View source for Earth:Dunedin Volcano

{{short description|An extinct volcano in South Island, New Zealand}}
{{Infobox rockunit
| name           = Dunedin Volcano
| image          = Organ pipes columnar joints Dunedin.jpg
| caption        = Basaltic columnar crystals in the "Organ pipes" formation
| type           = Igneous
| age            = {{Geological range|16|10|prefix=Miocene (|ps=)}}
| period         = Miocene 
| prilithology   = [[Earth:Basalt|Basalt]], [[Earth:Trachyte|trachyte]], [[Earth:Phonolite|phonolite]] and breccia
| otherlithology =
| namedfor       = [[Place:Dunedin|Dunedin]]
| namedby        =
| region         = Otago
| country        = New Zealand
| coordinates    = {{coord|-45.848|170.636|type:landmark_globe:earth_region:NZ|display=ti}}
| unitof         = 
| subunits       =
| underlies      = 
| overlies       = Waipuna Bay Formation
| thickness      =
| extent         =
| area           =
| map            ={{maplink|frame=yes
|frame-align=center
|text=Map of selected surface volcanic features of the South Island centred on the Otago Peninsula. {{Wikipedia:Map data/South Island Volcanics/key}}
|raw={{Wikipedia:Map data/South Island Volcanics}}
|frame-width=300 
|frame-height=300 
|frame-lat=-45.825
|frame-long=170.56
|icon=no
|zoom=10
}}
| map_caption    =
}}
{{main|Earth:Dunedin volcanic group}}
[[File:Lighthouse above cliffs at Taiaroa Head.jpg|150px|thumb|left|A layer of scoria covered by a basalt lava flow at Taiaroa Head]]
The '''Dunedin Volcano''' is an extensively eroded multi-vent [[Earth:Shield volcano|shield volcano]] that was active between 16 and 10 million years ago.<ref name="McDougallCoombs1973">{{cite journal|last1=McDougall|first1=Ian|last2=Coombs|first2=D. S.|title=Potassium-argon ages for the Dunedin volcano and outlying volcanics|journal=New Zealand Journal of Geology and Geophysics|volume=16|issue=2|year=1973|pages=179–188|issn=0028-8306|doi=10.1080/00288306.1973.10431451|doi-access=free}}</ref> It originally extended from the modern city of [[Place:Dunedin|Dunedin]], [[Earth:New Zealand|New Zealand]] to Aramoana  about 25&nbsp;km away. Extensive erosion has occurred over the last 10 million years and [[Earth:Otago Harbour|Otago Harbour]] now fills the oldest parts of the volcano.<ref>{{cite web |url=https://data.gns.cri.nz/ |title= New Zealand Stratigraphic Lexicon |last= |first= |date=|website= |publisher=GNS Science |access-date=|quote=}}</ref><ref name="VFSDV">{{cite web |url=https://www.gns.cri.nz/content/download/6194/33962/file/VolcanoFactSheets%20Dunedin.pdf |title= Volcano Fact Sheet |last= |first= |date=|website=www.gns.cri.nz |publisher=GNS Science |access-date=|quote=}}</ref> The remnants of the volcano form the hills around [[Earth:Otago Harbour|Otago Harbour]] (including [[Earth:Mount Cargill|Mount Cargill]], Flagstaff, Saddle Hill, Signal Hill, and Otago Peninsula).

==Geology==
Volcanism began in a shallow marine environment. The volcano became larger with flows of basalt and minor [[Earth:Trachyte|trachyte]] creating the bulk of the volcano. The total volume of eruptives has been estimated to be {{convert|150|km3|abbr=on}} but there was a much larger intrusive component at {{convert|600|km3|abbr=on}}.<ref>{{cite journal|first1=A. |last1=Pontesilli |first2= M. |last2=Brenna|first3= S. |last3=Mollo |first4= M. |last4=Masotta| first5= M. |last5=Nazzari |first6= P. |last6=Le Roux |first7=P. |last7=Scarlato |title=Trachyte-phonolite transition at Dunedin Volcano: Fingerprints of magma plumbing system maturity and mush evolution |journal=Lithos |volume = 408–409 |year=2022 |issue=106545 |page=106545 |issn=0024-4937 |doi=10.1016/j.lithos.2021.106545 |bibcode=2022Litho.40806545P |s2cid=244561166 |url=https://www.sciencedirect.com/science/article/pii/S0024493721005880 }}</ref> The final phase of eruptions are preserved as phonolite domes around [[Earth:Mount Cargill|Mount Cargill]].<ref name="Coombs2008">{{cite journal|first1=Douglas S. |last1=Coombs |first2= Christopher J. |last2=Adams |first3= Barry P. |last3=Roser |first4= Anthony |last4=Reay |year=2008| title=  Geochronology and geochemistry of the Dunedin Volcanic Group, eastern Otago, New Zealand|journal= New Zealand Journal of Geology and Geophysics|volume= 51|issue=3 |pages= 195–218|doi=10.1080/00288300809509860 |s2cid=129436943 |url=https://www.tandfonline.com/doi/pdf/10.1080/00288300809509860 }}</ref>

Traces of the old Dunedin Volcano are best seen in the Dunedin Botanic Garden's geology walk,<ref>{{Cite web|url=https://www.odt.co.nz/lifestyle/magazine/city-born-fire-and-rock|title=A city born of fire and rock|date=2018-11-19|website=Otago Daily Times Online News|language=en|access-date=2019-01-10}}</ref> the cliffs at Aramoana and the geomorphology of Mount Cargill which has preserved lava domes. Basaltic columns are also to be found as prominent features above Second Beach at St Clair and at Blackhead.

This '[https://www.youtube.com/watch?v=6n5wNAShgvI Dunedin Volcano Animation]' shows various locations around Dunedin with information on age of the rocks and general evolution of the Dunedin Volcano and more distant volcanism.

=== Origin and relationship to other volcanoes ===
New Zealand's South Island has many extinct volcanic centres with no yet fully agreed [[Earth:Volcanic arc|tectonic mechanism]] of formation and the Dunedin volcano is one of them. They extend in age from the [[Earth:Cretaceous|Cretaceous]] to the Pliocene and outcrop throughout Otago, Canterbury and on the [[Earth:Chatham Islands|Chatham Islands]]. The largest single eruptive centre is the [[Earth:Banks Peninsula Volcano|Banks Peninsula Volcano]] followed by the Dunedin Volcano, which is half its size, but is part of a much larger volcanic area, the [[Earth:Dunedin volcanic group|Dunedin volcanic group]].<ref name="Scott2020">{{cite journal|first1= James M. |last1=Scott |first2= Alessio |last2=Pontesilli |first3= Marco |last3=Brenna |first4= James D. L. |last4=White |first5= Emanuele |last5=Giacalone |first6= J. Michael |last6=Palin |first7=Petrus J. |last7=le Roux |year=2020 |title= The Dunedin Volcanic Group and a revised model for Zealandia's alkaline intraplate volcanism| journal= New Zealand Journal of Geology and Geophysics|volume= 63 |issue=4 |pages= 510–529 | doi=10.1080/00288306.2019.1707695 |s2cid=212937447 |url=https://www.tandfonline.com/doi/full/10.1080/00288306.2019.1707695}}</ref> The relationship in terms of age and magma melts within the Dunedin volcanic group has caused some revision of previous models as part of a larger synthesis that remains incomplete as it requires reanalysis and new analysis of many volcanics, as well as better information from off land studies. A model consistent with current evidence is that there has been intermittent melting of a middle [[Earth:Lithosphere|lithospheric]] [[Earth:Mantle (geology)|mantle]] with [[Earth:Metasomatism|metasomatism]] by hydrous [[Earth:Asthenosphere|asthenosphere]]-derived melts<ref name="Scott2020"/>

These volcanic centres can be dormant for tens of millions of years between eruptions. This implies that the mechanism of formation may be connected to the [[Earth:Lithosphere|lithosphere]] unlike some other intraplate volcanoes such as the [[Earth:Hawaiian Islands|Hawaii island chain]], which are rooted in the [[Earth:Asthenosphere|asthenosphere]]. One possible mechanism for the creation of these volcanoes is the flaking off of the base of [[Earth:Zealandia|Zealandia's]] lower lithosphere into the asthenosphere. Zealandia has a thin lithosphere as it has been extended while rafting away from Australia. If large sections of this already thin lithosphere sank into the asthenosphere, it would be replaced with hotter rock leading to decompression melting. This theoretically could cause volcanic activity that is locked to the moving lithosphere over many millions of years.<ref name="Hoernle 2006">{{cite journal|last1=Hoernle |first1=K |last2= White |first2=JDL |last3= van den Bogaard |first3=P |last4= Hauff |first4=F |last5= Coombs |first5=DS |last6= Werner |first6=R |last7= Timm |first7=C |last8=Garbe-Schönberg |first8=D |last9= Reay |first9=A |last10= Cooper |first10=AF |year= 2006 |title= Cenozoic intraplate volcanism on New Zealand: Upwelling induced by lithospheric removal |journal= Earth and Planetary Science Letters |volume=248 |issue=1–2 |pages=350–367 |doi =10.1016/j.epsl.2006.06.001 |bibcode=2006E&PSL.248..350H |url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X06004146}}</ref>

===Risk potential===
The Dunedin Volcano has the distinction amongst the [[Earth:South Island surface volcanism|South Island volcanics]] of having a potential magma melt pool still underneath it, as backed up by heat flow<ref name="Godfrey2001">{{cite journal|last1=Godfrey |first1=NJ |last2= Davey |first2=F |last3= Stern |first3=TA |last4= Okaya |first4=D |year= 2001 |title= Crustal structure and thermal anomalies of the Dunedin region, South Island, New Zealand |journal= Journal of Geophysical Research: Solid Earth |volume=106 |issue=30835–30848 |pages=30835–30848 |doi=10.1029/2000JB000006|bibcode=2001JGR...10630835G |doi-access=free }}</ref>  and surface helium measurements.<ref name="Hoke2000">{{cite journal|last1=Hoke |first1=L |last2= Poreda |first2=R |last3= Reay |first3=A |last4= Weaver |first4=SD |year= 2000 |title= The subcontinental mantle beneath southern New Zealand, characterised by helium isotopes in intraplate basalts and gas-rich springs |journal= Geochimica et Cosmochimica Acta|volume=64 |issue=14 |pages=2489–2507| doi=10.1016/S0016-7037(00)00346-X|bibcode=2000GeCoA..64.2489H }}</ref> Accordingly there may now be a gradually re-accumulating 10 million year melt which could, with a low risk of it happening, in due course become manifest as active surface volcanism again.<ref name="Godfrey2001"/> 
[[File:Otago Harbour panorama.jpg|center|500px|thumb|The Dunedin Volcano was originally centred on Quarantine Island (centre of the image) and is now highly eroded. There is speculation that it may have been up to 1000m high.<ref name="VFSDV"/> It is still close to the centre of the present magma hot spot underneath Portobello.<ref name="Godfrey2001"/><ref name="Scott2020"/>]].

==See also==
*[[Earth:Dunedin volcanic group|Dunedin volcanic group]]
*[[Earth:Banks Peninsula Volcano|Banks Peninsula Volcano]]

==References==
{{reflist}}

[[Category:Extinct volcanoes]]
[[Category:Polygenetic shield volcanoes]]

{{Sourceattribution|Dunedin Volcano}}