KUWAE (APPROXIMATE-TO-1425 AD) - THE FORGOTTEN CALDERA

In Vanuatu, Tongoa and Epi islands once formed part of a larger landmass, Kuwae, which was partly destroyed during a cataclysmic seismo-volcanic event that is recorded in local folklore. It led to the formation of a 12-km-long and 6-km-wide oval-shaped submarine caldera with two distinct basins and a total area of approximately 60 km2 at the level of the rim. The age of this eruption, 1420-1430 A.D., and the structure of the related collapse are discussed and a composite log (143 m) of the pyroclastics surrounding the caldera is presented. They comprise thick hydromagmatic deposits belonging-to a terminal hydromagmatic phase of the pre-caldera edifice, which grade upwards into two major sequences of pyroclastic flow deposits, clearly related to the caldera event. Collapse near the caldera edge was at least in the range 650 to 950 m, and may have been as much as 800 to 1100 m. The volume of rocks engulfed during the caldera formation is approximately 32-39 km3, suggesting the same volume of magma was erupted. Even if two coalescent collapse structures were formed, it is worth noting that the Kuwae caldera is not a reactivated structure, but the result of a single event of short duration which occurred in the first half of the Fifteenth century. This event is one of the seven biggest caldera-forming events during the last 10,000 years, and is comparable with the Santorini Minoan eruption and the Crater Lake eruption.