{{Short description|Volcanic caldera in New Zealand}} {{Use New Zealand English|date=April 2024}} {{Infobox mountain | name = Ohakuri Caldera | image = Ohakuri.Caldera.Rift.Zone.North.Island.NZ.jpg | image_caption = Approximate location and outline (white) of Ohakuri Caldera. Known surrounding Ohakuri ignimbrite (yellow) is approximately as shown. The relationship to the inactive southern portion (red dots) and active northern portion (red line) of the [[Horohoro Fault]] may be important. The old [[Taupō Rift]] (light yellow shading), modern Taupō Rift (light red shading) and [[Hauraki Rift]] (light purple shading) are shown for context. Landmarks such as [[Lake Taupō]], [[Lake Rotorua]], [[Mount Tarawera]] are shown for orientation. Within the caldera the dome Ngautuku is identified. | map = North Island | map_size = 130 | elevation_m = 629 | elevation_ref = | prominence = Ngautuku | location = [[North Island]] | country = [[New Zealand]] | region = [[Waikato]] | coordinates = {{coord|-38.378|176.019|type:mountain|format=dms|display=inline}} | range_coordinates = {{coord|38|21|53|S|176|02|05|E|display=inline,title}} | topo = | type = [[Caldera]] | age = {{Geological range|0.24|prefix=[[Pleistocene]] (|ref=)<ref name="Bégué2014">{{cite journal|first1=F. |last1=Bégué|first2=C. D. |last2=Deering|first3=D. M. |last3=Gravley |first4=B. M. |last4=Kennedy |first5=I. |last5=Chambefort |first6=G. A. R. |last6=Gualda |first7=O. |last7=Bachmann |title=Extraction, Storage and Eruption of Multiple Isolated Magma Batches in the Paired Mamaku and Ohakuri Eruption, Taupo Volcanic Zone, New Zealand |journal=Journal of Petrology |volume=55 |issue=8 |year=2014 |pages=1653–1684 |doi=10.1093/petrology/egu038 |url=https://academic.oup.com/petrology/article/55/8/1653/1486635|doi-access=free |hdl=20.500.11850/88102 |hdl-access=free }}</ref>}} | width = 5km | volcanic_zone = [[Taupō Volcanic Zone]] | last_eruption = 240,000 years ago | access = [[State Highway 1 (New Zealand)]] }} The '''Ohakuri Caldera''' (also spelled with a macron; '''Ōhakuri Caldera''') was formed in a paired single event eruption of Ohakuri [[ignimbrite]] and is located in the [[Taupō Volcanic Zone]] on the [[North Island]] of [[New Zealand]]. 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 [[Taupō Volcanic Zone]] about 240,000 years ago that included the formation of [[Lake Rotorua]] and eruption of the Mamaku ignimbrite.<ref name="Gravley2004">{{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}}</ref>
== Geography == 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.<ref name="Gravley2006">{{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 |url-access=subscription }}</ref>
=== 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 [[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.<ref name="Gravley2004"/> 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.<ref name="Gravley2004"/> This ignimbrite deposit has been reported to extend to about {{convert|15|km|abbr=on}} to the north and east.<ref name="Downs2020">{{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 }}</ref> To the north east there is definite presence {{convert|17|km|abbr=on}} away.<ref name="Loame2016">{{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 }}</ref>
==== 240,000 years ago Ohakuri paired eruption ==== [[File:Ohakuri.Rotorua.Paired.Eruptions.Rift.Zone.North.Island.NZ.jpg|thumb|295px|alt=Paired eruptions Rotorua(Mamaku) and Ohakuri |Key is as with other image on the page with addition of surrounding Mamaku (blue)<ref name="Bégué2014"/> and separate phases of Ohakuri [[ignimbrite]]s (yellow phase 1 and dark yellow phase 2).<ref name="Gravley2004"/>]] There is now good evidence that the 240,000 years ago the Ohakuri ignimbrite 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]].<ref name="Bégué2014"/> [[Ignimbrite]], up to {{convert|180|m}} thick was deposited in the surrounding area to the south of Rotorua.<ref name="Milner2001">{{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}}</ref>
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).<ref name="Gravley2004"/> Further [[geomagnetic excursion]] studies appear to constrain the entire sequence of twin eruptions to a period of decades or at most centuries.<ref name="Gravley2025">{{cite journal|first1=D.M. |last1=Gravley |last2=Hasegawa |first2=T. |last3=Mochizuki |first3=N. |last4=Kusu |first4=C. |last5=Okada |first5=M.|year =2025|title=A new paleomagnetic constraint on the duration of a twin caldera-forming eruption sequence: a potential solution to an enduring problem in decoding> VEI 7 eruption timescales |journal=Earth, Planets and Space |volume=77 |issue=1 |at=106|doi=10.1186/s40623-025-02179-9|doi-access=free |bibcode=2025EP&S...77..106G}}</ref> 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 (geology)|mantle]] common mush body, as paired events are being increasingly recognised.<ref name="Gravley2007">{{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|url-access=subscription}}</ref> 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.<ref name="Bégué2014"/>
{| class="wikitable" |+ 240 ka Ohakuri eruption sequence<ref name="Gravley2025"/> |- ! Unit !! Source Caldera !! Thickness at Ohakura !! Comment |- | - || Ohakuri || {{cvt|10|cm}} ||Precursor’ airfall ash overlaid by some soil |- | 1 || Ohakuri || <{{cvt|10|cm}} ||Airfall deposit - no significant time break beyond weeks or a few months. |- | 2 || Ohakuri || about {{cvt|30|cm}} ||Linian pumice-fall deposit that is observed below and above the thick Mamaku ignimbrite (unit 4) from Rotorua caldera so is simultaneous to the Mamaku eruption. |- | 4 || Rotorua || {{cvt|5|m}} ||Mamaku ignimbrite |- | 2 || Ohakuri || about {{cvt|30|cm}} ||Linian pumice-fall deposit that is observed below and above the thick Mamaku ignimbrite (unit 4) from Rotorua caldera so is simultaneous to the Mamaku eruption. |- | 5 || Rotorua || up to {{cvt|50|cm}} ||Fine-grained ash deposit from the Mamaku eruption, time-break of possibly days to weeks. |- | 6 || Ohakuri || up to {{cvt|100|m}} || Ohakuri ignimbrite. |} 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.<ref name="Gravley2004"/> 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.<ref name="Zachariasen2001">{{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 }}</ref> Understanding that there is volcanotectonic interrelationship lead to a complete reinterpretation of events in the [[Taupō Volcanic Zone]] in the last 250,000 years.<ref name="Gravley2007"/> {{#tag:mapframe|[{"type":"FeatureCollection","features": [ {"type":"Feature","properties": {"stroke-width":1,"stroke":"#00ff00","fill":"#00ff00", "fill-opacity":0.3, "title":"Postulated [[Rotorua Caldera]]"},"geometry": {"type":"Polygon","coordinates": [[[176.1503899,-38.0361103],[176.1613762,-38.0242113],[176.1998283,-38.0144744],[176.2218010,-38.0114990],[176.2739861,-38.0201544],[176.3138115,-38.0336766],[176.3462538,-38.0570679],[176.3647933,-38.0846365],[176.3586134,-38.1202981],[176.3441939,-38.1435228],[176.3154263,-38.1586193],[176.2869305,-38.1632084],[176.2625943,-38.1771188],[176.2495481,-38.1773887],[176.2210523,-38.1846752],[176.2052594,-38.1790080],[176.1949598,-38.1763091],[176.1912944,-38.1784099],[176.1758449,-38.1719324],[176.1641719,-38.1616752],[176.1562755,-38.1495267],[176.1463191,-38.1438567],[176.1353328,-38.1433167],[176.1257197,-38.1335956],[176.1209132,-38.1217125],[176.1277797,-38.1079365],[176.1362918,-38.0871325],[176.1366351,-38.0711877],[176.1452182,-38.0536175],[176.1469348,-38.0422622],[176.1503899,-38.0361103]]]} },{"type":"Feature","properties": {"stroke-width":1,"stroke":"#00ff00","fill":"#00ff00", "fill-opacity":0.3, "title":"Postulated Ohakuri Caldera"},"geometry": {"type":"Polygon","coordinates": [[[175.9985767,-38.3283332],[175.9963451,-38.3431446],[175.9965167,-38.3517608],[175.9999500,-38.3636063],[175.9965167,-38.3714126],[176.0119663,-38.3918663],[176.0298190,-38.4004767],[176.0490451,-38.3975170],[176.0852224,-38.3854582],[176.0828192,-38.3615031],[176.0828192,-38.3456184],[176.0736058,-38.3288136],[176.0691426,-38.3188479],[176.0615895,-38.3188479],[176.0547230,-38.3242349],[176.0454533,-38.3199253],[176.0304329,-38.3192519],[176.0154126,-38.3215415],[176.0073445,-38.3195213],[175.9987614,-38.3210028],[175.9985767,-38.3283332]]]} }]},{{Wikipedia:Map data/Northern North Island Volcanics}}] | frameless =1 | align =right|width=295 |height=500 |latitude=-38.16 |longitude=176.03 |icon=no |zoom=9 |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}} }}
==== Subsequent mineral potential ==== There has been interest in the mineral potential close to the western rim of the caldera.<ref>{{cite web|title=Ohakuri Epithermal Au-Ag Project|url=https://emxroyalty.com/site/assets/files/3991/emx-ohakuri-intro.pdf}}</ref>
==External links== *{{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}}
==See also== {{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]] }}
==References== {{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]]