Events During the Complex Plinian Phases of the c. AD 1305 Eruption of Tarawera, New Zealand

Plinian phases of the c.1305 AD Kaharoa eruption deposited a minimum of 5.1 km3 of tephra (2.3 km3 DRE) in two cycles involving 2 possibly 3 vents out of a total of 7 vents associated with the eruption. Each cycle is characterized by a general increase of column height and mass discharge rate in successive phases to a maximum, followed by a rapid decrease in both parameters. The 1st cycle produced six coarse pumiceous fall units (A through G, excluding C which is a co-pyroclastic flow density current), all dispersed to the southeast, under stable northwesterly winds. Thin (up to 6 cm) widely dispersed distinctive fine ash beds of uncertain origin are present between most of the coarse pumice beds. The initial eruptions of the 2nd cycle occurred during a sheared wind field thay may indicate an onset of a deviation from the normal weather pattern, producing the bi-lobate unit H. Four subsequent fall units (I - L) were deposited under variable wind fields predominantly from southerly directions predominantl. First-order comparisons with other plinian deposits suggests that the Kaharoa eruptions were of low to moderate intensities, with calculated column heights ranging from 16 to 26 km and wind speeds from 20 to > 30 m/s. Mass discharge rate varied by an order of magnitude from 8.8 x 106 to 9.8 x 107 kg/s and volumetric discharge ratefrom 4.0 x 103 to 4.4 x 104 m3/s. The minimum inferred durations for individual plinian events ranged from 2 to 5 hours with the exception of the widely spread unit K (~25 hours), interpreted from its poorly sorted, mixed nature and anomalous dispersal. to have been deposited from a series of low intensity eruptions with slightly varying wind directions. Total minimum duration of the coarse plinian fall is 64 hours. The timing of the plinian phase, and thus the onset of the eruption, can be constrained by the change in the predominant northwesterly wind pattern to the less frequent southerly winds which occur mostly in austral winter season.