CHAPTER TWO

Pumice Lands

T ~Tc!~~~~~~~~~: ~:i~~~~~~~~;~hn~S~~~~ ;:~e~~S~hi~~ :~~:n~~n~~~~~ to the Waikato basin and the Bay of Plenty. These are the 'pumice lands', so-called because their soils are derived almost entirely from pumice, the popular term for the distinctive soft, greyish-white, frothy, volcanic rock which mantles most of the land. On a geological time scale, these pumice landscapes are very young (only a few thousand years old) and the underlying volcanic rocks are generally less than a million years old.

Most of the volcanic activity has been confined to a depress'ion that cuts diagonally across the region for 250 km from White Island in the north-east to Mt Ruapehu in the south-west. This depreSSion, the Taupo Volcanic Zone (Fig. 2.1), is flanked by ranges consisting of much older greywacke and argillite rocks of non-volcanic origin. These stand above the volcanic landfonns as the densely forested Kaweka and Huiarau Ranges in the east and the Hauhungaroa and Rangitoto Ranges of the King Country in the west.

Within the Taupo Volcanic Zone itself, the most widespread landforms of volcanic origin are the huge plateaux formed by the eruption of volcanic gas and molten rhyolite 300 000 - 750 000 years ago. This volcanic debris erupted at temperatures sufficiently high to weld into a relatively soft but coherent rock (ignimbrite) upon cooling. In the eastern sector of the volcanic zone an ignimbrite sheet makes up the vast, flat Kaingaroa Plateau which slopes gently down from an altitude of 700 m near Lake Taupo to 300 m at Matahina. In the west the smaller ignimbrite plateaux - Mamaku, Tokoroa and West Taupo - are less uniform, but rhey too slope gently down to the north. Within this central upland rise the great rivers of the North Island - Waikato, Rangitaiki, Whakatane, Mohaka, Ngaururoro, Rangitikei and Wanganui - to flow down to the agriculturally important lowlands of the Waikato, Bay of Plenty, Hawke's Bay and Wanganui­Manawatu.

Development of the Pumice Lands The pumice lands have presented something of an enigma to land developers since the earliest days of colonisation. Access from the coastal lowlands was difficult, but this was a blesSing in some respects for it ensured th~t the dense podocarp forests of the King Country and western Urewera were not rapidly felled and burned to make way for agriculture as happened in the lowlands of the Waikato, Hawke's Bay and Manawatu. Indeed, the scenic importance of Lake Taupo and the geothermal centres for tourism was appreciated at an early stage. The three volcanic peaks of Tongariro, Ngauruhoe (Plate 2.1) and Ruapehu dominate the southern part of the region. Gifted to the Crown by the Ngati Tuwharetoa people as long ago as 1886, they form the nucleus of T ongariro National Park, the winter playground of the North Island.

Initially there were hopes for extensive pastoral farming with Merino sheep on rhe open tussock plains and shrublands of the Kaingaroa Plateau and upper basins of the Waikato River, but attempts to achieve this were plagued by a puzzling stock malady termed 'bush sickness'. Nutrient deficiencies in the soils derived from the pumice were the main cause and soil surveys during the mid-1930s eventually

33

Plate 2.1 (opposite)

Mt Ngauruhoe in eruption, 26 January 1974. The column of tephra reached c.2000 m above the summit and hot avalanches of andesitic ejecta can be seen flowing down the northern and western slopes. Ngauruhoe is the most active vent in the Tongariro Volcanic Centre, its classical, youthful, 900-m cone having been built up over the past 2500 years. Over 60 eruptive episodes have been recorded over the 140 years of European settlement.

34 The Living Mantle

Fig. 2.1

The Taupo Volcanic Zone extends from White Island to Mt Auapehu with volcanic activity concentrated in the three main vo lcanic cen tres of Tongariro, Taupo and Okataina (near Rotorua).

60km L-_~_~_---'!

established the association of bush sickness with soils derived from the Kaharoa and Taupo pumice eruptions. These soil parent materials were deficient in a number of trace elements important for animal health, chiefly cobalt, selenium, and copper.

Subsequently a major land-use debate ensued, leading government and private industry to establish large areas of the pumice lands in exotic forest (principally Pinus radiata), particularly during the depression years. By the late I 930s, however, the widespread use of cobaltised superphosphate had successfully controlled bush sickness and opened the way for the rapid pastoral development of the Ngakuru area and, later Reporoa and the Galatea basin.

With hindsight, problems with agricultural development of the pumice lands were probably for the best, for these exotic plantings during the depression were the basis of New Zealand's future exotic timber industry. Today, exotic forests cover 400 000 ha of the Rotorua-Taupo basin pumice lands, an important forest resource making up 40 percent of the entire exotic forest estate in New Zealand.

There is an increasing public call for an end to the milling of most of the remaining indigenous forests of the central North Island pumice lands, and it is therefore timely to contemplate how difficult it would be to reserve these indigenous forests if the SUitability of the pumice lands for exotic trees had not been recognised and acted upon.

Soil-forming Tephras To understand the soil pattern of the pumice lands it is necessary to appreciate the age, composition, and distribution of the material erupted from the Taupo Volcanic Zone. The largest and most devastating of these eruptions within recorded history was the Taupo eruption variously estimated at around A.D. 130 (by carbon dating), or around A.D. 186 (from historic Roman and Chinese records). From a vent close to the White Cliffs in the north-eastern corner of present-day Lake Taupo (Plate 2.2), 60 km3 of rhyolitiC pumice and ignimbrite were erupted, incinerating the forest and entirely remoulding the landscape. This was over 60 times the volume of the debris from the well-documented Mt St Helens erur.tion in North America in 1980, yet it is only a small fraction of the 15 000 km3 of volcanic rock and ash erupted since volcanism commenced in earnest within the zone about 1 million years ago.

Most of the erupted material is ignimbrite which solidified long ago and has been subsequently mantled with successive eruptions of less consolidated airfall volcanic material collectively termed 'tephra' (ranging in size from fine particles of ash, through gravels Qapilli) to large boulders). Although the volume (300 km3

)

of tephras erupted during the last 40 000 years is much smaller than the earlier landform-generating ignimbrites, it is the tephras which are important as the soil parent materials.

Scientific and engineering interest in these tephra showers is such that their distributions have now been mapped and their pedological properties better appreciated. To illustrate their widespread distribution, the 12 major tephras erupted during the past 40 000 years are shown on the North Island map in Fig. 2.2. The legend to Fig. 2.2 indicates that most of the tephras are rhyolitic in composition and were ejected in cataclysmic eruptions from the central part of the volcanic zone - Rotorua caldera, and the Okataina and Taupo Volcanic Centres (Fig. 2.1).

Pumice Lands 35

Plate 2 .2

The north-eastern shores of Lake Taupo. looking north from above Hatepe . The prominent finely dissected scrub-covered Ouaha Ridge inland of the White Cliffs consists of great depths of Taupo Pumice. which was probably erupted from a deep offshore basin within the Horomatangi Reef Elsewhere this characteristic pattern of rill erosion in Taupo Pumice has been largely masked by the growth of exotic forest plantations. A good example of a beach remnant can be seen in the foreground. cutting across Ouaha Ridge. It marks the level to which Lake Taupo rose (over 30 m above its present level) when the outlet was blocked after the Taupo eruption.

36 The Living Mantle

TEPHRA FORMATI ON VOLCANIC CENTRE LITHOLOGY AGE (YEARS) VOLUME (km3)

RotomahanaMud Hydrothermal 100 } 1.3

TaraweraAsh& Lapilli Basaltic 100

Okataina Rhyolitic

Taupo Pumice Taupo Rhyolitic

WaimihiaLapilii Taupo Rhyolitic 19

Okataina Rhyolitic 10

okataina Rhyolitic

RotomaAsh Okataina Rhyolitic 7350

Okataina Rhyolitic

RotoruaAsh Okataina Rhyolitic 13500

AokautereAsh Taupo Rhyolitic 19900 16 (Hydrothermal)

'MangaoniLapilii okataina Dacitic 30100 66

'RotoehuAsh Rhyolitic

'I metre depth

Fig. 2.2 (opposite)

The map shows the distribution of the twelve principal tephras (listed in the legend) erupted during the last 40 000

~~~~~~r~~e~t~e:~ih~ ~~~o~~~taina _____________ ,

_ _ eruptions from the Taupo and Okataina Centres were usually extremely violent and separated by long periods of volcanic inactivity during which soil development could take place on the tephra surface. In contrast the andesitic tephras from the Tongariro Centre were probably erupted intermittently with little time for topsoi l development on each relatively small shower. With this lack of paleosols to

- - act as markers they are consequently more difficul t to separate into l"~' "--',". ~----l-eruptions.

1 Ohakune Taupo

1 1 Reporoa Kawerau

Pumice Lands 37

TE NGAE SECTION

Tephfas

D Andesitic (Tongarifo)

D AhyolitiC(Taupo)

D Rhyolitic (plus minor basaltic) (AotofualOkataina)

1 White Island

38 The Livinn Manrle

However, much smaller amou nts of the more basic andesitic tephras have been erupted intermittently and less violently from the volcanic zone, interestingly enough from the northern (White Island. Whale Island and Mt Edgecumbe) and southern (fongariro Volcanic Centre) ends.

Yet the imprint upon our soils of volcanism in the Taupo Volcanic Zone extends far beyond the pumice lands. Older, fi ner tephras erupted from this zone cover much of the lowlands of the Waikato and Bay of Plenty. The major rhyolitic eruption that gave rise to the Kawakawa Tephra 20 000 years ago deposited several centimetres of fine ash as far away from its Taupo sou rce as Christchurch and the Chatham Islands; shards of volcanic glass found in Antarctica are considered to have been part of this same Kawakawa Tephra. Yet tephra from relatively minor andesitic eruptions can be carried great distances; during the I950s ash from an eruption of Mt Ngauruhoe was collected from car and house roofs in Well ington and Hamilton.

An obvious question is: 'How can all these different tephras be recognised?' The answer is complex, but most tephras have distinctive colours, or a developed topsoil (subsequently buried and now called a 'paleosol). or a distinctive particle size or mineralogy. The tephras have built up like layers in a huge volcanic landscape cake, with interfingering of the tephras from the three volcanic centres - Tongariro, Taupo and Okataina. This is illustrated by cross-section and photographs in Fig. 2.3 for 24 tephras erupted over the last 20 000 years along the axis of the Taupo Volcanic Zone.

Only the upper. most recent members of the tephra layers lie within the rooting zone of plants. and only five of them - Ngauruhoe Ashes. the Tarawera Formation (Tarawera Ash and Lapilli . and Rotomahana Mud). Kaharoa Ash. Taupo Pumice. and the o lder andesitic tephras collectively called 'Tongariro Ashes' - are entirely soil-forming (Fig. 2.4). There are sufficient differences in the physical and chemical properties of these soil-forming tephras to have a Significant effect on land use in the pumice lands.

The Soil Pattern of the Pumice Lands The volcanic soils of the pumice lands have a number of features that distingu ish them from the more typical soils formed on non-volcanic parent materials in other parts of New Zealand. They are generally deep. weakly weathered. coarse-textured and susceptible to erosion because of their low cohesion. Most of these properties reflect their youthfulness. for most of the soil parent tephras post-date the Waimihia eruption (3400 years ago). These young volcanic soils fall into two main groups shown in Fig. 2.4:

o pumice soils (see p. 52) formed from rhyolitic pumice from the Taupo and Kaharoa eruptions; these pumice soils are widespread, covering around I 500 000 ha (Fig. 1.6(a). p. 28);

o raw volcanic soils (see p. 52) which are very young soils formed in tephra (Fig. 1.6a). These very coarse. localised tephras (Ngauruhoe Ashes) are from the Tarawe ra eruption of 1886 and the pe riodic minor eruptions of Mts Ngauruhoe and Ruapehu since the Taupo eruption.

Both groups are shown in Fig. I .6(a). This figure also shows that there are relatively small. but sign ificant. areas within the Taupo Volcanic Zone (e.g. the Mamaku Plateau and the uplands of the south around Ohakune and Waiouru) where the soils are of volcanic origin but are much o lder and the tephras have weathered to volcanic loam s (described on p. -53). These soils are widespread in the loarnlands of the Waikato. Bay of Plenty. and Taranaki (Chapters 3 and 4).

Airfall tephras were deposited on both flat and hilly land; gullying of the fl at land led to redeposition of the tephra as alluvial terraces. while tephras eroded from the hill slopes built up deep deposits of pumice colluvium. In othe r places. such as the Galatea basin, fans spilt out of the ranges and onto the river terraces and valley floors. With this redistribution within the landscape, the soil parent tephras were modified (particularly in terms of their particle size and mixture with weathered non-volcanic rocks). Consequently the volcanic soils in the region differ according to their position in the landscape and the nature and degree. of development of the soil-forming tephras; the total range is immense. (The detailed pattern can be found in the soil survey publications listed in the bibliography.)

Fig. 2.4

Map of soil-forming tephras in central North Island and Taranaki. The tephras contributing to the top metre of soil are shown by both colour (surface tephra) and symbol (for significant underlying tephras).

Pureora to Lake Waikaremoana The variation of soils with changes in topography and climate can be appreciated by traversing the central part of the pumice lands, from Pureora in the west to Lake Waikaremoana in the east (Fig. 2.5). Taupo Pumice is the dominant soil· forming tephra in the west and central part of the traverse. The imprint of differences in climate and vegetation upon the airfall part of this tephra is well illustrated by the development sequence of the widespread Taupo, Oruanui and Tihoi soils on the rolli ng and hilly land west of the Waikato River.

Pumice Lands 39

SURFACETEPHRAS

DNgauruhoeTephra

D RotomahanaMud

DTaraweraAshandlapilii

DKaharoaAsh(Ka)

DTauPoPumice(Tp)

Burrell Tephra (B)

DStratfordTePhra(S)

UNOERlYINGTEPHRAS

Waimihia Formation (Wm)

WhakataneAsh(Wk)

MamakuAsh (Ma)

RotoruaAsh(Rr)

OkarekaAsh{Ok)

RotoehuAsh(Re)

UNOIFFERENTIATEOTEPHRAS

DTonllariroAsheS(TII)

DEgmontAshes(E)

D HamiltonandKauroaAshes

DWaikatorhYOlitittephras

o North Taranaki andesi!it tephras

40 The Liv;nn Mancle

Plates 2.3 - 2.5

The sequen~e of Tau~o, - Orua~ui a~~ Ti~oi soils shows a .trend of incre.asing soil development with an increase in leaching and differences In vegetation. Their position In the landscape IS shown in Fig. 2.5.

Plate 2 .3

The Taupo soils are the least developed; they are very widespread in the Taupo-upper Waikato basin at elevations of 200 - 450 m and rainfall of 1100 - 1200 mm. A black A horizon has developed under an indigenous vegetation of manuka, tutu, bracken and grasses, the characteristic shrub/ fernland induced when the Taupo eruption destroyed the earlier podocarp/hardwood forest. The 8 horizon is only 10 - 15 cm thick and there is usually a coarse-textured layer of Taupo Lapilli in the pale yellow­brown C horizon.

Plate 2.6

The Poronui soils are alluvial soils developed from water-depos ited Taupo Pumice in the Taupo district. Their natural vegetation is indigenous shrubland which can tolerate their coarse-textu red character (Plate 2.7) The layers of pumice alluvium in the subsoil indicate the great depth of Taupo Pumice that has been deposited and its susceptibi lity to erosion by water (Plate 2.7).

Plate 2.4

At higher elevations of 450 - 600 m, the Oruanui soils show more signs of soil development. Rainfall is now 1200- 1500 mm and the Bs horizon is much thicker (up to 50 em) and is redder in hue because of the accumulation of iron oxides and humus under the stronger leaching conditions. The A horizon is deeper (15 - 20 cm) and also black due to bracken fern which replaced much of the forest .

Plate 2.5

The end member of the sequence is the Tihoi soil which is widespread, particularly under podocarp/hardwood forest, on the flanks of the Rangitoto and Hauhungaroa Ranges above 550 m and under a rainfall of 1 500 - 2000 mm. A somewhat bleached E horizon (indicating eluviation of Fe and AI oxides) is found (particularly under rimu trees) and there is a thick (20 em) 8s horizon (indicating deposition of Fe and Al oxides) below this E horizon.

With increasing altitude, rainfall also increases and the Taupo, Oruanui and Tihoi soils become progressively leached (Fig. 2.5). Although the colour and depth of their horizons reflect this increase in leaching and the influence of different vegetation (plates 2.3 - 2.5), the three soils are too young and unweathered to exhibit significant differences in chemical (e.g. exchangeable nutrients) and physical (e.g. clay content, water-holding capacity) properties.

Both the Taupo and Oruanui soils have been largely cleared of fern, shrubland and indigenous forest and have been developed to pasture and exotic forest. The Tihoi soils still support areas of high-quality podocarpihardwood forest on the flanks of the Hauhungaroa and Rangitoto Ranges although here also large areas have been converted to pasture and exotic forest.

Further west, on the uplands of the King Country, the effect of the Taupo eruption was probably not as severe. Here Taupo Pumice is thinner (usually less than 50 cm) overlying large areas of older tephras. On hilly and steep slopes it may be absent altogether. These soils have composite properties, a pumice topsoil and a volcanic loam subsoil, and are mapped as a complex of Maroa, Tihia, Ngaroma. Piropiro and Waione soils. They are generally better for farming than the true pumice soils since plant roots can extract nutrients and moisture from the underlying finer-textured brown tephra.

After the Taupo eruption huge volumes of pumice dammed Lake Taupo and some of the rivers in the catchment. When these pumice dams were later eroded away water levels dropped, exposing stranded shorelines (Plate 2.2) and large areas of water-sorted pumice. The Whenuaroa soils in the Reporoa-Broadlands section of the Waikato catchment, or the Otamatea and Poronui soils further south. are typical of these alluvial pumice soils. They are susceptible to drought since the finer material has been washed out during deposition leaving coarser sands (Plate 2.6). Generally less weathered, they are low in available magnesium and (along with the flow tephra soils) have usually supported only a shrub and tussock vegetation rather than forest (Plate 2.7).

DUring the Taupo e ruption glOwing tephra avalanches ('nuees ardentes) swept across the landscape as incandescent mud flows. incinerating the forest and flOwing into valleys and depressions, filling them to a considerable depth with pumice. These 'flow tephras' can be recognised by their poor sorting and their compactness. Angular fragments of pumice are interlocked with sharp, unweathered mineral shards (Plate 2.8) to such an extent that they can act as a Significant barrier to root penetration (see Plate J 4.4).

Pumice Lands 41

Fig. 2.5

Rainfall and soil pattern along landform transect from Pureora to Lake Waikaremoana.

42 The Living Mantle

Plate 2.7

A view north across the headwaters of the Ripia and Rangitaik i Rivers to the exotic forests of the southern Kaingaroa Plateau. Here the airfall and flow Taupo Pumice has been redeposited as thi ck deposits of alluvium and colluvium. The fire­resistant manuka/monoao/bristle tussock vegetation in the foreground is typical of the relatively small areas of soils developed from Taupo Pumice now remaining in an unmodified state. The pattern of land development in the distance indicates appropriate uses of these coarse, deep soi ls once nutrient and erosion limitations are recognised. Note the deep gully erosion in the foreground.

Plate 2.8

Kaingaroa soils are pumice soils developed on deep flow tephra from the Taupo eruption. They cover a large part of the southern and western sectors of the Kaingaroa Plains (Fig . 2.5). The dark A horizon is sha llow and there is only a very weakly developed shallow B horizon. Most of the profile in the photograph consists of a deep, compact C horizon of angular pumice fragments held tightly in a matrix of sharp, unweathered shards of volcanic glass . This subsoil can be a significant barrier to tree -root penetration (Plate 14.4 ).

Devastating Taupo flow tephras swept over flat areas like the Kaingaroa Plateau, and across the Hauhungaroa Range to the west and into the King Country. Flow tephras infilled the upper Ngaururoro catchment in the northern Kaimanawa Range and even extended as far south as the T aruarau River and Ngamatea Swamp (Plate 2.9), just north of the highest plateau on the Taihape-Napier road. A common feature of the flow tephras is the presence of charred logs and pieces of charcoal such as that shown in the Taruarau soil at Ngamatea (Plate 2.10).

At lower elevations Atiamuri soils are found on the flow tephra, but the most widespread are the Kaingaroa soils (Plate 2.8) which cover large areas of the more elevated, moister Kaingaroa Plateau (Fig. 2.5). Both soils have very shallow A horizons, weakly developed shallow B hOrizons, and compact C horizons which restrict plant growth; as such. they are quite distinct from the more friable soils like Taupo and Oruanui. Despite the physical limitations of the flow tephra pumice soils, their best use is for exotic forestry (Plate 2.11), espeCially with the modern practice of deep-ripping to loosen their compact subsoil.

Towards the eastern margin of the Kaingaroa Plateau, Kaharoa Ash (which is typically whiter. harder and less vesicular than the softer Taupo Pumice) begins to dominate the upper part of the soil profile. These Te Rere and Pekepeke soils are more like the Taupo and Oruanui soils in their properties and they both support large areas of the huge Kaingaroa exotic forest plantations which extend as a sombre green swathe from the upper Rangitaiki to the Tarawera River.

Further east is the valley of the Whirinaki River, a major tributary of the Rangitaiki, at the junction of the ignimbrite sheets underlying the Kaingaroa Plateau and the older greywackes of the Huiarau Range (Fig. 2.5). Here lie some of the most impressive podocarp forests remaining in the North Island and it is interesting to speculate upon the role of volcanic eruptions. especially the Taupo eruption, in determining such a forest pattern (Plate 2.12). It is likely that the Taupo eruption destroyed the forest on the Kaingaroa Plateau but the Whirinaki valley may have been far enough away from the source for sufficient seed trees to have survived on the steeper slopes. The eruptions, in rejuvenating the soils of the valley floor and the roBing country on the northern and western margins, tended to reverse the successional trend towards hardwood forests (dominated by tawa, kamahi and rewarewa) which are found on the more developed hill soils. Today tall, dense podocarp forest dominates these easier slopes and can be considered to represent only an intermediate stage in the post-eruption vegetation recolonisation.

Pumice Lands 43

Plate 2.9

The rolling uplands of the Ngamatea Plateau (c. 1 000 m) are now largely established in pasture. The cutting has exposed the profile of the Ngamatea soil (thin, ai rfall Taupo Pumice over Tongari ro Ash). The Taruarau soil (thick, fl ow tephra Taupo Pumice over Tongariro Ash!. shown in Plate 2.10, is found at a slightly lower altitude (around the pine trees in the distance) where the nueeardente swept th rough from the southern Kaimanawa Range.

Plate 2.10

The Taruarau soil is developed in thick flow Taupo Pumice overlying Tongariro Ash. This profile on the Ngamatea Plateau shows t he white C horizon in Taupo Pumice perched above the weathered Tongariro Ashes (Plate 2.9), The wel l-developed, black A horizon is a characteristic of this soil, and other pumice soils developed from Taupo Pumice. The carbonised remains of trees (oriented in a north-west/south­east direction) can be found at the base of the Taupo Pumice, indicating the force of the nuee ardente which incinerated them.

Plate 2.11

Young pine trees established in Kaingaroa soils (Plate 2.8) at the southern end of the Kaingaroa Plateau. Despite the harsh climate of this upland area, and the coarse, compact nature of the subsoi l, the t rees appear to be growing vigorously and, as yet, show no sign of nutrient deficiencies.

44 The Ljvjng Mantle

Plate 2.12

In the Whirinaki valley, on the south­west margin of the Huiarua Range, Taupo Pumice and Waimihia Ash overlie varying depths of the andesitic Tongariro Ashes; Taupo Pumice deposits infilled the valley floors and provided an excellent rooting medium for the recolonisation of the podocarp forest, such as these dense stands of rimu, matai , totara , miro and kahikatea.

In the ranges to the east of the Whirinaki River the mantle of tephra is generally much thinner. It consists of Kaharoa Ash and Taupo Pumice over Whakatane Ash in the Urewera, and Taupo Pumice over Waimihia Ash in the southern Huiarau and Ahimanawa Ranges (Fig. 2.4). These Urewera steepland soils support the vast upland podocarp/hardwood forests of the western part of Urewera National Park. They have not been investigated or mapped in any detail , as their overriding importance for soil, flora and fauna conservation has never been questioned.

The soils in the easternmost part of the Urewera uplands lie on a complex jumble of softer calcareous sandstones and siltstones of Miocene age (Fig. 2.5). Here slopes are more gentle and much of the deposited tephra has resisted erosion. Rainfall in these cool uplands around Lake Waikaremoana (Plate 2.1 3) is up to 2500 mm per annum, with very heavy falls from occasional storms from the south and south-east. The risk of erosion is very high if this dense vegetation is disturbed and it is essential that these fragile soils be protected to avoid the enormous loss of biota and soil productivity which occurred through forest removal along the length of the eastern slopes of these axial ranges (Chapter 7).

Rotorua-Okataina Volcanic Centres The soil landscape pattern traversed across the Taupo basin is generally repeated in the Rotorua area; most soil parent materials are of rhyolitic tephra, with Kaharoa Ash particularly prevalent north and south-east of the Rotorua Lakes (Fig. 2.4). Taupo and Oruanui soils occur on similar landforms and similar soil development and leaching sequences occur (Plate 2.14). The coarse-textured Oropi and Oturoa soils are similar to Taupo and Oruanui soils and are found on Kaharoa Ash and Taupo Pumice around the northern margins of Lake Rotorua, at an altitude of 320 m and an annual rainfall of 1600 ~2000 mm (Plate 2_15). The Oropi soils are very susceptible to drought because of their coarse texture and, like the Oturoa soils, they have a low natural nutrient status which is readily rectified by fertilisers. Although the Oropi soils have been developed to good pasture land they are probably better suited to forestry. The Oturoa soils are more versatile (only slightly susceptible to drought) and have a horticultural potential (berry fruit and pip fruit orchards) in addition to their use for dairy and beef cattle.

Pumice Lands 45

Plate 2.13

Lakes Waikareiti (foreground) and Waikaremoana nestle among the tephra-smoothed uplands on the eastern side of the Huiarau Range within Urewera National Park. These Ruakituri and Matawai soils (Fig. 2.5) are podzolised pumice soils developed on Kaharoa Ash and Taupo Pumice overlying Waimihia Lapilli. The strata of the underlying Miocene-age sediments can be seen in the cliffs of Panekiri Bluff above Lake Waikaremoana in the middle distance . In this cool, moist environment around Lake Waikareiti, the forest is a mixture of red and silver beech, the altitudinal limit of rimu forest being reached somewhat lower around the shores of Lake Waikaremoana at gOO-m altitude

Plate 2.14

View south along the escarpment of the Horohoro rhyolite dome towards Ngakuru and the hills of the upper Waikato River around lake Ohakuri The Horohoro rhyolite dome marks the apex of the ignimbrite flows that make up the Mamaku Plateau to the north. Taupo and Oruanui soils (from Taupo Pumice) are found under the pasture at the foot of the escarpment while the summit plateau is a complex of Mamaku soi ls (podzolised volcanic loams)

Plate 2.15

The striking caldera of lake Rotorua, looking east across the northern end towards Lake Rotoiti. The formation of the Rotorua caldera was probably associated with the eruption of the Mamaku Ignimbrite around 170 000 years ago. Below the prominent escarpment, Dturoa soils have developed on old lake terraces and slopes that were covered by a larger lake which was once 100 m higher than the present Lake Rotorua. Beyond the escarpment, sandy Dropi soils have developed on the rolling tephra-covered land which can be seen sloping down towards the Bay of Plenty (Chapter 3)

46 The LivinS Mantle

Plate 2 .1 6

The summit of the Mamaku Plateau has been cleared of large areas of indigenous podocarp forest and established in pasture. A striking feature of the landscape is the large number of ignimbrite tors and conica l mounds . These are the eroded remnants of a jointed ignimbrite surface standing on a more resistant welded ignimbrite base. The landscape is mantled with Mamaku, Rotoma and Rotorua tephras, and the predominant soils are the podzolised Mamaku soils which developed under the indigenous

~~e~~a~~~ni; th is cooler, moist

To the west of the Rotorua caldera much of this rhyolitic pumice has been eroded away, allowing the underlying older Mamaku, Rotoma, Waiohau and Rotorua tephras to dominate the soil profile (Fig. 2.4). The climb of 250 m in altitude along State Highway 5 from the shoreline of Lake Rotorua at Ngongotaha to the broad summit of the Mamaku Plateau traverses a zone of Significant soil development where these older tephras make up most of the soil parent material. The annual rainfall increases from 1600 mm to 2400 mm and the soils (Ngakuru, Ngongotaha, Waiteti and Mamaku) correspondingly exhibit increasing podzolisation as the effect of leaching intensifies with the transition into a high rainfall, lower temperature, and mor-forming podocarp forest environment. Iron has accumulated in the B horizon of the Ngongotaha and Waiteti soils while the strongly podzolised Mamaku soils have the typical bleached E horizon of podzols and have some similarities to the Tihoi soils (Plate 2.5). These soils are volcanic loarns which have deep brown, friable subsoils and a characteristic greasy feel because of their high content of allophanic clays, typical products of the weathering of these older rhyolitic tephras in this wetter environment over the past 7000 -13 000 years.

The broad summit of much of the Mamaku Plateau is dotted with small, conical tors of more resistant ignimbrite (Plate 2.16). Although the higher altitude and cooler climate of these Marnaku soils means a shorter growing season, they still have a potential carrying capacity of I 5 stock unitslha. Nevertheless, any further forest clearance on the Mamaku Plateau is a matter of considerable public controversy (Plate 2.17). This is because there is now greater recognition of the importance for water and soil conservation and wildlife habitat of the large arc of upland indigenous forest stretching from the Kaimai Range through the Mamaku Plateau to its southern termination in the striking Horohoro Escarpment (plate 2.14).

In sharp contrast to these more developed soils in the west of the Rotorua Basin, the soils to the east are generally much younger. Here the landscape still bears the imprint of the only major eruption within the history of European settlement - the Mt T arawera eruption of 10 June 1886. The ejecta from the eruption was of two types:

D previously erupted rhyolitic material around the Rotomahana and Waimangu craters. This was intensely altered by hydrothennal activity and violently ejected to blanket a wide area on the eastern side of Lake Rotorua with up to 2 m of a pale olive grey sandy deposit, the Rotomahana Mud (Figs. 2.2 and 2.4);

D a basaltic tephra, the Tarawera Ash and Lapilli. This erupted from the main fissure on Mt Tarawera and swept in an arc to the north and east across large part of the Bay of Plenty (Figs. 2.2 and 2.4).

The Rotomahana soils (Plate 2.18) cover around 13 000 ha of rolling and hilly land between Lakes Rotorua and Tarawera. Because of the sandy and silty nature of their parent material (Rotomahana Mud), the hmy topography and the destruction of vegetation during the eruption, the Rotomahana soils have developed on a highly erodible landscape (Plate 2.19). Cultivation has smoothed out many of these rills and most of the Rotomahana soils have now been established in excellent pastures, particularly for dairying (Plate 2.20). Because of their hydrothermal origin, Rotomahana soils have a number of Significant differences in their soil chemistry when compared with most pumice soils. Their high levels of available molybdenum have been implicated in inducing copper deficiency (and consequent ill-thrift) in dairy cows. The soils are also deficient in boron (for root crops) and manganese (for celery and some fruit trees). The cobalt content of these soils is, however, generally considered sufficient for grazing animals, unlike that of the pumice soils associated with the Taupo and Kaharoa tephras.

In contrast to the Rotomahana soils, the Tarawera soils are very gravelly because of their coarse unweathered basaltic scoria. They are consequently drought prone and in summer the black scoria absorbs heat, thus further decreaSing the moisture status of the soil.

Pasture establishment has been difficult on the Tarawera soils and they are better suited to deep.rooted crops Oucerne or tree plantations) or retained in their indigenous vegetation of manuka or (where fire has been controlled) the mixed kanuka/tawa forest of the hill slopes.

Pumice Lands 47

Plate 2.18

Profile of a Rotomahana soil near Waimangu . A depth of 60 cm of grey Rotomahana Mud overlies Taupo Pumice with its characteristic dark topsoil . In contrast, the present topsoil is very shallow and there is little other profile development . The coarser nature of the Taupo Pumice subsoil is apparent and the gravel band below the buried topsoil is Taupo Lapilli

The Rotomahana Mud consisted of lake bottom sands and silts which had already been partially weathered. Consequently, these Rotomahana soils have much higher clay contents (up to 20 - 30 percent) than other raw volcanic soits. These clays consist largely of the more crystalline forms, such as mica, smectite and kaolin, as well as the ubiquitous amorphous clay of weathered volcanic soils -allophane.

Plate 2.17

On its western side where it slopes down to the Waikato basin, the Mamaku Plateau shows a striking pattern of parallel gullies which have been eroded in the underlying ignimbrite. The soils (Ngaroma) on the flat interfluves have developed in thin Taupo Pumice over older rhyolitic and andesitic tephras. Most of this landscape has now been converted to exotic forests after a long -standing controversy centring on the high value

~:t~r~ f~~:~r~~~i;~~ous forest for

48 The Living Mantle

Plate 2.19

View of the Waimangu region, looking north-east to Mt Tarawera, soon after the eruption of the Rotomahana Mud in 1886. Severe gullying has already commenced in these soft, unconsolidated lake-bottom sediments. The photograph shows the new Lake Rotomahana beginning to form; it has risen 24' m since the eruption. Mt Tarawera in the background was the source of the rhyolitic Kaharoa Ash of c.660 years ago and of the basaltic Tarawera Ash and Lapilli showers which immediately preceded the hydrothermal explosions from Lake Rotomahana. The Rotomahana Mud fell over an area of about 13 000 ha and buried the villages of Te Ariki, Moura and Te Wairoa, where over 150 persons died.

Plate 2.20

The Rotomahana Mud landscape today, in contrast to the immediate post­eruption landscape in Plate 2.19 . The rills in the soft material are still a distinctive feature of the landscape, where not obscured by trees.

Tongariro Volcanic Centre

At the southern extremity of the Taupo Volcanic Zone, the most dramatic volcanic landforms in New Zealand dominate the landscape. Whereas the soils of the TaupolRotorua area have mainly rhyolitic parent materials, the soils around the Tongariro Volcanic Centre (Fig. 2.1) have developed on andesitic tephras (some of it relatively young) in an upland and mountainous environment characterised by cold temperatures and high rainfalls (except for a significant triangular rainshadow area wedged between Mt Ruapehu and the arc of the Kaimanawa, Kaweka and north-west Ruahine Ranges). The dominant soil-forming tephras (Fig. 2.4) are the still-accumulating Ngauruhoe Ash (0 - 1800 years ago), Taupo Pumice (1800 years ago) and the so-called 'Tongariro Ashes', which consist of a series of andesitic tephras erupted from the Tongariro Volcanic Centre over the period 1800 -14000 years ago.

In the immediate vicinity of the T ongariro-Ruapehu volcanic massifs, Ngauruhoe and Waimarino soils are developed on Ngauruhoe Ashes over Taupo Pumice and T ongariro Ashes. They are coarse-textured, highly erodible raw volcanic soils of no commercial use because of their poor physical properties and the cold climate; however, they support a most attractive subalpine flora protected within the Tongariro National Park. These Ngauruhoe soils are best viewed where the Desert Road traverses the eastern edge of the so-called Rangipo Desert (a misnomer since these subalpine gravel fields have a relatively high annual rainfall of 1200 -1600 mm; nevertheless, this is only half of the precipitation at this altitude on the western slopes of Mt Ruapehu).

Because of the prevailing westerly winds, neither the Ngauruhoe Ashes nor Taupo Pumice have accumulated to any great extent to the south-west of the Ruapehu massif; here the T ongariro Ashes are the important soil parent materials giving rise to deep, volcanic loarns such as the moderately leached Ohakune soils. Despite the high altitude and cooler environment, the excellent physical properties of the O hakune soils and the plentiful rainfall make them highly suited to market gardening for vegetables such as carrots (Plate 2.21). The wetter, strongly leached Pokaka soils at the foot of the ringplain on the western side of Ruapehu support good pasture but are not as versatile as the Ohakune soils. The climate and mor~ broken topography dictate the retention of most of the Pokaka soils in indigenous forest.

To the east of Ruapehu, the Ngauruhoe soils merge into Waiouru and Ngamatea soils (Plate 2.9). Around Waiouru the landscape changes dramatically as the smoother landforms give way to the sharp-edged plateaux and escarpments that characterise the uplifted sedimentary rocks around the Rangitikei, Whangaehu and Mangawhero valleys. Yet these sandstones, siltstones and limestones have little effect on the soils, for the ash mantle is still thick enough to encompass the entire soil profile. The result is one of the most amazing and least known landscapes in the North Island - the only such area truly resembling parts of the South Island. These are the red tussock uplands (altitude 1000 - 1200 m) stretching across the vast open spaces of the upper Moawhango catchment (plate 2.22) to the Ngamatea Plateau at the foot of the rugged Kaweka Range and the Mangaohane Plateau wedged between the northern Ruahine Range and the canyon of the Rangitikei River (Plate 2.23).

Most of the volcanic soils lying on these plateaux have the typical volcanic loam attributes of depth, friable consistence, loamy texture and high moisture­holding capacity, their only limitations being climatic in that the growing season is much shorter. The most extensive soils of agricultural importance are the Moawhango soils. These lie at the limit of the Ngauruhoe Ashes and have only a thin covering of airfall Taupo Pumice over deep Tongariro Ashes sitting on sedimentary rocks (Plate 2.24). However, thick depOSits of Taupo Pumice from nuees ardentes which poured through the Kaimanawa Range and down the Taruarau Valley cover much of the Ngamatea Plateau. This is the only Significant area of pumice soils (Taruarau soils) in the southern part of the pumice lands (Plate 2.10). Where developed to pasture the Taruarau soils need careful management to keep a close vegetative cover in order to avoid frost-heave and subsequent erosion by wind and water.

Pumice Lands 49

Plate 2.21

Old andesitic Tongariro Ashes mantle the ringplains of the volcanoes of the Tongariro Volcanic Centre but have generally been covered by the younger, coarser tephra from the Taupo eruption and the recent eruptions of Mt Ngauruhoe (Plate 2.1). The Ohakune region in the south-west is in the shadow of Mt Ruapehu and here the Tongariro Ashes are still soil-forming. The deep, friable Ohakune soils are typica l volcanic loams (see Plate 2.24) which are suitable for the production of vegetables such as these carrots -despite the relatively high altitude 1600 mi.

50 The [ivins Mantle

Plate 2.22

The volcanic massif of Mt Ruapehu (left) and the active cone of Mt Ngauruhoe (right) dominate the landscape in the upper Moawhango catchment near Waiouru. Waiouru and Moawhango soils (Plate 2.24) have developed on the tephra-mantled surface of uplifted, faulted and dissected sedimentary rocks in the foreground. Red tussock, with the occasional patch of beech fore st which has survived historic fires, covers this survi ving remnant of the North Island's indigenous tussock grasslands. Most of these tussock lands have been converted to pasture (Plate 2.9). Those in this landscape have a precarious future as a military training ground, part of which is being inexorably

~~~t~i:t~~ by the exotic tree, Pinus

Raw volcanic soils Raw volcanic soils are a very heterogeneous group of soils, their common attribute being their volcanic origins and extreme youthfulness. They are of only local distribution around North Island volcanic centres that have been active in historic times (generally the last 800 years), e.g.:

o the Ngauruhoe soils around Mts Ruapehu and Ngauruhoe; o the Tarawera and Rotomahana soils of the Rotorua area; o the Burrell soils of the upper eastern slopes of Mt Taranaki; o the Rangitoto soils of Rangitoto Island in the Hauraki Gulf.

Their pare nt materials are generally andesitic tephras or basaltic lavas. They exhibit no B horizon development and generally only a shallow A horizon. Textures are sandy, clay contents very low, structures weak, bulk density is low and their water-holding capacity is generally so low that they are droughty and support limited floras. Many of these soils are protected within national parks or reserves.

The Rotomahana soils (Plate 2.(8) are an exception, exhibi ting more development because of the hydrothermal nature of the Rotomahana Mud.

At the southern limit of the pumice lands, the landscape becomes more angular and dissected, characterised by the terraces and hill country of the mid-Rangitikei River. The Tongariro Ashes are, however, still the dominant soil-forming materials on the flat terrace surfaces around Taihape (Ohakune soils) although the landforms are quite different (Chapter 6). Other major tephra showers, such as the Kawakawa Formation and the Taupo Pumice, were thinly distributed much further south. Most of these tephra deposits were subsequently eroded away and the underlying sedimentary rocks, and loess derived from both sources, are the dominant soil parent materials of the Rangitikei and Manawatu (see Chapter 6).

Our journey down from the volcanic pumice plateaux is not southwards, however. Rathe r, it begins high on the northern rim of the plateau, looking out across the fertile loamlands of the Waikato and Bay of Plenty.

Pumice Lands 51

Plate 2.23

At the southernmost corner of the Mangaohane Plateau, Tongariro Ashes still dominate the soil profile. Here Titapu soils occur in a higher, moister environment than the Moawhango soils (Plate 2.24) and support a vegetative cover of red tussock and subalpine herbs. A unique mountain cedar forest, fringed with mountain toatoa, has survived the historic fires which are thought to have induced the red tussock which dominates most of these plateaux.

Plate 2.24

The Moawhango soils are widespread in the tephra-covered uplands (below 1100 m) to the south-west of Mt Ruapehu. They have a trace of Taupo Pumice in the topsoil but T ongariro Ashes dominate all A, Band C horizons to a depth of nearly 2 m. Below the sharp break at 2 m depth is the underly ing, lighter -coloured, weathered sedimentary basement rock of Pliocene age. In their properties the Moawhango soils are similar to the volcanic toams of the Waikato and Taranaki.

52 The Livina Mantle

Distinguishing features of pumice soils and raw volcanic soils

PARENT MATERJAL AND DISTRJBUTION - rhyolitiC tephra, with at least top 50 em of profile developed in tephras which are between 660 and 3 500 years old. Predominant soil of volcanic plateau of central North Island; also scattered thro ughout Bay of Plenty and Hawke's Bay

PROFILE CHARACTERISTICS - the thickness and darkness of the A horizon is closely related to the vegetative cover, e.g. deep and black under bracken fern, shallow and grey under manuka scrub, brown under podocarplhardwood forest; the B ho rizon becomes more reddish-brown as the pumice soils become more leached (see Taupo-O ruanui-Tihoi soil leaching seque nce, p. 4 1). Strongly leached and podzolised profiles may show E, Bh, and Bs horizons.

TEXTURES - gene rally coarse, shOWing a gradation from gravelly sand near the eruption source to silty sand near the periphery of the ash showe r. Weakly weathered; altho ugh weathering rapidly in a moderate weathering zone, their youthfulness and the heterogeneous locatio n of the weathering products (as a skin around the grains o f pumice) give the soils an overall weakly weathered character.

CLAY CONTENT LO W - generally < 10%, consisting mainly of allo phane.

STRUCTURE WEAKLY DEVELOPED - crumb o r gra nular.

FRJABLE TO LOOSE CONSISTENCE - lack of weathering products such as iron oxides which tend to give a soil cohesion. Conseque ntly, very susceptible to erosion if vegetative cover incomplete.

BULK DENSITY LOW - 0.6..0.8 Tim'

PLANT-AVAILABLE MO ISTURE HIGH - highly porous yet readily avai lable moisture ranges fro m 22 - 30% of soil volume: owing to the highly vesicular nature of the pumice, sufficient mo isture is retained for the plant root to exploit while the free-draining properties confer advantages of soil ae ration . Where pumice soils are deep these physical properties allow deep­rooting plants like lucerne and Pinus radiata to tap large reserves of soil nutrients and moistu re.

SOIL ANIMAL POPULATIONS LOW - most soil animals and micro-organisms are concentrated in the topsoil ; the coarse textures and droughtiness of some of the pumice soils may be respo nSible for the difficulty of establishing earthwo rms in some areas.

IMPO RTANT N UTRJENTS LACKI NG - relative to other soil parent materials, the Taupo and Kaharoa rhyolitiC tephras are low in the major ele ments potassium, magnesium, calcium, phosphorus and sulphur as well as some important trace elements such as copper, cobalt and selenium. Molybdenum is generally high and has been implicated in copper deficiency in cattle grazing pasture on the Rotomahana soils.

USES OF PUMICE SOILS

Pumice soil s respond we ll to fertiliser additions of phospho rus, sulphur, potassium. magneS ium and trace elements: because of their low clay contents and weakly weathered nature it is easier to shift their nutrient balance by fertiliser management than it is with more weathered soil s in other parts of New Zealand .

Distinguishing features of volcanic loams PARENT MATERIAL AND DISTRIBUTION - airfall tephras (both rhyolitiC and andesitic).

generally between 3500 and 50000 yea rs old ; also from alluvium or loess containing a high proportion of this tephric material. W idespread in Bay of Plenty. Waikato, King Country and Taranaki.

PROFILE CHARACTERlSTICS - At Band C horizo ns. o A horizons moderately deep (15 -25 em), black to brown in colour; o B horizons quite deep (20 -60 em), commonly yellow-brown in colour but some red-brown

in higher rainfall areas.

TEXTURES - generally sandy loam or fin er (cf. coa rser textures of pumice SOils).

C LAY MINERALS - clay contents generally 10 - 25%. through weathering o f volcanic glasses. Allophane dominates the clay fraction and its content increases (and the halloysite content decreases) with increasing rainfall and leaching of the soil (see Table 3. 1). The content of silicon in so il solution appea rs to be quite important in govern ing whether allophane or halloysite is the predominant clay form. When wet the soils fee l slippery rather than sticky. and there is generally no evide nce of clay movement down the so il profile.

STRUCTURES - are strongly deve loped nut in the A horizon, but B horizo ns have weaker block structure (breaking to fine crumb or gra nular). They are particul arly res istant to puddling by the passage of farm ve hicles or animals.

VERY FRJABLE CO SISTENCE - of both topsoil and subsoil horizons.

BULK DENSITY LOW - generally less than 0.85 T/ml.

MOISTURE RETE NTION HIGH - although much of the water held is not read ily available to plants. The readily-ava ilable water retained by these soils (15 - 25% of so il volume), although reasonable, is not as high as that retained by the pumice soil s. Good drainage is ensured by the highly porous Band C horizons which contain an exceptionally high content of large pores.

ORGANIC MA TIER - stable mineral/o rganic complexes are a feature, particularly of topsoils. The carbon content is high (7 - 14%) and their relatively high carbon to nitrogen ratios indicate the res istance of this organic matter to bio logical breakdown

PHOSPHATE RETENTION VERY HIGH - particu larly in the subsoil, indicating the effect of topsoil organic matte r in decreasing the ability of amorphous clays to retain phosphate. The large capacity of the volcanic loams to adsorb phosphate is one of their most important characteristics and an understanding of the so rptio n behaviour of allophane in the soils is critical for predicti ng fe rti lise r (superphosphate) maintenance requirements.

NO SIGN IFICANT TRACE ELEMENT DEFICIENC IES - although cobalt is marginal in strongly leached volcanic loams such as Mai roa and Mamaku SO il s; low reserves of magneSium and potass ium; sulphur deficiencies un likely; responsive to liming where so il pH < 5.9.

HIGH POPULATIONS OF SOIL ORGANISMS - particularly in A horizon; earthworm populations are similar to those in soils from sedimentary parent rocks. Microbial biomass is generally high, grass grub is a problem in drought-prone pastures, and nematodes are a problem in both pastoral farming and horticulture.

USES OF VOLCANIC LOAMS

Volcanic loams are ge nerally of high value for food production because they are deep and have excellent phYSical properties (free-drainage, good structure, high plant-available moisture retention). They require regular mainte nance dreSSings of phosphatic fertilisers but, nevertheless, they are among our most ve rsat ile so ils for pasture growth (16 - 18 tonnes dry matterlha in Waikato and Taranaki), cropping (maize in Waikato) and horticulture (such as kiwi fruit, berry fruit, pip and stone fruit, citrus, asparagus, feijoas and tamarillos, espeCially in the Bay of Plenty).

Pumice Lands 53