Geology

Both the Ōkāreka Embayment and the Tarawera volcanic complex are inside the old Ōkataina Caldera, often termed the Ōkataina volcanic centre. The caldera forming its eastern boundary has been called the Haroharo Caldera,^[4] but as no major single event formed it, this is perhaps better regarded as a general categorisation term, explaining why some maps of the Horahora Caldera include the embayment.^[5] It is now regarded as a subsidiary volcanic part of the Ōkataina Caldera related to a mechanism of collapse and subsidence at the edges of a major event caldera due to lateral movement of magma.^[6] The Ōkataina Caldera has in the last 21,000 years contributed a total magma eruptive volume greater than about 80 km³ (19 cu mi).^[7][8]

Eruptions

The Northern Dome, just to the east of Lake Tikitapu formed 25,171 ± 964 years ago^[9] in the Te Rere rhyolite eruption which also had other vents in the Ōkataina Volcanic Complex.^[10] Such domes typically form over a vent that have an initial pyroclastic eruption and the vent(s) for this eruption lies under the Northern and Eastern domes. The Te Rere tephra deposits from the initial pyroclastic eruption had a volume of 5 km³ (1.2 cu mi)^[10] and are widely distributed.^[11] The DRE volume of all the Te Rere eruptions totals about 11.5 km³ (2.8 cu mi).^[9] All the vents in the Ōkareka Embayment lie on the Haroharo linear vent zone's western end.

The Rotorua eruption now dated at 15,635 ± 412 cal.yr BP,^{[9]: 15 [12]} was a two phase eruption commencing with a plinian eruption that deposited 2.6 cubic kilometres (0.62 cu mi) of material to the north-west from a vent now under the Trig 7693 dome and that lasted no more than 4 days.^[2] Significant ash cover was towards the Rotorua area (hence the name) but ash fall was as far away as Auckland^[2] with a total ash volume of 6 km³ (1.4 cu mi) including later minor ash deposits.^[10] The largely degassed magma body then in a dominantly effusive rhyolite dome forming process built up Trig 7693 and Middle Dome to the south east of the Ōkāreka Embayment over several years, but no more than 6, to a total volume of 0.62 cubic kilometres (0.15 cu mi).^[2]