The fol/owing three papers formed a symposium on "Vulcanicity and VegetatiOl' in the

Rotorua District" at the 1962 Conference.

GEOLOGY OF THE ROTORUA DISTRICT

J. HEALY

N.2.c'Geological Survey, D.S.1.R., Rotorua

INTRODUCTION

The Rotorua district is 3500 square milesin area, extending north from Maroa andMurupara to the Bay of Plenty coastbetween Matata and Tauranga. The TaupoVolcanic Zone, which lies as a belt north-east from Ruapehu to White Island,passesthrough the centre of the area. In thisaccount the rocks have been divided into anumber of groups on a lithologic basis, andtheir distribution and approximate agerange are shown in Figure 1. These groups donot include late Quaternary volcanic ash,which mantles the eniire area superficiallyand in places is more than 40 ft thick.

PHYSIOGRAPHY

Broadly speaking, the district consists oftwo extensive ignimbrite plateaux betweenwhich the Taupo Volcanic Zone extends asa 20-mile wide belt between Maroa and theWhakatane Graben. On the extreme east arethe ranges reaching from Te Whaiti to thesea at Whakatane. They reach altitudes upto 3500 ft. but fall gradually towards thenorth. They are bounded by fault scarps oftwo intersecting systems - a north-southset of dextral transcurrent faults, and a setof north-east faults of normal type, down-thrown to the north-west and parallel tothe Taupo Volcanic Zone. Both systemsform prominent high fault scarps along theeastern side of the Galatea and Waiohaubasins.

The Kaingaroa Plateau has a remarkablyflat surface 1800 ft. above sea level atKaingaroa. To the north there is a 200 ft.scarp around the erosional margin of thetopmost ignimbrite sheet, and the remainderof the plateau is more dissected and fallsgradually towards Te Teko, where it endsagainst the lower Rangitaiki Plains. Theyouthful volcanic cone of Edgecumbe risesfrom the western side of the plateau, which

has a sharp descent into the WhakataneGraben and Reporoa lowlands, but on theeast it slopes gently down to the RBngitaikiRiver. This flows along the faul: anglebetween plateau and ranges, tlowing inignimbrite gorges between the tectonicbasins of Galatea, Waiohau and Te Mahoe.

North-west of the Taupo Volcanic Zonethe Patetere Plateau is gently arch, d alonga north-south axis at an elevation of 2000 ft.,but terminates on the north against theKaimai Ranges and Whakamarama Plateau,also an ignimbrite plateau. On the west itdescends towards the Tokoroa Plat"au, andon . the east it gives way to the ;(aharoaPlateau, which north of Lake Rotorua is alittle over 1500 ft. above sea level hut fallsto 1200 ft. north of Rotoma and 1(00 ft. atMatata. On the north this plateau descendsgradually to sea level in Makettl Basin.Tauranga Basin is a tectonic fe,lture inwhich the post-gracial rise of sea-level pro-duced a drowned valley system \vhich isnow Tauranga Harbour, cut off frolll the seaby large sand spits and bars. Make:u Basinis occupied by swamp, and WhakataneGraben has been filled by Holocene volcanicdetrital material.'A remnant of the KaharoaPlateau remains between RotolUa andOkataina, Rotorua Caldera being a basinwithin the plateau. ,

The Taupo Volcanic Zone contains variedstructural and volcanic relief. Maroa Vol-canic Centre is a cluster of rhyolite domesand flows, 10 miles in diameter; from whichthe slopes fall to north-west and south-eastinto Atiamuri and Reporoa valley!; respec-tively. Both valleys extend north-east for 15

miles at the feet of the flanking ignimbriteplateaux, and between them are a series ofnorth-east ranges. These are faul t blockstilted away from Ngakura valley, which isa central graben.

Farther north-east are the huge volcanic- piles of Tarawera and Haroharo. 1 hese ar~

""....

<c:t'"(')>z-(')->-i"'

>z"<[!Jtn

~-o.z,

LEGEND

Alluvium, .sand-dun~s. swamps,

etc.

Alluvium, lake sediments, tuffs.

Rhyolitic lavas.

Pumice flows (non-welded).

Andesitic and docitic lavas and

ignimbrites.

Lake sediments and interbeded

tuffs.

Miscellaneous non~morine

pumiceous sediments.

Marine sandstones.

Ignimbrites (welded types).

Greywocke.-- - -,",

FIGURE 1. Geological map of the Rotorua area

HEALY: (;EOLOGY OF ~OTORUA DISTRICT 55

rhyolite lavas of the Okataina VolcanicCentre, extruded on the floor of HaroharoCaldera, within which Rotokakahi, Tikitapu,Okareka, Okataina, the eastern end ofRotoiti, Rotoehu and Rotoma are marginallakes dammed by the lavas, and Taraweralake is a remaining part of the floor blockedoff by coalescing flows from Tarawera andHaroharo. Two miles downstream from theoutlet, the Tarawera River plunges over themargin of one of the flows, and enters thelower Tarawera Valley.

North-east of Tarawera volcano the TaupoZone narrows abruptly and enters theWhakatane Graben. This is bounded bysteep fault scarps south of Matata and atAwakeri. Seaward the graben probablyenters White Island Trench (Fleming 1952).

ROCKS OF THE DISTRICT

The oldest rocks, usually known as "grey-wackes", are sandstones, argillites and con-glomerates, which form the ranges east ofthe Rangitaiki River, and two small rangesnear Otamarakau. Jurassic fossils have beenfound at Awakeri and east of Taneatua,though the rocks may locally be LowerCretaceous elsewhere. These are the base-ment rocks of the region. Beneath theKaingaroa Plateau they are believed on geo-physical evidence to lie above sea level(Beck & Robertson 1955), but in the TaupoVolcanic Zone are faulted down to muchgreater depths (Modriniak & Studt 1959).

The most widely distributed rocks areignimbrites. These are believed to be theproducts of large scale fissure eruptions,probably from ring fractures (Healy 1962).The material, of ash, lapilli and blockgrade, spread as pyroclastic pumice flowsand settled by compaction after coming torest hot enough to retain plasticity and tobecome welded in the lower parts. Onesheet, the Matahina Ignimbrite (5 on theeastern side), flowed at least 70 miles, andindividual sheets had volumes which insome cases exceeded' 50 cubic miles.

The ignimbrites vary in colour from pinkand mauve to grey and even black. Theymay contain abundant phenocrysts ofplagioclase and quartz, or very few; andgenerally the upper and softer zones contain

a greater amount of pumice,

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metimescompletely recrystallized during t Ie coolingprocess, which weather to give t, e rock ahoney-combed appearance. The ldest oneis the Te Kopia Ignimbrite (1 , a darkglassy, crystal-rich type which orms thebase of the Paeroa Fault scarp. T e PaeroaIgnimbrite (2), which forms t e highestpart of the Paeroa Range and the lock westof it, is a light grey quartz-rich type similarto the Rangitaiki Ignimbrite bu( differingin containing abundant biotite. 'J!he Rangi-taiki Ignimbrites form the bas~ of theKaingaroa Plateau and outcrop

I

on bothflanks. On top of the plateap. are theKaingaroa Igni.mbrites (6), including anupper hard, fine-grained type

1th minor

plagioclase and no quartz, and a I wer darkgrey to black, glassy type contai 'ng lumpsof white and grey-streaked pumi"".

The Waiotapu Ignimbrite (3) lis red topink in colour, contains no quarjtz, and atWaiotapu contains abundant c<!>arse flat-tened lenticles. On the Patetere Piateau andfarther west it is generally pinkl and finergrained. The Matahina and Mam<l.ku ignim-brites (5) were spread in the spape of ahuge circle centred on Rotorua qtldera andOkataina Volcanic Centre, from 'iihich theywere probably erupted. The UPPer parts ofthese two ignimbrites are soft and pumice-ous and the lower parts dense arid welded,and both contain quartz.

Those described above are rpyolitic todacitic in composition, but igni~brites ofthe Whakamarama and Kaimail area aredacite and those of the Papamci>a Rangeswest of Te Puke are andesitic.1 They areolder than the ignimbrites erupte<!l from theTaupo Volcanic Zone. i

Some ignimbrites are extremely pumice-ous and are not welded, though

I they maybe well compacted, and some, eriIpted intowater, were interbedded with iake sedi-ments. Rocks of this type form th~ southernhalf of the Paeroa Range (oldest Iformationof the Huka Group-Grindley 1959), andolder ones form the block we!!t of theNgakura Graben (Ohakuri Forination ofGrindley). Still younger, loosely ~ompactedpumice breccias formed by Ji>yroclasticflows from the Maroa Volcanic <Centre (1)and from vents in and around th~ Okataina

56 VULCANICITY AND VEGETATION

Volcanic Centre (2) are the youngest bedsof this type with any extent. These includethe Waitahanui Breccias of Grange (1937)and Hapacangi Rhyolite Pumice and MihiBreccia of Grindley (1959).

The final stage of the volcanism at the twocentres was the eruption of rhyolite lavasin the forra of domes, and there are notableaccumulations of rhyolite at Maroa andOkataina. In general the rhyolites of theouter. rings west of both centres are olderthall'thos(' within the centres, and are con~sidered to have been erupted from surround-ing 'ring fractures. In Rotorua Caldera thecentral domes are similarly younger thanthose of ':he north and west rim. All areincluded in the Haparangi and Patetererhyolites of Grange (1937). The oldest rhyo-lites are :,hose of the Tauranga, Papamoaand Kaimai' district, west of the TaupoVolcanic Zone. The oldest rhyolites areusuaJly lithic and spherulitic, whereas theyounger ones have undergone little erosionand still,retain their, glassy obsidianitic andpumiceou:iouter skins. The youngest domeat,Maroa ,s Puketarata, which may be about10,000 yeas old. At ,Tarawera the youngestdomes ar(' about 900 years old. '

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At Maaawahe 'and near Matata areandesites which possibly form part of alarge; volcanic complex almost entirelyburied'by later sediments and ignimbrites:'Together with andesite near Te Puke and!lorth of the Kaimai Ranges these couldpossible be correlated with andesites of theCoromandel Range which are of Mioceneage. They are hornblende andesites, whereasEdgecumbe is a young hyperstheneandesite similar in composition to the rocksof White Island and Whale Island. It maybe little older than 1000 years, beingcovered only by Kaharoa Ash about 900years old. Maungaongaonga and Maunga-kakaramEa are young hornblende dacitevolcanoes of late Pleistocene age atWaiotapu.

The only dated sediments younger thanthe Mesozoic basement rocks are marinesandstoms and conglomerates at Matata.There are several hundred feet of well com-pacted sandstones:. containing fossils ofupper Castlecliffian age. Younger conglom-erates containing pebbles of greywacke and

ignimbrites pass' up into 1 pumiceous tuffsover:' which Jies the Matahina Ignimbrite,taken to be of Hawera age. Siltstones andsandstones with lignites ,and interbeddedpumiceous tuffs border the Tauranga Har-bour, and include the Tauranga beds ofHenderson and Bartrum (1913). Includedalso are sands, conglomerates and tuffs atMaketu, Matata and Awakeri. They range inage from Castlecliffian' to Hawera. YoungerHawera siltstones, sandstones and tuffswere laid down in lakes jin the Ngakuru,Horohoro and Rotorua areas. The bulk ofthe Holocene sediments is alluvial pumice.

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LATE QUATERNARY VOLCANIC ASH.

Late Quaternary volcanic ash mantles theentire district. In the south this consistsalmost entirely of pumice ash erupted fromnear Taupo (Baumgart 1954, Baumgart &Healy 1956, Healy in press), but it alsocOIitains ash erupted from Puketarata andelsewhere in the Maroa tVolcanic Centre.The remaining part of: the district iscovered byash erupted fr6m vents either inor around the Okataina ,Volcanic Centre.The upperm'ost beds consist of' a series ofbrown pumice ash ana lapilli layers(Grange 1937, Vucetich! & Pullar 1962):Beneath these are two thick pumice lapilliarid blocks' beds which aggregate 60 ft. atRotoehu; 12 ft. at Te PUKe, AO ft. at Puke-hi ria and 25 ft. at Awakeii: These brecciasdb""ri'ot appear to rest on the Haroharorhyolites within the Okatilina Centre. Else-where they are underlain by older ashmembers and breccias, a good section ofwhich is exposed in the cliffs east ofOtamarakau. The ash beds pass down hereinto pumice flows mentioned earlier. Thelatter were probably erupted during the lastinterglacial stage, because valleys were cutin them during the last glaciation. Youngerterraces within the vaJleys can be distin-guished by the presence or absence of someof the ash formations. I

GEOLOGICAL HISTORY

During the latter part of the Tertiary, theTaupo Volcanic Zone was elevated andstripped of any marine Tertiary cover it hadreceived. :There is no clear evidence of thedate of the beginning of volcanism, but

HEALY: GEOLOGY OF ROTORUA DISTRICT

pumice outwash is first noted in Opoitiansediments (Pliocene). Subsequent ignim-brite eruptions were accompanied by sub-sidence within the Taupo Volcanic Zone, butthe early history is complex and not yetunderstood. In the surrounding plateauxthere is evidence that periods of erosionintervened between major ignimbrite out-pourings, but within the Zone sedimentationmay have continued in depressions andlakes formed as a result of the subsidence.

Within the large circular area betweenMokai and Waiotapu sedimentation tookplace in one or more lakes intermittentlyduring the latter part of the Pleistocene,following cauldron subsidence on a largescale. Subsequently intrusion appears tohave arched up the central part along anorth-east axis, probably reaching itsgreatest elevation where the Maroa Centreis now located. Lacustrine deposits were laiddown, but eruptions at intervals depositedalso bands of tuff, and ocasional pyroclasticflows laid down pumice breccias. Thedeposits were repeatedly tilted and dis-located because of the uparching, thenfaulted down along the crest as a result ofthe withdrawal of magma at the time oferuptions.

Finally the eruption of pumice flows (1)produced marked subsidence in the Maroaarea, followed by the extrusion of rhyolitedomes accompanied by pumice explosiveeruptions. The youngest domes are on theeastern side associated with branches of thePaeroa Fault, which has the youngest andhighest scarp of the faults east of NgakuruGraben, indicating that the intrusive anderuptive processes, although concentratedalong a north-east axis through Maroa, hada progressive tendency towards the south-east. This has also been suggested by Grind-ley (1959).

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Nearer Rotorua the general sequence ofevents was similar, but there the intrusionhas not been associated idth a large foldedand faulted structure such as NgakuruGraben and neighbouring fault blocks. Theeruption of the, Mamaku, Matahina andKaingaroa Ignimbrites presumably pro-duced calderas at Rotorura and Haroharo.Since then there have been further' extru-sions of rhyolite within Rotonia'HCaldera,

57

and a number of benches indi\:ate pro-gressively lower lake levels. The; topmostone, about 300 ft. above present ];,ke level,does not occur on Mokoia Island, which is

,

youi1ger.I

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In the Okataina Centre eruptivl' activitycontinued. During the last interglacial stagethere were large eruptions of pumice aspyroclastic flows to the north imd east.These were accompanied by caldera forma-ti!'>!).'and, the partial draining; of, LakeROtorua. Presumably the drainage from theenlarged caldera flowed into the WhakataneGraben, though it was not then ,Is deep asnow. The digitate arms of Rotoel)u are theresult of headward erosion during thisstage. Finally the eruption of rhyi,lite lavasand domes at Haroharo, Tarawera andother centres commenced, acom)Janied byintermittent ash eruptions. Tl)e coarselapilli and block beds may hav,l been anearly product of this series. As the domesaccumulated the centre of th~ calderabecame filled and lakes formed in the re-,maining marginal hollows.

I

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Sediments and water laid tuffs whichoverlie the coarse pumice lapilli 'and blockbeds along the north shores of Rotoiti,Rotoehu and Rotomalakes proqably post-date the rise of Haroharo volcan() and indi-cate that the lakes rose in level and wereconnected to Lake Rotorua t,':mporarilyuntil the Kaituna River spilled over to thenorth and drained the lakes down to their,

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present levels. However, som<; warpingseems to have occurred, because, the aboveyoung sed.\lIlents and tuffs read, a higherh,vel at the west end of Rotoiti than they doat the east. Also tectonic activity! continuedin the Whakatane Graben, ,the ,downwardmovement being offset by alluvia1ion.About900 years ago the three domes or Tarawerawere extruded and there was i a violentpumice eruption which showered KaharoaAsh over the district., One or milre pumicelahars swept down the Taraweni River andbuilt a large fan on which Ka,verau nowstands, and which forms a wi:ll definedterrace up the river. In 1904 part'of this wasremoved by a flood caused by' the lQweringof. the outlet of Lake Tarawera,!whiChhadbeen. raised by an unknown amlmnt ,,af thetirt\~"bf the 1886 eruptioh of Ta~iweiit:' The

58 VULCANICITY AND VEGETATION

1886 eruption also blocked the valley lead-ing from Rotomahana to Tarawera lake andbrought into existence a much larger andmore elevated Rotomahana., The domebuilding and associated explosive eruptivephase of the Okataina Volcanic Centreactivity presumably still continues.

REFERENCES

BAUMGART,L. L., 1954. Some ash showers of theCeniral North Island. NZ. 1. Sci. Tech. 35B:457",,7.

BAUMGART,!. L., and HEALY..J" 1956. Recent volcani-:: city at faup? New Zealand. Proc. 8th Padf.

" SCt. Con gr. 2.113-25. ,

BECK, A. C., and ROBERTSON,E. I., 1955. Geothermal, steam for power in New Zealand (comp. L. 1., Grange). In Geology and Geophysics. Bull. N.Z.I,

Dep. SeL Industr. Res. 117.,

FLEMING, C. A., 1952. The White Island Trench; a: submarine graben in the Bay of Plenty, New,

Zealand. Proc. 7th Pacit. Sci. Congr. 3: llG-213.GRANGE,L. 1., 1937. The geology of the Rotorua-

Taupo Subdivision. Bull. Geol. Surv. N.Z. (n.s.), '37. I

GRINDLEY,G. W., 1959. Sheet N8S, Waiotapu. Geol.Map of N.Z., 1:63,360. GovI.' Printer, Wellington.

HEALY, J" 1962. Structures' and volcanism in theTaupo Volcanic Zone, New Zealand. Amer., Geophys. Un., Geophys. Mon. 6: 151-157.

HEALY,J., in press. Dating of the younger vol-canic eruptions of the Taupo region, New Zea.land in Bull. Gool. Surv. N.Z. (n.s.) 73 (I).

HENDERSON,J., and BARTRUM,J. A., 1913. The geologyof the Aroha Subdivision, "Hauraki, Auckland.Bull. Geol. N.Z. (n.e.) 16. !

MODRINIAK,N., and STUDT,F. E., t959. Geologicals~ruc;ture and volcanism of the Taupo-Taraweradrstnci. NZ. 1. Geol. Geophys. 2: 654-<\84.

VUCETICH,C. G., and PULLAR,W. A., in press. Holo-cene ash stratigraphy of' the Rotorua 'andGisborne district. in Bull. Geo. Surv. N.Z. (n.s.)73 (2).

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VULCANICITY AND INDIGENOUS VEGETATION IN THE,

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ROTORUA DISTRICT:

J. L. NICHOLLS

Forest Research Institute, Rotorua

INTRODUCTION

Although most of the country roundRotorua is climatically and edaphically suit-able for forest, only a third was covered byabout the middle of last century. There wasone 'major tract on high land between thecoastal lowlands and the upper Thames(Waihou) and Waikato valleys; and a fewoutliers on high, ground within inlandvalleys; The Kaingaroa plateau was nearlydevoid of forest. Throughout the districtthere were small patches of forest in thegorges flan king the uplands, in folds of thehills in generally' open country, and clumpsof swamp forest on the plains. Apart fromdeep'swamps, sand dunes, and the nearlybare summits of Mt'. Tarawera, the rest wasmainly scrub and fern; with heath andtussock in the south. '

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, This pattern suggests that a nearly univer-sal forest had been reduced by fire, probablyby the Polynesians living in middle Waikatovalley, along the Bay of Plenty, and aboutthe Rotorua lakes., Fire was their only means

of clearing land, but they could not controlit. Frequent burning wasi'necessary for ashifting' cultivation, to retain fern land forthe staple fern root, and to maintain com-munications. In recorded; local traditionslarge fires are rarely mentioned, but earlyEuropeans 'commented on them and on thenatives' casual attitude towards them.

The recent dating of the last violent erup;tions from the east Taupo "olcanic centre atonly 1;800 years ago sugges!ed that the thenforest had been devastated' and had notsince returned. But there is pedologicalevidence of former more widespread forestson soils of the Taupo suite (Vucetich, un-pub.). Furthermore, forests now in this areaare on southerly (the wetter) faces: theaspect expected from fires, but not fromvolcanic blasts from the direction of Taupo.A general "post-Taupo" fOl;-est would prob-ably have been densely stocked with rimu,miro, matai, totara, and maires (Dacrydiumcupressinum, Podocarpus I ferrugineus, ,P.spicatus, P. to tara, and Gynmelaea spp.) ; 'itwould have grown largely, on coarse soils