Richard Musgrove
Dr Richard Musgrove is a zoologist and science journalist. He writes for Cosmos and Double Helix.
A 1200 tonne piece of limestone was transported at least 200m inland by a tsunami as it hit Tonga’s main island, Tongatapu, 7000 years ago. The Tongan rock is one of the largest known wave-transported boulders in the world.
The landslide that probably triggered the tsunami was “most likely underwater, but could also be from a nearby volcanic island, or even from Tongatapu,” says University of Queensland PhD candidate, geologist Martin Köhler, who was guided to the boulder by local farmers.
Other islands could have been hit too. “Landslide tsunamis generally don’t travel as far as earthquake tsunamis. It could have reached Fiji, maybe, or Samoa or the other Tongan islands,” says Köhler.
Co-author Dr Annie Lau, a coastal geomorphologist, says Tonga has a long history of tsunamis triggered by volcanic eruptions and earthquakes along the underwater Tofua Ridge and the Tonga Trench.
“Tonga’s most recent tsunami in 2022 killed 6 people and caused a lot of damage,” says Lau.
But humans did not witness the event that moved Maka Lahi (“Big rock” in Tongan) — their migration out to these islands wouldn’t happen for another 4000 years. The 1200 tonne boulder stands today surrounded by tropical jungle, 200m inland at 39m above sea level.
Köhler estimates Maka Lahi, which measures 14 x 12x 6.7 metres, would have needed a 50m tsunami to get to where it is today. It could have taken no more than 90 seconds for the wave to dislodge it Maka Lahi from its cliff-edge origin and move it to its resting place, he says.
The find provides new historical context and insights into the risks and power of such tsunamis.
“We had been surveying the southern side of the island of Tongatapu looking along the coastal cliffs at evidence of past tsunamis,” Köhler told Cosmos.
“It was the last day of the fieldwork, late in the day, and we were talking to some farmers when they directed us to this boulder.
“I was so surprised; 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.
“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.”
Other, albeit smaller, limestone boulders dot the island. Several have been dated to the 15th Century, Köhler says. “But there are more boulders on the southern side of the island, which haven’t been researched until now.”
Köhler worked out Maka Lahi’s age by analysing uranium-thorium ratios in the limestone’s weathered crust. Uranium-thorium dating is commonly used on calcium carbonate-bearing materials, making use of uranium’s known rate of decay to daughter-element thorium. The ratio between the two elements, measured with a mass spectrometer, gives an approximate age.
“Understanding past extreme events is critical for hazard preparation and risk assessment now and in the future, says Lau.
“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.
“The analysis strengthens our understanding of wave transportation of rocks to improve coastal-hazard assessments in tsunami-prone regions around the world.”
“Next up is a more detailed tsunami modelling to model the volume of water, and also model where the tsunami would travel,” says Köhler.
The research is published in Marine Geology.
Tongan rock
