Ohakuri Caldera

The Ohakuri Caldera lies mainly to the east and north of the Ātiamuri Dam 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 {{convert|300|m}} 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 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 to the east is likely more related to the Maroa Caldera.{{Cite journal |doi=10.1080/00288306.2006.9515169 |last1=Gravley |first1=D. M. |last2=Wilson |first2=C. J. N. |authorlink2 = Colin J. N. Wilson|last3=Rosenberg |first3=M. D. |year=2006 |last4=Leonard |first4=G. S. |s2cid=129012659 |title=The nature and age of Ohakuri Formation and Ohakuri Group rocks in surface exposures and geothermal drillhole sequences in the central Taupo Volcanic Zone, New Zealand |journal=New Zealand Journal of Geology and Geophysics |volume=49 |pages=305–308 |url=http://www.royalsociety.org.nz/publications/journals/nzjg/2006/026/ |issue=3 }}

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. 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. This ignimbrite deposit has been reported to extend to about {{convert|15|km|abbr=on}} to the north and east.{{cite book|chapter=Geologic map of the Paeroa Fault block and surrounding area, Taupo Volcanic Zone, New Zealand |chapter-url=https://www.usgs.gov/publications/geologic-map-paeroa-fault-block-and-surrounding-area-taupo-volcanic-zone-new-zealand |year=2020 |doi=10.3133/ofr20201021 |first1=Drew T. |last1=Downs |first2=Graham S. |last2=Leonard |first3=Colin J. N. |last3=Wilson|first4=Julie V |last4=Rowland |title=Open-File Report |s2cid=216218170 }} To the north east there is definite presence {{convert|17|km|abbr=on}} away.{{cite thesis|first1=Remedy Charlotte |last1=Loame |title=Using a tephrostratigraphic framework to determine the past 40,000 yrs of fault rupture and paleohydrothermal activity on the east strand of the Whirinaki Fault, Ngakuru Graben, central Taupo Volcanic Zone |year=2016 |url=https://researchcommons.waikato.ac.nz/bitstream/handle/10289/10537/thesis.pdf }}

File:Ohakuri.Rotorua.Paired.Eruptions.Rift.Zone.North.Island.NZ.jpgs (yellow phase 1 and dark yellow phase 2).]]

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. Ignimbrite, up to {{convert|180|m}} thick was deposited in the surrounding area to the south of Rotorua.{{cite thesis|last1=Milner |first1=David M |title=The structure and eruptive history of Rotorua Caldera, Taupo Volcanic Zone, New Zealand |url=https://ir.canterbury.ac.nz/handle/10092/12722 |year=2001}} 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 {{convert|30|km}} 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.{{cite journal|first1=D.M. |last1=Gravley|first2=C.J.N.|last2= Wilson|first3= G.S.|last3= Leonard|first4=J.W. |last4=Cole|title= Double trouble: Paired ignimbrite eruptions and collateral subsidence in the Taupo Volcanic Zone, New Zealand|journal= GSA Bulletin| year=2007|volume=119 |issue=1–2 |pages=18–30 |doi=10.1130/B25924.1|url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/119/1-2/18/125353/Double-trouble-Paired-ignimbrite-eruptions-and}} The maximum outflow dense-rock equivalent (DRE) of the Ohakuri ignimbrite is {{convert|100|km3|mi3}} which means the combined eruptions produced {{convert|245|km3}} of material.

It has been postulated that the drainage of the linked deep magma mush body between Rotorua and Ohakuri resulted in more than {{convert|250|m}} 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. 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 {{convert|0.14|mm}}/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.{{cite journal|first1=Judith |last1=Zachariasen |first2=Russ |last2=Van Dissen |title=Paleoseismicity of the northern Horohoro Fault, Taupo Volcanic Zone, New Zealand |journal=New Zealand Journal of Geology and Geophysics |doi=10.1080/00288306.2001.9514946 |year=2001 |volume=44 |issue=3 |pages=91–40|s2cid=131520830 |doi-access=free }} Understanding that there is volcanotectonic interrelationship lead to a complete reinterpretation of events in the Taupō Volcanic Zone in the last 250,000 years.

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|text=Map centered between the Ohakuri caldera and Rotorua caldera (both light green shading) so as to show selected surface volcanic deposits including all the present surface Mamaku and Ohakuri ignimbrites. {{Wikipedia:Map data/Northern North Island Volcanics/key}} }}

There has been interest in the mineral potential close to the western rim of the caldera.{{cite web|title=Ohakuri Epithermal Au-Ag Project|url=https://emxroyalty.com/site/assets/files/3991/emx-ohakuri-intro.pdf}}

• {{cite web|title=The Ohakuri pyroclastic deposits and the evolution of the Rotorua-Ohakuri volcanotectonic depression |url=https://ir.canterbury.ac.nz/bitstream/handle/10092/5465/gravley_thesis.pdf |year=2004 |first1=Darren MClurg |last1=Gravley |access-date=2022-08-17}}

{{columns-list|colwidth=20em|

• {{annotated link|Geology of New Zealand}}
• {{annotated link|Geothermal areas in New Zealand}}
• {{annotated link|Geothermal power in New Zealand}}
• List of volcanoes in New Zealand
• {{annotated link|North Island Volcanic Plateau}}
• Taupō Volcanic Zone
• Taupō Volcano
• Volcanology of New Zealand

}}

{{reflist}}

Category:Taupō Volcanic Zone

Category:Rotorua

Category:Calderas of New Zealand

Category:VEI-6 volcanoes

Category:Volcanoes of Waikato

Category:Rotorua Volcanic Centre

Category:Pleistocene calderas