{{Short description|Volcanic caldera in New Zealand}} {{Use New Zealand English|date=April 2024}} {{Use dmy dates|date=July 2019}} {{Infobox mountain | name = Reporoa Caldera | image = | image_caption = | elevation_m = 592 | elevation_ref = | map = New Zealand North Island | map_caption = | label_position = right | listing = | location = Taupō Volcanic Zone, New Zealand | coordinates = {{coord|38|25|00|S|176|20|00|E|type:mountain_scale:100000|format=dms|display=inline,title}} | coordinates_ref = | topo = | width = {{convert|10|km|abbr=on}} | length = {{convert|15|km|abbr=on}} | type = Caldera | age = {{Geological range|0.281|prefix=Pleistocene(|ref=<ref name=Downs2014>{{cite journal| last1=Downs| first1=D.T.| last2=Rowland| first2=J.V.| last3=Wilson| first3=C.J.N.| last4=Rosenberg| first4=M.D.|last5=Leonard| first5=G.S.| last6=Calvert| first6=A.T.|year= 2014|title= Evolution of the intra-arc Taupo-Reporoa basin within the Taupo Volcanic Zone of New Zealand| journal=Geosphere| volume=10| issue=1| pages=185–206| doi=10.1130/GES00965.1| doi-access=free}}</ref>{{rp|198}}) }} | last_eruption = 1180 (?), 2005 hydrothermal | first_ascent = | easiest_route = }} [[File:Paeroa-Ngapouri-Rotomahana Faults.jpg|thumb|left|alt=Okataina Volcanic Centre relationships to other nearby volcanic and tectonic structures|The Reporoa Caldera is to the east of the Paeroa Fault and south of the Ngapouri-Rotomahana Fault. It is accordingly south of the Okataina Volcanic Centre (approximate thick white border) and occupies a low land area between this and the Waikato River]] The '''Reporoa Caldera''' is a {{cvt|10| by |15|km}} caldera in New Zealand's Taupō Volcanic Zone located in the Taupō-Reporoa Basin. It formed some 280,000 years ago,<ref name=Downs2014/>{{rp|197}} in a large eruption that deposited approximately 100 km<sup>3</sup> of tephra, forming the Kaingaroa Ignimbrite layer.<ref name=Nairn1994>{{cite journal | first1 = I.A.| last1= Nairn | first2=C.P.| last2=Wood |first3=R.A. | last3=Bailey | date = December 1994 | title = The Reporoa Caldera, Taupo Volcanic Zone: source of the Kaingaroa Ignimbrites | journal = Bulletin of Volcanology | volume = 56 | issue = 6 | pages = 529–537 | doi = 10.1007/BF00302833 | bibcode=1994BVol...56..529N | s2cid=128947835 }}</ref> The ignimbrite sheet extends up to {{convert|15|km|abbr=on}} to the east.<ref name="Spinks2005">{{Cite web|title= Rift Architecture and Caldera Volcanism in the Taupo Volcanic Zone, New Zealand |url=https://ir.canterbury.ac.nz/handle/10092/4944| year=2005 |last1=Spinks |first1=Karl D. }}</ref>
In April 2005, a large hydrothermal explosion occurred near a cow paddock within the Taupō-Reporoa Basin, destroying some trees, temporarily blocking a nearby stream and creating a {{cvt|50|m}} crater at {{coord|38.533832|S|176.172010|E|format=dms|display=inline}}.<ref name="Simpson2014">{{cite web|title=Insight into the 2005 hydrothermal eruption at South Orakonui, Ngatamariki Geothermal Field, New Zealand from calcite microthermometry |url=https://www.researchgate.net/publication/282938468 |access-date=2022-10-16 |year=2014 |first1=Mark P. |last1=Simpson |first2= Michael D. |last2=Rosenberg |first3= Andrew J. |last3=Rae |first4= Greg |last4=Bignall |first5= Bruce W. |last5=Mountain |first6=Duncan |last6=Graham |first7= Debra |last7=Chappell }}</ref> A similar explosion happened in the area in 1948, and smaller explosions have happened in the years between.<ref>[http://thestar.com.my/news/story.asp?file=/2005/4/20/latest/20050420102446&sec=Latest Geothermal eruption in New Zealand paddock leaves big crater], ''The Star'', 20 April 2005.</ref><ref name="Simpson2014"/> At the time of the last eruption, it was believed to be within the caldera, but these recent eruptions are not technically within the now known area of the caldera.<ref name=Downs2014/>{{rp|197}}
==Geography== {{maplink|frame=yes|frame-align=left|frame-width=180|frame-height=250|frame-long=176.22|frame-lat=-38.53|zoom=8|raw={{Wikipedia:Map data/Reporoa Caldera}} |text=The Reporoa Caldera is outlined approximately in red. Also shown are approximate sizes of the two temporary Lake Reporoa's created transiently after the 232 ± 10 CE Hatepe eruption, the first about {{cvt|15|m}} higher than the later created when the volcanic dam of Lake Taupō broke.<ref name=Manville2001/><ref name=GreenLowe2025>{{cite book|last1=Green |first1=J.D. |last2=Lowe |first2=D.J. |year=2025|chapter=Lakes in the past – paleolimnology and paleolakes|title=Hidden gems of the Waikato: the history, ecology and management of the Waikato lakes – Ō Tātou Roto: He Taonga Tuku Iho |editor-last1=Özkundakci|editor-first1=D. |editor-last2=Grainger |editor-first2=N. |editor-last3=Dean-Speirs|editor-first3=T.|publisher=Waikato Regional Council and Te Tumu Whakaora Taiao – the Environmental Research Institute (University of Waikato) |location=Hamilton |pages=55-79}}{{rp|loc=Figure 3.14B Short-lived post-Taupō-eruption paleo Lake Reporoa}}</ref><ref>{{cite journal|last=Manville |first=V.|year=2002|title=Sedimentary and geomorphic responses to ignimbrite emplacement: readjustment of the Waikato River after the A.D. 181 Taupo eruption, New Zealand |journal=The Journal of Geology |volume=110 |issue=5 |pages=519-541|doi=10.1086/341596}}</ref> The Taupō-Reporoa Basin includes both the lakes and caldera and is not shown specifically. }} The Reporoa Caldera is located in the northern part of the Taupō-Reporoa Basin, which extends north-east from Lake Taupō in the south to the Waiotapu geothermal area, and whose southern features are distinct from the caldera.<ref name=Downs2014/>{{rp|189}} The Waikato River where it leaves Lake Taupō enters the south east corner of the basin and is flanked by geothermal areas developed for power near Wairakei. In the north-west, the basin rises up to the Paeroa Range whose western limit is defined by the Paeroa Fault scarp.<ref name=Downs2014/>{{rp|189}} The hills of the eastern basin margin are mostly covered by the Kaingaroa Forest. The Waikato River loops to the north before exiting the basin at its south-west but is always south of the caldera.
The basin has been the location of several temporary lakes during its volcanic history and one long term lake that lasted about 200,000 years.<ref>{{cite journal|last1 =Rosenberg|first1 =MD|last2 =Wilson|first2 =CJ|last3 =Bignall|first3 =G|last4 =Ireland|first4 =TR|last5 = Sepulveda|first5 =F|last6 =Charlier|first6 =BL| title = Structure and evolution of the Wairakei–Tauhara geothermal system (Taupo Volcanic Zone, New Zealand) revisited with a new zircon geochronology| journal = Journal of Volcanology and Geothermal Research|date=15 January 2020| volume = 390| number =106705| doi =10.1016/j.jvolgeores.2019.106705}}{{rp|loc=Tables 1 & 2}}</ref> The long term lake was Lake Huka which was destroyed in the 25,600 years ago Oruanui eruption.<ref name=Manville2004>{{Cite journal | doi = 10.1080/00288306.2004.9515074| title = The 26.7 ka Oruanui eruption, New Zealand: A review of the roles of volcanism and climate in the post‐eruptive sedimentary response| journal = New Zealand Journal of Geology and Geophysics| volume = 47| issue = 3| pages = 525–546| year = 2004| last1 = Manville | first1 = V. | last2 = Wilson | first2 = C. J. N. | author-link2 = Colin J. N. Wilson|doi-access=free}}</ref>{{rp|541}}
After the Oruanui eruption there is evidence for a '''Lake Reporoa''' temporary lake in the Reporoa Basin which has some uncertainty about its height at various times.<ref name=Manville2004/> The highest shoreline terrace is about {{convert|360|m|abbr=on}} and lake deposits are up to {{convert|400|m|abbr=on}} above today's sea level. This lake may have partially drained before the main break-out flood from Lake Taupō, and was certainly destroyed during the break-out flood that followed the Oruanui eruption,<ref name=Manville2004/>{{rp|535}}, at which time its level may have been about {{convert|320|m|abbr=on}} above present sea level.<ref name=GreenLowe2025/>
After the Taupō Hatepe eruption about 1,800 years ago, the largest recent Lake Reporoa formed .<ref name=Manville2001>{{cite book|last1=Manville |first1=V| title=Sedimentology and history of Lake Reporoa: an ephemeral supra‐ignimbrite lake, Taupo Volcanic Zone, New Zealand in Volcaniclastic sedimentation in lacustrine settings| editor1=James D. L. White|editor2= N. R. Riggs | publisher=Wiley| date= 2001-04-18| pages=109–40|isbn=1444304267}}</ref>{{rp|109}} This lake occupied up to {{convert|190|km2|abbr=on}} of the basin impounding about {{convert|2.5|km3|abbr=on}} of water.<ref name=Manville2001/>{{rp|109}} In due course its volcanic dam was breached with a resulting initial flood down the Waikato River. This flood is known to have been before the later breaching of a further upstream ignimbrite dam, also formed by the same eruption, at present outlet of the Lake Taupō.<ref name=Manville2001/>{{rp|109}} This greater breach released about {{convert|20|km3|abbr=on}} of water,<ref name=Manville2009>{{cite journal| last1=Manville| first1=V.| last2=Segschneider| first2=B.| last3=Newton| first3=E.| last4=White| first4=J.D.L.| last5=Houghton| first5=B.F.| last6=Wilson| first6=C.J.N.| year=2009 | title= Environmental impact of the 1.8 ka Taupo eruption, New Zealand: Landscape responses to a large-scale explosive rhyolite eruption| journal=Sedimentary Geology| volume=220| issue=3–4| pages=318–336| doi=10.1016/j.sedgeo.2009.04.017}}</ref>{{rp|327}} which temporarily flooded for no more than a week, some of the area of the former Lake Reporoa to {{cvt|303|m}} above present sea level.<ref name=Manville2001/>{{rp|109}}
===Geology=== {{#tag:mapframe|[{"type":"FeatureCollection","features": [ { "type": "Feature", "properties": { "stroke-width":1, "stroke": "#00ff00", "fill": "#00ff00", "stroke-opacity":0.3, "fill-opacity":0.3, "title": "Reporoa Caldera" }, "geometry": { "type": "Polygon", "coordinates": [ [ [176.3178402,-38.4693710 ],[ 176.3271528,-38.4663637 ],[ 176.3350922,-38.4668510 ],[ 176.3407570,-38.4675062 ],[ 176.3474518,-38.4692366 ],[ 176.3570648,-38.4728317 ],[ 176.3676220,-38.4677330 ],[ 176.3757974,-38.4614881 ],[ 176.3870198,-38.4498401 ],[ 176.3940150,-38.4392524 ],[ 176.3966757,-38.4224098 ],[ 176.3924700,-38.4079508 ],[ 176.3912469,-38.4000645 ],[ 176.3936287,-38.3933881 ],[ 176.3952810,-38.3891667 ],[ 176.3982636,-38.3860215 ],[ 176.3910538,-38.3819175 ],[ 176.3870627,-38.3845414 ],[ 176.3806683,-38.3849114 ],[ 176.3722998,-38.3836668 ],[ 176.3724500,-38.3754918 ],[ 176.3715810,-38.3710338 ],[ 176.3707978,-38.3585669 ],[ 176.3608736,-38.3551680 ],[ 176.3558418,-38.3632107 ],[ 176.3500214,-38.3640183 ],[ 176.3472050,-38.3698730 ],[ 176.3415724,-38.3692001 ],[ 176.3365513,-38.3779477 ],[ 176.3277537,-38.3825230 ],[ 176.3157374,-38.3825566 ],[ 176.3050944,-38.3848778 ],[ 176.2979275,-38.3982315 ],[ 176.2898165,-38.4055968 ],[ 176.2871557,-38.4250322 ],[ 176.2863833,-38.4359246 ],[ 176.2895590,-38.4468152 ],[ 176.2942368,-38.4551838 ],[ 176.2996012,-38.4621401 ],[ 176.3108450,-38.4675164 ],[ 176.3178402,-38.4693710] ] ] } } ] },{{Wikipedia:Map data/Northern North Island Volcanics}}] |align=right| frameless =1 |text=Map to show surface volcanic deposits of Kaingaroa ignimbrite and Reporoa Caldera (light green shading). {{Wikipedia:Map data/Northern North Island Volcanics/key}} |width=300 |height=290 |latitude=-38.49 |longitude=176.33 |icon=no |zoom=9 }} It was first recognised as a volcanic caldera in 1994<ref name=Nairn1994/>{{rp|1}} and later work has defined it as a single major event caldera within the Taupō Rift.<ref name="Beresford2000"/>{{rp|471}} Initially the age was thought to be 0.23 Ma<ref name="Beresford2000">{{cite journal | doi = 10.1080/00288306.2000.9514903 | author = S. W. Beresford | author2=J. W. Cole | year = 2000 | title = Kaingaroa Ignimbrite, Taupo Volcanic Zone, New Zealand: evidence for asymmetric caldera subsidence of the Reporoa Caldera | journal = New Zealand Journal of Geology & Geophysics | volume = 43 | issue = 3 | pages = 471–481 | s2cid = 129243910 | doi-access = }}</ref>{{rp|pp=471,473}} but is now thought to be about 50,000 years older at 281 ± 21 ka BP.<ref name=Downs2014/>{{rp|197}} This age is more consistent with argon–argon dating on the tephra, ignimbrite and samples from the later smaller rhyolite dome eruptions that happened after caldera formation.<ref name=Downs2014/>{{rp|197}} One of these was at Deer Hill, which is just to the south of the caldera margin and is dated at 264 ± 4 ka BP. Within the southern area of the caldera proper Pukekahu (263 ± 5 ka BP), and Kairuru (247 ± 2 ka BP) formed.<ref name=Downs2014/>{{rp|pp=197,203}} Gravitational<ref name=Soengkono2012>{{cite web| last1=Soengkono| first1=S.| year=2012| title=Gravity Modelling Of Reporoa Basin, Eastern Taupo Volcanic Zone (TVZ), New Zealand: In Proceedings of the 34th New Zealand Geothermal Workshop, Auckland, New Zealand|url=https://www.geothermal-energy.org/pdf/IGAstandard/NZGW/2012/46654final00065.pdf |access-date=2023-09-01}}</ref>{{rp|pp=1–6}} and magnetic<ref>{{cite web| last1=Soengkono| first1=S.| last2=Hochstein| first2=M.P.| year=1996| title=Interpretation of Magnetic Anomalies over the Reporoa Geothermal Field, Taupo Volcanic Zone, New Zealand: In Proceedings of the 18th New Zealand Geothermal Workshop, Auckland, New Zealand|url=https://www.geothermal-energy.org/pdf/IGAstandard/NZGW/2012/46654final00065.pdf |access-date=2023-09-01}}</ref> studies have been used to define the caldera to being north of the present Waikato River course in the Taupō-Reporoa Basin.<ref name=Kaya2014>{{cite journal| last1=Kaya| first1=E.| last2=O'Sullivan| first2=M.J.| last3=Hochstein| first3=M.P.| year=2014| title=A three dimensional numerical model of the Waiotapu, Waikite and Reporoa geothermal areas, New Zealand| journal=Journal of Volcanology and Geothermal Research| volume=283 |pages= 127–142| doi=10.1016/j.jvolgeores.2014.07.008}}</ref>{{rp|loc=Fig. 1.(a)}}<ref name=Downs2014/>{{rp|187}} A relative gravity low does exist south-west of the river and caldera in the Mihi area and has been called the Mihi volcanic depression, although there is presently no evidence of a related volcanic event.<ref name=Soengkono2012/>{{rp|4}} The Reporoa Caldera is associated with three geothermal fields. These are the active Reporoa geothermal field in the caldera, the Waiotapu geothermal area north of the caldera rim, and the Broadlands thermal area to the south.<ref name=Kaya2014/>{{rp|loc=Fig. 1.(a & b)}}
==See also== * List of volcanoes in New Zealand * {{cite gvp|vnum=241060|name=Reporoa}}
==References==
{{reflist}}
Category:Taupō Volcanic Zone Category:Calderas of New Zealand Category:VEI-7 volcanoes Category:Rotorua Lakes District Category:Volcanoes of Waikato Category:Pleistocene calderas Category:Geothermal areas in New Zealand