The nature and dynamics of the Rotorua eruptive episode, Okataina Volcanic Centre, Taupo Volcanic Zone

The 15.7 cal yrs B.P. Rotorua eruptive episode is the latest expression of western Okataina volcanism. Okataina is one of the world's most active rhyolitic volcanoes and this eruptive episode provides an ideal opportunity to examine the nature, dynamics and impacts of a large magnitude explosive eruption from this volcanic centre. This re-investigation of the Rotorua eruptive episode has lead to the grouping of the deposits into two distinct phases of eruption. The Rotorua Tephra and Eastern Dome (previously thought to be Te Rere-related) are termed 'Rotorua A' phase. Trig 7693 andMiddle domes (previously linked to the source of the Rotorua Tephra) have been grouped with the Upper Rotorua Tephra into the. 'Rotorua B' phase. These groupings are predominantly based on mineralogy and geochemistry on samples of pyroclastic and dome deposits. The eruptive sequence began with a small initial ash without a basalt trigger, followed soon after by a plinian fall deposit (1.62 km³) directed to the NW. Dome growth ensued (Eastern dome), creating an extensive rhyolite dome (0.72 km³) in the Okareka Embayment. Soon after Eastern dome growth (based on the absence of paleosol development) the Rotorua B phase was initiated. This phase of predominant effusive dome growth lead to the construction of the coalesced Trig 7693 and Middle domes. Periodic vulcanian explosions and dome collapses lead to the localised dispersal to the S and E, of pyroclastic density currents and fall. A new age of 12941 ± 75 ¹⁴C years BP was obtained from charcoal in a surge deposit at a proximal site. This results in a revised age for the Rotorua eruption of 15 700 cal yrs BP. The eruption duration is estimated to exceed 15 yrs, based on a total dome volume of 1.37 km³, assuming a growth rate of 3 m ³/s. To the NW, the Rotorua A plinian deposit (Ap-1 to Ap-10 sub units) is overall normally graded, however cm-scale bedding is evident throughout the deposit. The grain size ranges from a block-sized, basal sub unit to a medium ash, upper. The plinian tephra is biotite-poor compared to the distinct Rotorua B deposits (previously 'Upper Rotorua Tephra'), which is biotite-rich. Whole rock geochemistry on the Rotorua A pyroclastics, forms an apparent trend, suggesting the magma supplying the explosive phase was compositional zoned prior to eruption and was subsequently disrupted immediately prior to or during eruption. The Rotorua A and B phases plot as distinct clusters in all binary element plots. Intensive parameters calculated on Fe-Ti oxides highlights the distinction between Rotorua A (Rotorua Tephra and Eastern Dome) and Trig 7693 and Middle domes. Isotopic evidence indicates the Trig 7 693 and Middle domes were supplied by a separate magma batch to the Rotorua A phase, suggesting small ( <1 km³) magma batches were residing in close proximity. An eruption of this size today would cause the total destruction of Rotorua City (home to >50,000 people) due to ~ 1.5 m of volcanic ejecta accumulating in this region. This would cripple the central North Island for weeks to months. Due to the length of the eruption, mitigation would be required for an extended period after the initial phase of the eruptive episode. Livestock would be severely affected from blanketing tephra, along with the complete collapse of intensive forestry operations in the Rotorua District. National and international flights from Hamilton and Auckland airports would be heavily disrupted causing major economic losses. Respiratory problems and contaminated water intake would continue for a long period after the cessation of volcanic activity.
Type of thesis
The University of Waikato
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