VUCETICH AND PULLAR: SOILS 65 The Rotomahana shower was a most un- Salisb.); III, Effect of temperatuJ'e and soH usual volcanic event, but nevertheless strik- conditions. Aust. J. Bot. 7: 279-294. ingly demonstrated the resilience of indigen- CRANWELL,L. M., and MOORE,L. 8., 1936.1 The occur.' ous vegetation. rence of kauri in montane forest on Te Moe- hau. N.z. J. Sci. Tech. 18: 531-543. I Ash showers may not always have becn KIRK. T., 1872. Notes on the flora of the Like district catastrophic for vegetation, and even where of the"North' Island. Trans. N.Z. Il1st. 5: 322- forest may have been obliterated by ash 345. I flows a return may not invariably have MASTERS, S. E., HOLLOWAY,J. T., and MsKELVEY"P. involved a long time and protracted plant J., 1957. The national forest survey (1 New Zea- successions. Long-lasting changes may be land, 1955, Vol. 1. Gavt. Printer, Wellington. considered probable only where soils werc MILLENER,L. H., 1953. How old is the velgetation on radically changed. Rangitoto Island? Rept. 2nd Ann.1 Mtg., N.z. Ecot. Soc. 17-18. REFERENCES NICHOLLS,J. L., 1959. The volcanic erupiions of Mt. BIELESKI,R. L., 1959. Factors affecting growth and Tarawera and Lake Rotomahana Imd effects distribution of kauri (Agathis australis on surrounding forests. N2. J. For. ,8: 133-142. ASH BEDS AND SOILS IN THE ROTORUA DISTRICT C. G. VUCETICH and W. A. PULLAR Soil Bureau, Department of Scientific and Industrial Research, Christchurch: and Whakatane INTRODUCTION Named Beds , Tarawera scoria (and Rotomahana mud) erupted During the Late Quaternary, volcanic 1886 Kaharoa Ash 810+ 70' eruptions of the explosive or paroxysmal Taupo Pumice 1700+ 1501 type (Taylor, 1953) occurred in the central Taupo Subgroup, members 9-13 North Island about centres, which for con- Waimihia Ash 3420:t:70' Rotokawau Ash venience, are designated Okataina, Waita- Whakatane Ash hanui, Maroa, and Tokaanu (Fig. 1). The Mamaku Ash resulting ejectamenta formed thick layered Rotoma Ash beds largely of rhyolitic pumice. Most is Taupo Subgroup, members 16--18 8850+ 1000' Waiohau Ash known about the distribution of beds from Rotorua Ash the Okataina and Waitahanui cruptive Rerewhakaaitu Ash centres where named beds have been Unnamed Beds "'- mapped in the Rotorua, Bay of Plenty, and "'X' Beds"; (Ash and lapilli) three in number. in Gisborne districts (Vucetich and Pullar, in Rerewhakaaitu locality and believed of restricted distribution; not mapped because of I few expo- press). Little is known about the Maroa and sures. Tokaanu centres except that they are con- *"Pinkish-brown Beds"; multiple, of lash grade, sidered to have been important contributors weakly weathered; mainly northern p:trt of Kai~ earlier in the Holocene and that they were ngaroa Forest, Wairakei, Waiotapu and :~otorua. also active contemporaneously with the *"Mauve Beds"; three in number' TIainly ".on Mamaku Plateau. , Taupo Pumice eruptions. , . ""'f'.,., "Yellow-block Bed"; mainly along Bay of Plenly and. in ,Gisborne District; not traced . south and STRATIGRAPHY west of Rotorua. I * The "pinkish-brown" and "mauve" bids are not The following ash beds have been recog- seen in stratigraphical order in one section and nised; the age where known is given in may be coeval. Mapping of these together with years before 1950: lower.-beds is proceeding.. ,;. I 66 VUJ.l"'A!\JTrTTV Af\JO ,"Whlte--block. Bed"; mainly along Bay uf Plt::Jlty; Produt.:lS of ash-flow uroppcd by glowing not traced west and south of Rotorua. (Both of the clouds or nuees ardentes are often wide- CiUUVC lUUY be a correlative of Ka:wcrnu Brcccit1.) "Grey-banded Bed"; wide distribution from west spread and more difficult to plot because of of Tauranga to Gisborne 'but southernmost limits violent eruption occurring simultaneously not kn01Nn.' (CoJ:'J:"elative of Rotoiti Bre<'c-i~.)'~[ trom a number of vents:. The cloud~ tr~vPllf"CI ,"Red Beds" or "ginger tuffs" '(Hamilton correla- long distances, transporting vast quantities tive); cOrI'Hated with known Hamilton beds west of Taurang;l and at Matamata; traced along Bay of of pumice and ash at temperatures sufficient- Plenty to Ciisbomei perhaps the most widespread ly high to char wood in their passage across , of aU a~h-fijll beds, I the landscape. Deposits from one eruptive I episode vary according IO LOpography; In 1. N.Z. lAC. 10. N.Z.J. ScL Tech., B36; 371-4 depressions and on valley floors, particu- ,(1954). I larly towards the foot of hill slopes. they l. N.£.. HC. 1. N.L.J. 5C1. Tech., 113.:5; 127-5 (19.:'13). 3. N.Z. 14C.179 N.Z.J. Geol. Geophys., 2: 208-41 are characteristically thick pumice breccias N.Z. ItC. 165 } (1'15'1) froquontly containing charco~l, ~nd on clopoc, they arc ach brocciac Q good dQ~l thinner but of irregular thickness. ERUPTIVE EVENTS I I Vegetatjon wa~ a]~o drowned or buried The ash column records events separated by water and alluvium impounded in tem- by considerable time intervals. Each event porary lakes. I c.i1u~cd c(n:nplctc or portia.! {' dC:5truction; of From ash-fall plans prepared by Vucetich the vegetation within an inner zone and, and Pullar (in pres:s:) the area covered by dm1ng the ensuing period I of quiescence, veg,etatioll established itself and soils began ash more than 24 in. thick is 5,000 square n'lilc.3 j 1TIOrc than 18 in., 7,000 .3quarc milc,:,; to lorm. ] n the preceding cold period (Last Glaciation) the ash column shows erosion and nJ.on; than 12 in., 12,000 "quarc milc", breaks mid it would appear that a number I in the combined districts of Rotorua, Taupo, of orupti~)n.c took plo.cQ ,"vhon tho. upland~ Bay of Plenty, and Northern Hawke's Bay were bad, of vegetation and there was ex- (FIg. I). tcn3ivc clo~ion. The critical thickness of cold ash required I to destroy vegetation cannot be given with Detailed studies of the Taupo Pumice and precision. On Mt. Egmont A. P. Druce (pers. :soils dtai;/cd frOID it are uscd as a bash for comm.) observes that the Burrell Ash the pattei'n of forest destruction and for a deposited cold some 300 years ago had the suggested history of the forest during the following effect on cedar and Hall's totara; 13 In. [lUck compleIe desIrucIlon Late Quaternary. 12-15 in. almost complete destruction 9-12 in. partial destruction hnC<"J.OI of '\'ULO UIT nnd. '\':COC'ThTIOH ThUUli:l:S (1000) levun:s ..hi:l.. ..ltt::: fUlt:::Sl Wi:lS DESTRUCTION not destroyed on Mt. Edgecumbe where Tarawera scoria and ash' fell 14 in. thick. In Ihe viclnlIy of the vents, blast ami If a conservative estimate of ash thickness glowing, avalanches' were the principal for complete destruction is 18 in., then des- agents of destruction and shower-bedded truction on at least one occasion occurred depo!i:itli: beyond are cnnc,;:idprprl to h~vf" I una iUI ct:u::::ct uf 7,000 ~y'UctH::: utik:s. f~1l9n l"l:\lA Qnrt tn h~"p Idl1Prtvpgpt~tinn hy ,burial. This principle is well illustrated in TAUPO PUMICE ERUPTIONS the products of Jhe Kaharoa eruption where , t.:harrt::u lug5 al~ :st:::eu in locali5cd ash.flow A :scdc:s of cl-uptions:: frol11 ccntl.cs aL deposits dropped by glowing avalanches on Waitahanui, Maroa, and' Tokaanu and the' lower slopes of Mt. Tarawera" but the occurrtng at short mtervals are known as ash-fall deposits lurther away do not con- the Taupo Pumice Eruptions. They termin- tain charcoal. Charcoal occurs sparingly in ated with particularly violent explosions ash-fall deposits and is usually to be found from vents which appearito have extended in contact with or within the buried soil beyond the centres named and with ash- rather thi~n in the overlying ash bed5. flQW material pend.5ting for long d15tal1ct:::t. , VUCETICH AND PULLAR: SOILS 67 ERUPTIVE CENTRES 12" lSOAIrICH ...-18" ISOPACH FIGURE 1. Area in which forest was destroyed by vulcanicity on at least one occasion during the Holocene. Destruction is postulated to be complete with deposits greater than 18 in. thick and partial with those 12-18 in. thick. The timetable, form of eruption, and pat- Phase 2 tern of vegetation destruction is given below Widespread deposits of Taupo Lapilli with particular reference to the Waiotapu- mainly from Waitahanui are shower bedded Oranui locality. and caused localised complete de,;truction by burning and blast, and by burial in an Phase 1 outer zone. Large trees remained standing Initial ash-fall deposits from centres near through 18 in. thick deposits. Waitahanui; Hatepe Ash, "putty coloured layer", and Rotongaio Ash caused localised Phase 3 destruction mainly by burial. After a short interval a violent eruption 68 VULCANICITY AND VEGETATION of the ":hyolitic block member" gave rise to , TEMPERATURE.,., . OF ASH-FLOW , . a wide, pread thin bed of rhyoliticlap,illi. " This eruption ~ppears to have its sequeni1' " ,Ash-flows must have had a substantial the Maroa centre where, on slopes near burning effect on the, vegetation, but while Orakei{orako, rhyolite ash and lapilli and the minimum temperature required to char pumice lapilli form thick, mainly loose, wood is 250"C, there is no sign of baking deposit,! of irregular thickness. In valley of a buried soil and this would suggest bottom,' massive breccias up to 15 ft thick lowest temperatures at the base of deposits are cha:acterised by discontinuous vertical and a temperature gradi,ent within the over- fissures with inclusions of loose ash and' lying ash.