Farmers lead Aussie research team to 'unreal' discovery on island
Australian-based researcher Martin Köhler had spent the day on the island of Tongatapu looking at smaller rocks they'd found using satellite images. The University of Queensland PhD candidate was working to understand how big the tsunamis were that thrust them from cliffs on the southern end of the island, further inland.
'On the last day of fieldwork in the late afternoon, we were talking to some farmers, and they said, 'Why aren't you looking at the much larger boulder further inl›and?',' Köhler recalled as he spoke to Yahoo News on Thursday.
'We were really interested, because a larger boulder further inland meant a much larger tsunami.'
Köhler and his team followed the farmers past their crops of bananas, cassava, sweet potatoes, yam, and taro roots. When he stood at its base, he was shocked by its size.
A photo taken later shows Köhler looking very small compared to the 1,200-tonne rock, which measured in at 14 metres long, 12 metres wide, and 6.7 metres high.
'I really couldn't believe the size and where it is — 200 metres inland is quite far for tsunami boulders. It was unreal,' he said.
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The University of Queensland team used computer modelling to determine how big the wave that carried the boulder from the edge of a cliff to the island's highest point.
They calculated that a 50-metre-high wave, that was sustained for 90 seconds, would have been needed to shift the rock. While Tonga is still regularly hit by extreme weather events, a wave this big hasn't hit the island since humans arrived.
Co-author Dr Annie Lau noted the 2022 tsunami killed six people and caused widespread damage. 'Understanding past extreme events is critical for hazard preparation and risk assessment, now and in the future,' she added.
The research has been published in the journal Marine Geology.
It's not the first sizeable tsunami rock found on Tongatapu. Another well-known boulder is considered a historical landmark on the island's northeastern coast. While it's bigger – measuring 30m long by 10m high – it wasn't moved ashore until 1917.
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