Holocene volcanic geology, volcanic hazard, and risk on Taveuni, Fiji

Abstract

The Holocene volcanic geology of Taveuni has been mapped in order to produce a volcanic hazard and risk assessment for the island. Taveuni is the third‐largest island of the Fiji group and home to 14 500 people. At least 2.7 km3 of olivine‐alkali‐basalt magma was erupted from over 100 events throughout the Holocene. Vents are concentrated along a northeast‐striking rift zone that is parallel to other regional structural trends. There is an overall trend of younging southward along the rift. Holocene lavas and tephras are grouped within six newly defined eruptive periods, established on a basis of radiocarbon dating. Within these periods, 14 tephra layers, useful as local marker horizons, are recognised. At least 58% of Holocene eruptions produced lava flows, while almost all produced some tephra. Individual eruption event volumes ranged between 0.001 and 0.20 km3 (dense rock equivalent). Many eruptions involved at least some phases of phreatic and/or phreato‐magmatic activity, although dominant hydrovolcanic activity was limited to only a few events. A volcanic hazard map is presented, based on the Holocene geology map and statistical analyses of eruption recurrence. The highest levels of ground‐based and near‐vent hazards are concentrated along the southern portion of the island's rift axis, with the paths of initial lava flows predicted from present topography. Tephra fall hazards are based on eruption parameters interpreted from mapped Holocene tephra layers. Hawaiian explosive‐style eruptions appear to be a dominant eruptive process, with prevailing low‐level (<3 km) southeasterly winds dispersing most tephra to the northwestern quadrant. Vulnerable elements (population centres, infrastructure, and economy) on Taveuni have been considered in deriving a volcanic risk assessment for the island. A number of infrastructural and subdivision developments are either under way or planned for the island, driven by its highly fertile soils and availability of freehold land. The area identified with the present greatest volcanic risk, in south Taveuni, is also where rapid and ongoing population growth is centred. The results from this study may be used within planning strategies to minimise vulnerability of future developments on the island as well as contributing to national and community‐level emergency or contingency plans.