Hidden within the greenery of southern Tongatapu sits a boulder so giant that locals barely observed it was misplaced. Geologists now say this limestone large, generally known as Maka Lahi – Tongan for “Massive Rock” – is the heaviest cliff-top boulder ever recorded.
The rock’s huge presence stands as silent proof {that a} highly effective and devastating tsunami slammed into Tonga roughly 7,000 years in the past.
Big rock discovered inland
College of Queensland PhD researcher Martin Köhler discovered the discover virtually by likelihood. “We had been surveying the southern aspect of the island of Tongatapu, trying alongside the coastal cliffs for proof of previous tsunamis,” Köhler mentioned.
Late one afternoon farmers pointed him inland towards an overgrown rise greater than 200 meters from the ocean.
“I used to be so stunned; it’s situated far inland and outdoors of our fieldwork space and should have been carried by a really large tsunami. It was fairly unbelievable to see this large piece of rock sitting there lined in and surrounded by vegetation,” he added.
Köhler’s workforce used drone imagery and laser scans to construct a 3D mannequin. The block measures roughly 14 meters lengthy, 12 meters broad, and almost 7 meters excessive.
With these dimensions, researchers estimate it weighs round 1,180 tons – making it heavier than three Boeing 747 jets.
Chemical fingerprints match the coral reef limestone that kinds the close by shoreline, pinpointing its authentic perch on a cliff over 30 meters above the ocean degree.
Monster tsunami vs. huge boulder
How do you budge such a monster? Numerical wave-flume experiments present that an peculiar storm – even a Class 5 cyclone – lacks the muscle.
The one situation that works is a landslide-generated tsunami producing a single wall of water a minimum of 50 meters excessive and lasting 90 seconds.
That torrent would have smashed the boulder off its ledge, hurled it inland at velocities topping 20 meters per second, and deposited it 39 meters above at present’s sea degree – precisely the place it rests.
How previous is the tsunami boulder?
Uranium-thorium courting of flowstone on the boulder’s shaded face locations the occasion at about 6,900 years in the past, early within the Holocene epoch.
Co-author Annie Lau, a coastal geomorphologist on the College of Queensland, hyperlinks the invention to Tonga’s lengthy geological report.
“Tonga’s most up-to-date tsunami in 2022 killed six folks and precipitated plenty of harm. Understanding previous excessive occasions is vital for hazard preparation and danger evaluation now and sooner or later,” she mentioned.
“The findings on the Maka Lahi boulder are the proof of a tsunami within the Pacific area within the Holocene epoch which started round 11,700 years in the past.”
Tsunamis depart clues
The evaluation enhances our understanding of how waves transfer rocks, serving to enhance coastal hazard assessments in tsunami-prone areas worldwide.
Radiocarbon proof suggests the wave might have been triggered by a large submarine landslide or volcanic flank collapse alongside the close by Tofua arc – processes nonetheless lively at present.
In 2022, Hunga Tonga’s eruption despatched shockwaves worldwide and triggered almost 20-meter-high tsunami waves.Maka Lahi demonstrates that the area can generate tsunamis greater than twice that measurement.
Folklore meets physics
Native legends describe the demigod Māui hurling rocks throughout the ocean whereas chasing birds. These tales now seem to protect cultural reminiscence of occasions that science is barely starting to quantify.
One other megaboulder, Maui Rock, sits on Tongatapu’s western shore and was seemingly thrown inland by a Fifteenth-century tsunami.
Collectively, the 2 stones bracket Tonga’s historical past of uncommon however catastrophic waves.
Future tsunami menace
Most Tongatapu residents stay on the coastal plain. Current evacuation maps assume worst-case run-ups of about 20 meters. A 50-meter wave would overtop these protected zones and inundate the capital, Nuku‘alofa.
Updating hazard fashions would require incorporating low-probability, high-impact occasions just like the one which moved Maka Lahi.
Early-warning techniques can detect earthquake tsunamis minutes after they begin, however landslide-triggered waves depart little time for alerting the general public.
That makes long-term training and clearly marked evacuation routes essential. Figuring out the utmost historic run-up helps engineers resolve how excessive to construct protecting berms and the place to find vital infrastructure.
What the tsunami boulder tells us
Sea-level rise will increase the attain of any future tsunami. Maka Lahi moved when sea ranges matched at present’s, that means future tsunamis may attain even farther as seas rise.
As local weather change intensifies coastal erosion and triggers extra landslides, the frequency of maximum waves may rise.
The examine holds broader significance past Tonga, since huge cliff-top boulders additionally seem alongside coastlines from the Mediterranean to the Caribbean.
Few, nevertheless, have undergone the exact courting and stream modeling used on Maka Lahi. Its exceptional measurement, elevation, and inland place make it a vital reference level for enhancing world tsunami modeling.
Massive rock with a much bigger message
Standing within the brush beside the limestone behemoth, Köhler mirrored on the chain of occasions that unveiled it: “We made a 3-D mannequin after which went again to the coast and located the spot the boulder may have come from, on a cliff over 30 meters above the ocean degree.”
What appeared like a farmer’s curiosity turned out to be a deep-time archive of ocean energy.
The rock provides a stark reminder that even essentially the most tranquil coastlines have confronted tsunami waves highly effective sufficient to reshape the bottom beneath our ft.
Understanding that historical past will not be merely an instructional train; it’s a prerequisite for surviving the following colossal surge.
The examine is printed within the journal Marine Biology.
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