Discovery of the world's largest cliff-top boulder: Initial insights and numerical simulation of its transport on a 30–40 m high cliff on Tongatapu (Tonga)

Abstract

This study provides the first scientific investigation of the Maka Lahi boulder, a large limestone cliff-top boulder, measuring 14 × 12 × 6.7 m and weighing approximately 1180 t, located 200 m inland at 39 m elevation on the southern coast of Tongatapu, Tonga. The boulder is one of the largest known wave-transported boulders worldwide. Fieldwork conducted in 2024 revealed its presence, geomorphic setting, and karstification features. Utilizing numerical modelling, we established that wave heights of approximately 50 m and periods of 90 s were required to transport the boulder from its cliff-edge origin, suggesting that its emplacement likely resulted from a landslide-triggered tsunami event. U/Th dating of flowstone on the surface is indicative of a minimum age of 6891 cal yr BP and contributes critical evidence of a significant early Holocene tsunami event in Tonga. Notably, this represents the earliest known Holocene tsunami in the Pacific, offering new insights into the long-term history of extreme wave events in the region. This research strengthens the understanding of megaclast transport mechanisms and their implications for coastal hazard assessments in tsunami-prone regions.