The Ohakuri Caldera (also spelled with a macron; Ōhakuri Caldera) was formed in a paired single event eruption of Ohakuri

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

and is located in the

Taupō Volcanic Zone The Taupō Volcanic Zone (TVZ) is a volcano, volcanic area in the North Island of New Zealand. It has been active for at least the past two million years and is still highly active. Mount Ruapehu marks its south-western end and the zone runs n ...

on the

North Island The North Island ( , 'the fish of Māui', historically New Ulster) is one of the two main islands of New Zealand, islands of New Zealand, separated from the larger but less populous South Island by Cook Strait. With an area of , it is the List ...

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

. Its significance was first recognised in 2004, as the geology of the area had been misunderstood until then. The paired eruption resulted in a very large eruption sequence in the

about 240,000 years ago that included the formation of

Lake Rotorua Lake Rotorua () is the second largest lake in the North Island of New Zealand by surface area, and covers 79.8 km2. With a mean depth of only 10 metres it is considerably smaller than nearby Lake Tarawera in terms of volume of water. It i ...

and eruption of the Mamaku ignimbrite.

Geography

The Ohakuri Caldera lies mainly to the east and north of the

Ātiamuri Dam Ātiamuri Power Station is a hydroelectric power station on the Waikato River, in the North Island of New Zealand. It is the third of eight hydroelectric power stations on the Waikato River. The station can easily be seen from State Highway 1 b ...

and extends almost to the Ōhakuri Dam. Its borders are ill-defined, particularly the northern and eastern borders, possibly because later volcanotectonic activity has completely replaced landforms that could have at one stage included a lake extending almost from Lake Rotorua to this caldera. Its western border is believed to be defined by the valley of the Mangaharakeke stream that the main highway uses and towards the north west of Ātiamuri the caldera floor extends at just below the level above sea level. Ngautuku is a dome at the south western aspect of the caldera. The much larger Maroa Caldera complex is to the south with its northern border on the

Waikato River The Waikato River is the longest river in New Zealand, running for through the North Island. It rises on the eastern slopes of Mount Ruapehu, joining the Tongariro River system and flowing through Lake Taupō, New Zealand's largest lake. It th ...

so the two caldera borders are adjacent. However, the older Whakamaru Caldera almost certainly crosses the present river course and overlaps the Ohakuri Caldera to a degree. The Waikato River course follows roughly the borders of these two caldera but the thermal area of

Orakei Korako Orakei Korako is a highly active geothermal area most notable for its series of fault-stepped sinter terraces, located in a valley north of Taupō on the banks of the Waikato River in the Taupō Volcanic Zone, New Zealand. It is also known as " ...

to the east is likely more related to the Maroa Caldera.

Geology

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

which is found to the east on the faultline of the

Paeroa Fault The Paeroa Fault is a seismically active area in the Taupō District, Waikato Region of the central North Island of New Zealand. Geology North of Lake Taupō, volcanic ignimbrite at least thick, and called the Paeroa Ignimbrite (dated to 339 ...

, 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. Ohakuri ignimbrite, which has been characterised as a deposit radiating in decreasing thickness from the

Ātiamuri Ātiamuri is a former hydro village in the central North Island of New Zealand. It lies alongside State Highway 1 about 27 km south of Tokoroa and 38 km north of Taupō. It is bordered by the Waikato River and surrounded by pine planta ...

area arises from the most significant eruption of the Caldera. This ignimbrite deposit has been reported to extend to about to the north and east. To the north east there is definite presence away.

240,000 years ago Ohakuri paired eruption

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 The Rotorua Caldera is a large rhyolitic caldera that is filled by Lake Rotorua. It was formed by an eruption 240,000 years ago that produced extensive Pyroclastic rock, pyroclastic deposits. Smaller eruptions have occurred in the caldera since ...

Ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

, up to thick was deposited in the surrounding area to the south of Rotorua. 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). The pairing separated by 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. The maximum outflow dense-rock equivalent (DRE) of the Ohakuri ignimbrite is which means the combined eruptions produced of material. It has been postulated that the drainage of the linked deep magma mush body between

Rotorua Rotorua () is a city in the Bay of Plenty region of New Zealand's North Island. It is sited on the southern shores of Lake Rotorua, from which it takes its name. It is the seat of the Rotorua Lakes District, a territorial authorities of New Zea ...

and Ohakuri resulted in more than of vertical displacement on the

Horohoro Fault The Horohoro Fault is in the old Taupō Rift of the central North Island of New Zealand and is associated with the spectacular Horohoro Cliffs. Geology Just south of Lake Rotorua, Horohoro mountain, a rhyolite dome, has very high cliffs in an ...

scarp and formed the Paeroa Graben, coincident to the north with the

Kapenga Caldera The Kapenga Caldera (also known as the Kapenda Volcanic Centre) in New Zealand’s Taupō Volcanic Zone lies in a lowland area immediately south of Lake Rotorua through the Hemo Gap in the Rotorua Caldera rim. Features of the caldera have develop ...

between it and the

to the east. This is an area known as the Horohoro Cliffs escarpment and displaced Mamaku ignimbrite from the

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 /year and has been assigned by some as the outer western fault of the modern

Taupō Rift The Taupō Rift is the seismically active rift valley containing the Taupō Volcanic Zone, central North Island of New Zealand. Geology The Taupō Rift (Taupo Rift) is a intra-arc continental rift resulting from an oblique convergence in the H ...

although most think this is further to the east. Understanding that there is volcanotectonic interrelationship lead to a complete reinterpretation of events in the

in the last 250,000 years.

Subsequent evolution

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

External links

References