Ōkataina Caldera

The caldera contains the first major event Matahina Caldera, the large Haroharo Caldera, and the Rotomā Caldera. The Ōkareka Embayment is another, now in-filled caldera.

The caldera has seen six eruptions in the past 10,000 years, most recently the 1886 Mount Tarawera eruption in the caldera's southeastern corner. The caldera contains two major lava dome complexes, Haroharo in the north and Mount Tarawera in the south. Other volcanoes connected with the caldera include Putauaki (Mount Edgecumbe) [3] and the maar crater of Lake Rotokawau which is most likely to have formed from a basaltic dike extrusion associated with the common magma mush body.[4]

While most New Zealand volcanoes produce small eruptions relatively frequently, Ōkataina's volcanoes tend to erupt very violently after intervals of centuries. As such, they pose significant potential threats to the Bay of Plenty Region but are also the most significant volcanic risk in New Zealand.[3] During the last 20,000 years, pyroclastic and lava eruptions have occurred of several types; low-silicate basalt eruptions, high-silicate rhyolite eruptions, and the rarer intermediate andesite and dacite eruptions. The most common magma type at Ōkataina is rhyolite.[3]

The reason for the various types relate to the underlying arc volcanism which is driven initially by large inputs of basaltic melt (from in this case the subducted Pacific Plate) which often never reach the surface due to a relatively high magma density compared to the surrounding Australian Plate crust. Usually, these intrusions cool in the crust and either solidify to gabbroic plutons or generate more evolved magmas with higher silicate content that separate and ascend to then erupt as rhyolite, dacite, or andesite but can also cool without erupting to form felsic plutons. Any basaltic magmas that do reach the surface will have traversed this complicated crustal region and often erupt as a dyke. The explosive nature of any secondary rhyolite eruptions after this basaltic melt priming is related to rhyolite's viscosity further complicated by its accumulation time as it is less able to find its way to the surface compared to say the more fluid andesite.[5] The sub-surface architecture is known to be made up of discrete rhyolitic melt-mush pockets that erupt compositionally distinct magmas within single eruptions. Little is known of the evolution of the primary basaltic magmas that generate these more evolved rhyolitic magmas. Heat and volatiles are assumed to be transferred between basalts and rhyolites. Basaltic-rhyolitic magma interaction definitely happens and will be a factor in the many different eruption styles that have occurred. Sometimes basalt appears to lead the eruption, at other times it has been postulated that tectonic earthquakes are the final enabler of an eruption.[5][6]

It is likely that the volcanic history of the area began some 625,000 years ago.[7] The caldera was formed by at least five huge eruptions between 400,000 and 50,000 years ago, causing the collapse of the ground. The most significant collapse event with an eruptive volume of 150 cubic kilometres (36 cu mi) was 280,000 years ago[8] and associated with eruption of the Matahina Ignimbrite which covers over 2,000 km² (770 sq mi).[1] The shape of the Matahina caldera was then modified (and buried/destroyed) by eight smaller eruptions which occurred between 50,000 and 24,000 years ago. The paired 61,000 ± 1500 years ago[9] Rotoiti eruption and Earthquake Flat eruption (previously timed 47,400 or 65,000 years ago) at far ends of the caldera had eruptive volumes of 120 cubic kilometres (29 cu mi) and 10 cubic kilometres (2.4 cu mi) respectively.[1] Between this eruption and 21,000 years ago over 81 km³ (19 cu mi) of Mangaone silicic plinian tephras or pyroclastic flow deposits occurred but eruptive centres can not be assigned. However one of these events can be assigned to the Kawerau Ignimbrite eruption of 33,000 years ago as a location within the central part of the Matahina Caldera at level of the Puhipuhi Basin. [1] Volcanoes within the caldera are known to have erupted eleven times in the last 21,000 years, with all but two of those eruptions being rhyolite.[10][3] Of these eruptions, two, both at Tarawera, occurred within the last 2000 years (in 1886 and c. 1314AD). The most explosive of the eruptions in the last 21,000 years is likely to have been that of Haroharo in about 5500 BCE, which ejected some 17 cubic kilometres of magma.[3] During the same period Ōkataina volcanos have contributed a total magma eruptive volume of about 80 km³ (19 cu mi) in all its eruptions.[10][11] In summary the more significant eruptions have been:[8][7][1]

Faults are not defined under this very active caldera but the existence of at least one paired eruption at the far north and south extremes of the caldera 61,000 years ago at Earthquake Flat and at Rotoiti suggest potential volcanicotectonic interaction. The active Paeroa Fault terminates at the caldera edge and the active Ngapouri-Rotomahana Fault is just to the south. The two recently active main vent alignments in the Ōkataina Caldera being the Horahora and Tarawera vents are parallel with these identifiable faults outside the caldera, however the faults are not on the exact vent line.[1] In the last 9,500 years four of the seven major ruptures of the Manawahe Fault have been associated in time with an volcanic eruption of the Okataina volcanic centre. This fault is just to the east of Lake Rotoma at the boundary between the tectonic Whakatane and the magmatic Ōkataina segments of the Taupō Rift. These are the Whakatane eruption of about 5500 years ago, the Mamaku eruption of about 8000 years ago and at least two fault ruptures in before or during the Rotoma eruption of 9500 years ago.[8] Similarly the Ngapouri-Rotomahana Fault and Paeroa Fault have multiple ruptures associated in time with volcanism including immediately prior to the Mamaku and Rotoma rhyolite eruptions in the case of the Paeroa Fault and of the Ngapouri-Rotomahana Fault immediately prior to the Kaharoa eruption.[6] At least 30% of major Taupō Volcanic Zone eruptions have now been associated with significant local fault ruptures within 30 km (19 mi) of the eruption.[8]