Ohakuri Caldera

There is evidence of local volcanic activity before 240,000 years ago and not all might have been due to events in the adjacent Maroa Caldera. Possibly Pokai ignimbrite which is found to the east on the faultline of the Paeroa Fault, actually came from a caldera eruption that may have been co-located with the present Ohakuri Caldera about 275,000 years ago, but this is speculation.[2] Ohakuri ignimbrite, which has been characterised as a deposit radiating in decreasing thickness from the Ātiamuri area arises from the most significant eruption of the Caldera.[2] This ignimbrite deposit has been reported to extend to about 15 km (9.3 mi) to the north and east.[4] To the north east there is definite presence 17 km (11 mi) away.[5]

Approximate location and outlines (white) of the paired single event Rotorua and Ohakuri calderas with approximate known surrounding Mamaku (blue)[1] and Ohakuri ignimbrites (yellow phase 1 and dark yellow phase 2).[2] The relationship to the inactive southern portion (red dots) and currently active northern portion (red line) of the Horohoro Fault is shown. Relationships also shown to old Taupō Rift (light yellow shading), modern Taupō Rift (light red shading) and Hauraki Rift (light purple shading). The present landmarks of Lake Taupō and Mount Tarawera are labelled for orientation

There is now good evidence that the 240,000 years ago Ohakuri eruption was a paired eruption within days/weeks of the very slightly earlier, slightly larger, northerly eruption from the same mush body feeding the Rotorua Caldera.[1] Ignimbrite, up to 180 metres (590 ft) thick was deposited in the surrounding area to the south of Rotorua.[6] Between Rotorua and Ohakuri crosssections of the ash and ignimbrite from the two eruptions have been able to be sequenced completely and have relationships that can only be explained by a sequence of eruptions separated on occasions by days or less (e.g. no rainfall between eruptions).[2] The pairing separated by 30 kilometres (19 mi) was possibly through tectonic coupling of separate magma bodies that co-evolved from a lower in the mantle common mush body, as paired events are being increasingly recognised.[7] The maximum outflow dense-rock equivalent (DRE) of the Ohakuri ignimbrite is 100 cubic kilometres (24 cu mi) which means the combined eruptions produced 245 cubic kilometres (59 cu mi) of material.[1] It has been postulated that the drainage of the linked deep magma mush body between Rotorua and Ohakuri resulted in more than 250 metres (820 ft) of vertical displacement on the Horohoro Fault scarp and formed the Paeroa Graben, coincident to the north with the Kapenga Caldera between it and the Paeroa Fault to the east.[2] This is an area known as the Horohoro Cliffs escarpment and displaced Mamaku ignimbrite from the Rotorua Caldera eruption by this amount, presumably shortly after the eruption. This fault, in the present day, while active has a much lower displacement rate of the order of 0.14 millimetres (0.0055 in)/year and has been assigned by some as the outer western fault of the modern Taupō Rift although most think this is further to the east.[8] Understanding that there is volcanotectonic interrelationship lead to a complete reinterpretation of events in the Taupō Volcanic Zone in the last 250,000 years.[7]

There has been interest in the mineral potential close to the western rim of the caldera.[9]