Unraveling the Source of Large Erratic Boulders on Tonga: Implications for Geohazards and Mega-Tsunamis

Large erratic boulders located near shorelines are sometimes linked with paleo tsunamis and associated geohazards. Perhaps the finest modern examples of tsunami-derived erratic boulders are the coral boulders deposited by the great 1883 Krakatau eruption, which generated a >40 m high near-field tsunami that devastated the Sunda Strait. Geophysical analysis of tsunami-derived erratic boulders offers insight into the size, energy, frequency, and trigger-mechanism of past mega-tsunamis. In the wake of the 2004 Sumatra earthquake and tsunami, there has been a significant effort to find, document, and analyze large erratic boulders that may represent paleo mega-tsunami deposits in order to constrain the size, frequency, and location of these events.

With this goal in mind, a team of geophysicists at the University of Texas traveled to Tonga last November in search of rumored reports of large out-of-place erratic boulders located along the western flank of Tongatapu. During their expedition, the researchers found a 3 km chain of massive coral boulders that had been deposit 100-400 m inland. Our analysis suggests these boulders may represent the largest known tsunami deposits on Earth. Radiometric dating and structural/sedimentary interpretation indicates these boulders may have been emplaced recently (Holocene). Preliminary results from wave modeling also suggest an unconventional wave-triggering mechanism. In particular, our analysis adds credence to the concept that submarine slides and volcanic eruptions may trigger Earth's largest tsunamis.