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 has a sharp descent into the Whakatane Graben and Reporoa lowlands, but on the The Rotorua district is 3500 square miles east it slopes gently down to the RBngitaiki in area, extending north from Maroa and River. This flows along the faul: angle Murupara to the Bay of Plenty coast between plateau and ranges, tlowing in between Matata and Tauranga. The Taupo ignimbrite gorges between the tectonic Volcanic Zone, which lies as a belt north- basins of Galatea, Waiohau and Te Mahoe. east from Ruapehu to White Island,passes through the centre of the area. In this North-west of the Taupo Volcanic Zone account the rocks have been divided into a the Patetere Plateau is gently arch, d along number of groups on a lithologic basis, and a north-south axis at an elevation of 2000 ft., their distribution and approximate age but terminates on the north against the range are shown in Figure 1. These groups do Kaimai Ranges and Whakamarama Plateau, not include late Quaternary volcanic ash, also an ignimbrite plateau. On the west it descends towards the Tokoroa Plat"au, and which mantles the eniire area superficially . and in places is more than 40 ft thick. on the east it gives way to the ;(aharoa Plateau, which north of Lake Rotorua is a little over 1500 ft. above sea level hut falls PHYSIOGRAPHY to 1200 ft. north of Rotoma and 1(00 ft. at Matata. On the north this plateau descends Broadly speaking, the district consists of gradually to sea level in Makettl Basin. two extensive ignimbrite plateaux between Tauranga Basin is a tectonic fe,lture in which the Taupo Volcanic Zone extends as which the post-gracial rise of sea-level pro- a 20-mile wide belt between Maroa and the duced a drowned valley system \vhich is Whakatane Graben. On the extreme east are now Tauranga Harbour, cut off frolll the sea the ranges reaching from Te Whaiti to the by large sand spits and bars. Make:u Basin sea at Whakatane. They reach altitudes up is occupied by swamp, and Whakatane to 3500 ft. but fall gradually towards the Graben has been filled by Holocene volcanic north. They are bounded by fault scarps of detrital material.'A remnant of the Kaharoa two intersecting systems - a north-south Plateau remains between RotolUa and set of dextral transcurrent faults, and a set Okataina, Rotorua Caldera being a basin of north-east faults of normal type, down- within the plateau. , thrown to the north-west and parallel to the Taupo Volcanic Zone. Both systems The Taupo Volcanic Zone contains varied form prominent high fault scarps along the structural and volcanic relief. Maroa Vol- eastern side of the Galatea and Waiohau canic Centre is a cluster of rhyolite domes basins. and flows, 10 miles in diameter; from which the slopes fall to north-west and south-east The Kaingaroa Plateau has a remarkably into Atiamuri and Reporoa valley!; respec- flat surface 1800 ft. above sea level at tively. Both valleys extend north-east for 15 Kaingaroa. To the north there is a 200 ft. miles at the feet of the flanking ignimbrite scarp around the erosional margin of the plateaux, and between them are a series of topmost ignimbrite sheet, and the remainder north-east ranges. These are faul t blocks of the plateau is more dissected and falls tilted away from Ngakura valley, which is gradually towards Te Teko, where it ends a central graben. against the lower Rangitaiki Plains. The youthful volcanic cone of Edgecumbe rises Farther north-east are the huge volcanic from the western side of the plateau, which - piles of Tarawera and Haroharo. 1 hese ar~ ....""
LEGEND
< c: Alluvium, .sand-dun~s. swamps, t'" (') etc. -z> Alluvium, lake sediments, tuffs. -(') Rhyolitic lavas. >-i "' Pumice flows (non-welded). >z Andesitic and docitic lavas and " ignimbrites. < [!J Lake sediments and interbeded tn
tuffs. ~ -o. Miscellaneous non~morine z,
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 Volcanic a greater amount of pumice, metimes Centre, extruded on the floor of Haroharo completely recrystallized during t Ie cooling Caldera, within which Rotokakahi, Tikitapu, process, which weather to give t,~e rock a Okareka, Okataina, the eastern end of honey-combed appearance. The ldest one Rotoiti, Rotoehu and Rotoma are marginal is the Te Kopia Ignimbrite (1 , a dark lakes dammed by the lavas, and Tarawera glassy, crystal-rich type which orms the lake is a remaining part of the floor blocked base of the Paeroa Fault scarp. T e Paeroa off by coalescing flows from Tarawera and Ignimbrite (2), which forms t e highest Haroharo. Two miles downstream from the part of the Paeroa Range and the lock west outlet, the Tarawera River plunges over the of it, is a light grey quartz-rich type similar margin of one of the flows, and enters the to the Rangitaiki Ignimbrite bu( differing lower Tarawera Valley. in containing abundant biotite. 'J!he Rangi- taiki Ignimbrites form the bas~ of the North-east of Tarawera volcano the Taupo Kaingaroa Plateau and outcrop on both Zone narrows abruptly and enters the flanks. On top of the plateap.I are the Whakatane Graben. This is bounded by Kaingaroa Igni.mbrites (6), including an steep fault scarps south of Matata and at upper hard, fine-grained type th minor Awakeri. Seaward the graben probably plagioclase and no quartz, and a I wer dark enters White Island Trench (Fleming 1952). grey to black, glassy type contai 'ng lumps of white and grey-streaked pumi"".1 ROCKS OF THE DISTRICT The Waiotapu Ignimbrite (3) lis red to The oldest rocks, usually known as "grey- pink in colour, contains no quarjtz, and at wackes", are sandstones, argillites and con- Waiotapu contains abundant carse flat- glomerates, which form the ranges east of tened lenticles. On the Patetere Piateau and the Rangitaiki River, and two small ranges farther west it is generally pinkl and finer near Otamarakau. Jurassic fossils have been grained. The Matahina and Mam Volcanic Centre (2) are the youngest beds ignimbrites pass' up into 1 pumiceous tuffs of this type with any extent. These include over:' which Jies the Matahina Ignimbrite, the Waitahanui Breccias of Grange (1937) taken to be of Hawera age. Siltstones and and Hapacangi Rhyolite Pumice and Mihi sandstones with lignites ,and interbedded Breccia of Grindley (1959). pumiceous tuffs border the Tauranga Har- bour, and include the Tauranga beds of The final stage of the volcanism at the two Henderson and Bartrum (1913). Included centres was the eruption of rhyolite lavas also are sands, conglomerates and tuffs at in the forra of domes, and there are notable Maketu, Matata and Awakeri. They range in accumulations of rhyolite at Maroa and age from Castlecliffian' to Hawera. Younger Okataina. In general the rhyolites of the Hawera siltstones, sandstones and tuffs outer. rings west of both centres are older were laid down in lakes jin the Ngakuru, thall'thos(' within the centres, and are con~ Horohoro and Rotorua areas. The bulk of sidered to have been erupted from surround- the Holocene sediments is alluvial pumice. ing 'ring fractures. In Rotorua Caldera the 1- ' " central domes are similarly younger than , i LATE QUATERNARY VOLCANIC ASH those of ':he north and west rim. All are . included in the Haparangi and Patetere Late Quaternary volcanic ash mantles the rhyolites of Grange (1937). The oldest rhyo- entire district. In the south this consists lites are :,hose of the Tauranga, Papamoa almost entirely of pumice ash erupted from and Kaimai' district, west of the Taupo near Taupo (Baumgart 1954, Baumgart & Volcanic Zone. The oldest rhyolites are Healy 1956, Healy in press), but it also usuaJly lithic and spherulitic, whereas the cOIitains ash erupted from Puketarata and younger ones have undergone little erosion elsewhere in the Maroa tVolcanic Centre. and still,retain their, glassy obsidianitic and The remaining part of: the district is pumiceou:iouter skins. The youngest dome covered byash erupted fr6m vents either in at,Maroa ,s Puketarata, which may be about or around the Okataina ,Volcanic Centre. 10,000 yeas old. At ,Tarawera the youngest The upperm'ost beds consist of' a series of ' domes ar(' about 900 years old. " brown pumice ash ana lapilli layers At" Maaawahe 'and near Matata are ( Grange 1937, Vucetich! & Pullar 1962): andesites which possibly form part of a Beneath these are two thick pumice lapilli large; volcanic complex almost entirely arid blocks' beds which aggregate 60 ft. at buried'by later sediments and ignimbrites:' Rotoehu; 12 ft. at Te PUKe, AO ft. at Puke- Together with andesite near Te Puke and hi ria and 25 ft. at Awakeii: These breccias !lorth of the Kaimai Ranges these could db""ri'ot appear to rest on the Haroharo possible be correlated with andesites of the rhyolites within the Okatilina Centre. Else- Coromandel Range which are of Miocene where they are underlain by older ash age. They are hornblende andesites, whereas members and breccias, a good section of Edgecumbe is a young hypersthene which is exposed in the cliffs east of andesite similar in composition to the rocks Otamarakau. The ash beds pass down here of White Island and Whale Island. It may into pumice flows mentioned earlier. The be little older than 1000 years, being latter were probably erupted during the last covered only by Kaharoa Ash about 900 interglacial stage, because valleys were cut years old. Maungaongaonga and Maunga- in them during the last glaciation. Younger kakaramEa are young hornblende dacite terraces within the vaJleys can be distin- volcanoes of late Pleistocene age at guished by the presence or absence of some Waiotapu. of the ash formations. I The only dated sediments younger than the Mesozoic basement rocks are marine GEOLOGICAL HISTORY sandstoms and conglomerates at Matata. During the latter part of the Tertiary, the There are several hundred feet of well com- Taupo Volcanic Zone was elevated and pacted sandstones:. containing fossils of stripped of any marine Tertiary cover it had upper Castlecliffian age. Younger conglom- received. :There is no clear evidence of the erates containing pebbles of greywacke and date of the beginning of volcanism, but HEALY: GEOLOGY OF ROTORUA DISTRICT 57 pumice outwash is first noted in Opoitian and a number of benches indi\:ate pro- sediments (Pliocene). Subsequent ignim- gressively lower lake levels. The; topmost brite eruptions were accompanied by sub- one, about 300 ft. above present ];,ke level, sidence within the Taupo Volcanic Zone, but does not occur on Mokoia Island, which is the early history is complex and not yet youi1ger. , " ' understood. In the surrounding plateaux I there is evidence that periods of erosion In the Okataina Centre eruptivl' activity intervened between major ignimbrite out- continued. During the last interglacial stage pourings, but within the Zone sedimentation there were large eruptions of pumice as may have continued in depressions and pyroclastic flows to the north imd east. lakes formed as a result of the subsidence. These were accompanied by caldera forma- ti!'>!).'and, the partial draining; of, Lake Within the large circular area between ROtorua. Presumably the drainage from the Mokai and Waiotapu sedimentation took enlarged caldera flowed into the Whakatane place in one or more lakes intermittently Graben, though it was not then ,Is deep as during the latter part of the Pleistocene, now. The digitate arms of Rotoel)u are the following cauldron subsidence on a large result of headward erosion during this scale. Subsequently intrusion appears to stage. Finally the eruption of rhyi,lite lavas have arched up the central part along a and domes at Haroharo, Tarawera and north-east axis, probably reaching its other centres commenced, acom)Janied by greatest elevation where the Maroa Centre intermittent ash eruptions. Tl)e coarse is now located. Lacustrine deposits were laid lapilli and block beds may hav,l been an down, but eruptions at intervals deposited early product of this series. As the domes also bands of tuff, and ocasional pyroclastic accumulated the centre of th~ caldera flows laid down pumice breccias. The became filled and lakes formed ,in the re- deposits were repeatedly tilted and dis- maining marginal hollows. I . - I located because of the uparching, then -', faulted down along the crest as a result of ." Sediments", and water laid tuffsI which the withdrawal of magma at the time of overlie the coarse pumice lapilli 'and block eruptions. beds along the north shores of Rotoiti, Finally the eruption of pumice flows (1) Rotoehu and Rotomalakes proqably post- produced marked subsidence in the Maroa date the rise of Haroharo volcan() and indi- area, followed by the extrusion of rhyolite cate that the lakes rose in level and were domes accompanied by pumice explosive connected to Lake Rotorua t,':mporarily eruptions. The youngest domes are on the until the Kaituna River spilled over to the north and drained the lakes down, to their eastern side associated with branches of the ' Paeroa Fault, which has the youngest and present levels. However, som<; warping highest scarp of the faults east of Ngakuru seems to have occurred, because, the above Graben, indicating that the intrusive and young sed.\lIlents and tuffs read, a higher eruptive processes, although concentrated h,vel at the west end of Rotoiti than they do along a north-east axis through Maroa, had at the east. Also tectonic activity! continued a progressive tendency towards the south- in the Whakatane Graben, ,the ,downward east. This has also been suggested by Grind- movement being offset by alluvia1ion.About , ' ' 900 years ago the three domes or Tarawera ley (1959). were extruded and there was i a violent Nearer Rotorua the general sequence of pumice eruption which showered Kaharoa events was similar, but there the intrusion Ash over the district., One or milre pumice has not been associated idth a large folded lahars swept down the Taraweni River and and faulted structure such as Ngakuru built a large fan on which Ka,verau now Graben and neighbouring fault blocks. The stands, and which forms a wi:ll defined eruption of the, Mamaku, Matahina and terrace up the river. In 1904 part'of this was Kaingaroa Ignimbrites presumably pro- removed by a flood caused by' the lQwering duced calderas at Rotorura and Haroharo. of. the outlet of Lake Tarawera,!whiChhad Since then there have been further' extru- been. raised by an unknown amlmnt ,,af the sions of rhyolite within Rotonia'HCaldera, tirt\~"bf the 1886 eruptioh of Ta~iweiit:' The 58 VULCANICITY AND VEGETATION 1886 eruption also blocked the valley lead- FLEMING, C. A., 1952. The White Island Trench; a ,: submarine graben in the Bay of Plenty, New ing from Rotomahana to Tarawera lake and Zealand. Proc. 7th Pacit. Sci. Congr. 3: llG-213. brought into existence a much larger and GRANGE,L. 1., 1937. The geology of the Rotorua- more elevated Rotomahana., The dome Taupo Subdivision. Bull. Geol. Surv. N.Z. (n.s.) , '37. I building and associated explosive eruptive GRINDLEY,G. W., 1959. Sheet N8S, Waiotapu. Geol. phase of the Okataina Volcanic Centre Map of N.Z., 1:63,360. GovI.' Printer, Wellington. activity presumably still continues. HEALY, J" 1962. Structures' and volcanism in the Taupo Volcanic Zone, New Zealand. Amer. , Geophys. Un., Geophys. Mon. 6: 151-157. HEALY,J., in press. Dating of the younger vol- REFERENCES canic eruptions of the Taupo region, New Zea. land in Bull. Gool. Surv. N.Z. (n.s.) 73 (I). BAUMGART,L. L., 1954. Some ash showers of the HENDERSON,J., and BARTRUM,J. A., 1913. The geology Ceniral North Island. NZ. 1. Sci. Tech. 35B: of the Aroha Subdivision, "Hauraki, Auckland. 457",,7. Bull. Geol. N.Z. (n.e.) 16. ! BAUMGART,!. L., and HEALY..J" 1956. Recent volcani- MODRINIAK,N., and STUDT, F. E., t959. Geological :: city at faup? New Zealand. Proc. 8th Padf. s~ruc;ture and volcanism of the Taupo- Tarawera " SCt. Con gr. 2.113-25. , drstnci. N Z. 1. Geol. Geophys. 2: 654-<\84. BECK, A. C., and ROBERTSON,E. I., 1955. Geothermal VUCETICH,C. G., and PULLAR,W. A., in press. Holo- , steam for power in New Zealand (comp. L. 1. cene ash stratigraphy of' the Rotorua 'and , Grange). In Geology and Geophysics. Bull. N.Z. Gisborne district. in Bull. Geo. Surv. N.Z. (n.s.) I, Dep. SeL, Industr. Res. 117. 73 (2). , , " ' ' VULCANICITY AND INDIGENOUS, VEGETATION IN THE ' ROTORUA DISTRICT: J. L. NICHOLLS Forest Research Institute, Rotorua INTRODUCTION of clearing land, but they could not control it. Frequent burning wasi'necessary for a Although most of the country round shifting' cultivation, to retain fern land for Rotorua is climatically and edaphically suit- the staple fern root, and to maintain com- able for forest, only a third was covered by munications. In recorded; local traditions about the middle of last century. There was large fires are rarely mentioned, but early one 'major tract on high land between the Europeans 'commented on them and on the coastal lowlands and the upper Thames natives' casual attitude towards them. (Waihou) and Waikato valleys; and a few The recent dating of the last violent erup; outliers on high, ground within inland tions from the east Taupo "olcanic centre at valleys; The Kaingaroa plateau was nearly only 1;800 years ago sugges!ed that the then devoid of forest. Throughout the district forest had been devastated' and had not there were small patches of forest in the since returned. But there is pedological gorges flan king the uplands, in folds of the evidence of former more widespread forests hills in generally' open country, and clumps on soils of the Taupo suite (Vucetich, un- of swamp forest on the plains. Apart from pub.). Furthermore, forests now in this area deep'swamps, sand dunes, and the nearly are on southerly (the wetter) faces: the bare summits of Mt'. Tarawera, the rest was aspect expected from fires, but not from mainly scrub and fern; with heath and ' volcanic blasts from the direction of Taupo. tussock, in the, south. ." , A general "post-Taupo" fOl;-est would prob- , This pattern suggests that a nearly univer- ably have been densely stocked with rimu, sal forest had been reduced by fire, probably miro, matai, totara, and maires (Dacrydium by the Polynesians living in middle Waikato cupressinum, Podocarpus I ferrugineus, ,P. valley, along the Bay of Plenty, and about spicatus, P. to tara, and Gynmelaea spp.) ; 'it the Rotorua lakes., Fire was their only means would have grown largely, on coarse soils