, hypoxic tents cannot fully replicate the tough conditions and and lack of oxygen in the mountains. It's not just mountaineers who put hypoxic tents to good use. All manner of sports people, from swimmers to footballers, use simulated altitude environments to expand their physical capabilities and access the benefits of altitude training. 'If you can transport and utilize oxygen better, that leads to better performance benefits, and literally just more ability to consume oxygen,' says Oestrike. "Most of the elite athletes, they have a hypoxic chamber at home," adds Millet. "It's not a tent, but it's a real chamber. It's the same idea, you decrease the oxygen concentration in the room." By training and sleeping at altitude, athletes can increase what's known as their hemoglobin mass, which boosts the amount of oxygen they can utilize during exercise. As Millet explains, this requires considerably more time than mountain acclimatization: "Every 100 hours sleeping in the tent, you will increase your hemoglobin by one percent." Every member of the US Olympic rowing quad who won gold at the Paris 2024 games reportedly slept in hypoxic tents for six weeks before racing. Although they're designed for elite athletes, hypoxic tents can be useful to outdoor enthusiasts of all abilities, helping to build fitness and intensify workouts. When it comes to climbing, Oestrike suggests that hypoxic tents are even more useful to amateurs than professionals. 'Most people who are committed to an 8,000m (26,247ft) peak know what they're getting into, they know what the training looks like, and they know what it takes to prepare. 'High-level athletes are already training super intensively. So the person that's more modest and maybe needs to lose a little weight, they're going to see a greater upside.' 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