By Dr Annie Lau

In June 2022, I gave a lecture at RGSQ on what big coastal boulders can tell us about hazard history. I ended the lecture sharing my excitement about visiting Tonga for an upcoming project as I will then have an opportunity to visit the world’s second largest* boulder, Maui Rock, in the Kingdom of Tonga. Little did I know that we would end up discovering** the next largest boulder in the world two years later!

Martin Köhler and “Maka Lahi”

The new research published in Marine Geology is led by my PhD student Martin Köhler. After completing our July 2024 Tonga field trip to meet collaborators and look for tsunami sands, Martin decided to check out some cliƯ-top boulders he identified from satellite images in his post-field trip free time. With his brother Daniel Köhler, a Geography major student visiting from Germany, they measured boulders (up to 300 tons) that were visible from the satellite. When they had finished work and started to pack up, the owner of the farmland asked, “Why don’t you measure the bigger rock over there (further inland)?” Curious, Martin and Daniel followed the farmer into the dense bush and saw “Maka Lahi” (meaning Big Rock). The rock measures 14 x 12 x 6.7 m, estimated to weigh almost 1200 tonnes. It was not identifiable from the satellite because it is completely covered by trees – the area just looks like a patch of taller trees. Martin carefully took a small sample from the surface of the rock, along with all the field data, back to UQ for age-dating and further analyses.

The results show that a wave of around 50 meters high and lasting around 90 seconds was needed to dislodge it from its cliƯ-edge origin and move it to its resting place. These wave characteristics are consistent with what landslide tsunamis can produce. The surface sample, a “flowstone” that was formed when rainwater dissolved the limestone of Maka Lahi and precipitated a new layer of calcium carbonate on the surface, returned an age of about 7000 years. Significantly, although Maka Lahi (14 x 12 x 6.7 m) is not as big as Maui Rock (15 x 11 x 9m), it is located at 39 m on top of the high cliƯ on the south coast of Tongatapu, while Maui Rock was on a lower platform of 9 m on the west coast of the same island. That means the waves that moved Maka Lahi reached higher on a cliƯ to dislodge this rock. Additionally, the age diƯerence between the two boulders also suggests they were moved by two separate, but both extremely powerful, wave events.

So, what does this finding mean for hazard management in Tonga? Simply put, the existence of Maka Lahi shows the high cliƯ shoreline can also be inundated by large tsunamis. If people feel significant ground movement or hear unusual sounds of rock collapse or waves, they should move inland swiftly and ideally to grounds over 40 m in elevation. The higher the better.

* The largest wave-transported boulder is “Obiishi” in Okinawa of Japan. Its estimated mass is 3400 tonnes.

** “Discovery” here only refers to discovering for science. In fact, we could not have achieved this without the local knowledge generously shared by the Teisina Family who owns this land. It makes one wonder how long ago Maka Lahi was first discovered by local people!