Deposition and generation of multiple widespread fall units from the c. AD 1314 Kaharoa rhyolitic eruption, Tarawera, New Zealand

Abstract

The c. AD 1314 Kaharoa eruption from Tarawera volcano, North Island, New Zealand, was a complex, long-duration, multi-episode rhyolitic event. Up to 13 discrete sub-Plinian to Plinian-style phases occurred in two stages from at least two different vents along an 8-km-long fissure. The early explosive stage produced seven pumice lapilli fall units dispersed solely to the southeast of the volcano, while a later stage produced six fall units dispersed almost entirely to the north and northwest. The duration of fallout for these 13 lapilli units is estimated to have totaled about 1.5 days. The presence of massive-textured ash layers between many of the lapilli fall units suggests that longer-duration, pyroclastic density-current producing phases and more quiescent fallout intervals occurred between the intense explosive phases of the eruption (total duration of ash fallout ~ 7.5 days). The ash deposition extended the minimum duration of the explosive episodes to at least 9 days, based on settling rates of the ash. Possibly the explosive phase of eruptive activity lasted for ~ 2 weeks, including some brief hiatuses and pyroclastic density-current deposition, and was followed by ~ 4 years of mainly lava-dome-forming activity. At least 15 km³ (> 7.3 km³ dense rock equivalent, DRE) of tephra was erupted during the main explosive episodes, and at least 9.1 km³ DRE during the whole eruption, more than doubling the previous estimate. This may be an underestimate by up to 50 % of the fallout volume. Eruption column heights for the two most widely dispersed fall units are estimated to have ranged from 25–28 km above the vents and durations of individual lapilli fall unit-forming phases ranged from 1.5 to 4 h, but the ash-producing phases lasted much longer (individually up to 30 h) and represent a longer-lived shut-down to each eruptive episode. Time-averaged mass discharge rates are estimated to have varied by more than one order of magnitude during the main Plinian-style phases, from 10⁷ to 10⁸ kg/s. However, the explosive phases were unsteady, as indicated by fluctuating grain-size with stratigraphic height in the lapilli fall units and common capping ash beds. The north-northwesterly dispersal of the later stage fall units, and the possible duration of ~ 4 days for these episodes, indicate an unusual period of southerly to southeasterly winds. Therefore, the main explosive part of the Kaharoa eruption is most likely to have occurred during the austral winter to spring months. The Kaharoa eruption style is also shared by the three previous rhyolite-dominated eruptions that have built Tarawera volcano over the past 22,000 years. Knowledge of the widespread pumice and ash fallout from the Kaharoa eruption is important in quantifying the hazard from this type of extended-duration explosive activity, and has implications for future similar rhyolitic activity in New Zealand.