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Research Shows Tonga's Maka Lahi Boulder's Journey
New research theorises that Tonga's Maka Lahi boulder was moved more than 200 metres inland by a tsunami around 7,000 years ago.
Maka Lahi, meaning "Big Rock", is a limestone boulder measuring 6.7 metres tall and 14 metres wide, and weighing approximately 1180 tonnes. It is the second-largest boulder in Tonga, after Maui Rock, and sits at an elevation of 39 metres.
The University of Queensland's School of the Environment PhD candidate Martin Köhler is one of the authors of the study published in Marine Geology.
He said that he and his team were directed to the boulder local farmers.
"We had been surveying the southern side of the island of Tongatapu, looking along the coastal cliffs at evidence of past tsunamis," Köhler said.
"[The boulder] is located far inland, outside of our field work area, and must have been carried by a very big tsunami.
"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."
The researchers used numerical modelling to establish that wave heights of around 50 metres, and lasting around 90 seconds, would have been needed to dislodge it from its cliff-edge origin and move it to its resting place.
This suggests its emplacement "likely resulted from a landslide-triggered tsunami event", researchers said.
Several wave-transported boulders (weighing 0.6-1555 t) had previously been reported on Tongatapu.
"The exceptional characteristics of the Maka Lahi boulder, including its dimensions, estimated mass of approximately 1180tn and its high elevation, rank it among the three largest coastal boulders globally and the largest cliff-top boulder in the world."
Co-author Dr Annie Lau said Tonga had a long history of tsunamis triggered by volcanic eruptions and earthquakes along the underwater Tofua Ridge and the Tonga Trench.
"The findings we have reported on the Maka Lahi boulder are the evidence of a tsunami in the Pacific region in the Holocene epoch, which began around 11,700 years ago," she said.
"Understanding past extreme events is critical for hazard preparation and risk assessment now and in the future."
The study said like other coastal boulders on Tongatapu such as Maui Rock on the west coast and the Haveluliku boulders on the east coast, Maka Lahi is also interpreted within a mythological framework as one of the "Maui throwing stones" believed to be deposited by the demi-god Maui, while chasing chicken(s) from 'Eua to Tongatapu, as described in a Tongan legend.
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