VOLCANIC ASH AND ITS CLAY MINERALOGY AT ... - Science Direct

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VOLCANIC ASH AND ITS CLAY MINERALOGY AT CAPE HOSKINS, NEW BRITAIN, PAPUA NEW GUINEA

P. BLEEKER and R.L. P A R F I T T

Division of Land Use Research, CSIRO, Canberra, A.C.T. (Australia) Chemistry Department, University of Papua New Guinea, Boroko (Papua New Guinea) (Accepted for publication November 11, 1973)

ABSTRACT Bleeker, P. and Parfitt, R.L., 1974. Volcanic ash and its clay mineralogy at Cape Hoskins, New Britain, Papua New Guinea. Geoderma, 11 : 123--135. Volcanic ash falls, the oldest laid down approximately 2500 years ago, are described and correlated wherever possible. Most of these ash deposits are thought to have been derived from the Garbuna volcano, located about 40--50 km from Cape Hoskins. Clay mineral data show with increasing age a weathering sequence in the beds in which allophane changes to halloysite. Charcoal data indicate that large amounts of halloysite are present in fossil soil horizons dated between 300 and 2000 years B.P. This compares with a date of 8000--9000 years from Japan, whereas data from St. Vincent indicate that halloysite can be formed under humid tropical conditions within 4000 years.

INTRODUCTION

The Cape Hoskins area, located on the north coast of central New Britain (Fig.l), is one of several areas in Papua New Guinea where Andosols occur, including other parts of New Britain, Bougainville and the environs of Mount Lamington and Mount Victori. Only the ash-fall layers of M o u n t Lamington have been described (Ruxton, 1966). The area consists of one active and ten extinct volcanos which have been described by Blake and Bleeker (1970). The active volcano is Pago, also known as Bango, which is located inside the caldera of Witori volcano. The volcanoes form mountains rising up to 1300 m above sea level. Most of the area is covered by rain forest. The vegetation of Witori and Pago volcanoes has been described by Paijmans (1973). The average annual rainfall on the coast ranges from 3400 to 4000 mm, almost half of which falls during the first three months of the year. Inland, however, the rainfall is much higher. The soils are developed on several metres of thick-bedded tephra* de* Tephra: volcanic products fragmented by a volcanic eruption.

124 PAPUA NEW GUINEA

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~

~

~I

Cape Hoskm$

w,rc~J /~c

i,~ , ./Hot

\

C

D

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I

,? ~

Fig.1. Locality of the Cape Hoskins area with sites studied.

posits interbedded with buried horizons. Most of these soils according to the 7th Approximation (United States Soil Conservation Service, 1960) belong to the Umbrandept great soil group. This paper describes and correlates some of these young volcanic ash beds, studying their mineralogical properties and their source. SOILS DATA

Soils in three localities of the Cape Hoskins area (labelled A, B and C in Fig.l) were examined in detail. Of these, A and B are on the lower flank of Witori volcano, whereas C occurs in the southwestern part of the area and was sampled because a recently built road had exposed several metres of tephra material. Cross sections of soil pits and the exposure are given in Fig.2, 3 and 4. In these figures field textures and Munsell colour notations have been given with ~4C dates. Samples analysed for clay minerals are indicated. Because it is virtually impossible to disperse allophane soils completely and to measure the exact a m o u n t of clay, no data on granulometric analyses have been given. Correlations between ash beds within profile sections* have been indicated only in clear cases. Work carried out by Ward (1967) in New Zealand has indicated also that it is difficult to correlate ash layers in relatively small areas * This term refers to a section in which several ash beds representing one or more ash showers are exposed.

125

because no profile section ever shows the full sequence of beds. Ward attributed this correlation problem mainly to erosion shortly after an eruption destroying the protective vegetation. However, several other factors could influence changes in profile sections. Variation in thickness of the beds is caused mainly by topography. Distance from the centre of eruption can also influence thickness and grain size of the deposit as will the prevailing wind during the eruption. Differences in weathering can cause variations in colour, texture and clay mineral composition in beds deposited during the same eruption. T h e Lavege road area

Six pits were dug along a line 4.2 k m in length in a south--west direction towards Witori volcano from a point 5.1 km south of Galilo village on the track to Lavege village. A cross section of four pits is shown in Fig.2, pits 3 and 6 being omitted since the upper parts of the profile sections had obviously been disturbed. Pits 1 and 2 show a great similarity in the sequence of their beds, except for the two beds overlying the Witori pumice which vary in both texture and colour. In pit 4 the correlation between the beds of the other pits is more difficult although this pit also has a clearly developed Alb horizon. However, this contains many more pumice fragments and also has a slightly different clay mineral composition (Table I). Beds underlying this Alb horizon also vary in colour and/or texture except for the layer overlying the Witori pumice. In pit 5 no buried A horizon is present, the only correlation with pit 4 being in the Witori pumice layer and the two overlying beds.

SO 8]0-+ 110B.P-

150

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Abrup{ boundary Clear boundary Gradual boundary Dif fvse boundary Few pumice fragmer~s Common p~mi~ fragments

F i g . 2 . P r o f i l e s e c t i o n s o f t h e L a v e g e a r e a . g = g r a v e l ; s = s a n d ; 1 = l o a m ; si = s i l t ; c = c l a y ; f = f i n e ; m = m e d i u m ; c = c o a r s e . T h e s e s y m b o l s a r e a l s o u s e d i n c o m b i n a t i o n s : gls = gravelly l o a m y sand, etc.

126

TABLE I Clay minerals and radiocarbon data Soil pit and sample

Depth (cm)

Minerals of the clay fraction allophane

halloysite

other minerals

Radiocarbon data (B.P.)

B

AB

A

++++ ++ 0 ++++ + 0 0

0 ++ ++ 0 ++ ++ +

0 0 0 0 + 0 0

0 0 ++ 0 0 ++ +++

(Q),(C) (Q),(C) 0 (Q),(C) (Q),(C) 0 (Q),(C)

100--110

0

+

0

+++

0

0--3 65--70 130--135 250--260

+++ 0 0 0

+ ++ + +

0 0 0 0

0 ++ +++ +++

0 0 0 0

320 ± 170 -Probably >2500 years

0--10 76--84 140--170 220--240 370--390 730--760

0 0 0 0 0 0

++ 0 ++ ++ + +

++ +++ 0 0 0 0

0 0 ++ ++ +++ +++

(Q) (C) (K),(M),(V) (K),(V) (K),(V) (K),(V)

--1990 ± 90

Lavege Pl--la lb 2 P4-1 2 3 4 5

0--10 20--30 70-75 0--10 30--40 50--60 75--85

-830 ± 110 --PresnmablyasP2, sample 2 --

Rikau P28--1 2 P27--1 2

Silopy Road P16--1 2 3 4 5 6

K e y t o c l a y m i n e r a l s : + = l i t t l e ; ++ = m o d e r a t e ; +++ = h i g h ; ++++ = v e r y h i g h . 0 = not detectable; ( ) indicates trace amounts; Q ~ quartz; C = cristobalite; K = kaolinite; M = montmorillonite; V = vermiculite.

T h e R i k a u area

In the Rikau area, a sequence of three pits was dug (Fig.3) and supported with some auger observations. These pits cover a distance of approximately 3 km between P28 and P27. Pits 28 and 43 are located on the northern part of the gentle-sloping lower slope of Witori volcano, whereas P27 occurs on the foot slope of Lollo volcano. There is a reasonable degree of similarity between the beds of P28 and P43 but correlation with P27 is less clear, except for a dark sandy ash layer occurring below the A horizon and the Galilo pumice layer which decreases in thickness towards the foot slope of Lollo volcano.

127

0 --

charcoal

data

P 28

lay mineral

(samples)

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Fig.3. Profile sections o f the Rikau area. For legend see Fig.2.

The Silopy road area

A newly built road in the southwestern part of the area has exposed several road cuttings of tephra including several buried horizons. Fig.5 shows part of one of these sections, whereas a more detailed description is given in Fig.4. Five buried horizons have been recognized, and samples 2--6 were taken from these horizons. CLAY M I N E R A L S

Materials and methods

Clay minerals were separated using the field moist soils and dispersed with NaOH at pH 10 (i.e., 10 . 4 M) using a 100 W ultrasonic probe for 5 min. The clay fraction was separated by centrifugation. In preliminary experiments, treatment of the dispersed clay from an A1 horizon (P16) with boiling 6% H202 caused a decrease in the intensity of infrared bands at 970 and 500 cm--', which is where alumina-rich allophane and amorphous alumina show infrared absorption. This suggests that dissolution of these materials takes place during the peroxide treatment. New Zealand

:128

c~a'/minera[s cl~y minerals P16 (sampLes) P16con~ ['IOYR2/2~- 1 ~ [ IILigh~ Ligh'tolive

(samples]-~00

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100-

150

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- 500

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650

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8130

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Fig.5

129

workers also have found that prolonged peroxide treatment of A horizons causes considerable dissolution of alumina, presumable as oxalates formed from the organic matter (K.S. Birrell, personal communication, 1970). Deferration in addition to peroxidation also dissolves amorphous material from volcanic-ash soils (Mitchell et al., 1964; Wada and Greenland, 1970), therefore these pretreatments were avoided and the clay fraction was dispersed with NaOH at pH 10 using an ultrasonic probe. Infrared spectra were obtained on 1% KBr discs of air-dry clays, and for electron microscopy clay suspensions were dried onto collodion-covered carbon-coated grids. Oriented clay samples were examined on glass slides with a Phillips PW 1050 X-ray diffractometer. X-ray p o w d e r photographs were taken using a Phillips PW 1024 camera. Because large amounts of amorphous material mask the infrared spectra and X-ray diffraction lines of small amounts of crystalline 1:1 and 2:1 clay minerals, the samples were treated with NaOH (0.5 M, 90°C, 15 min) to remove amorphous materials when the electron micrographs showed that crystalline clay minerals were present. Diffraction was repeated using the diffractometer, porous tiles and standard chemical treatments. All samples contained large amounts of 'amorphous clay minerals, and it was only possible to make a rough estimate of the a m o u n t of these minerals in the clay fraction by means of X-ray, infrared, and electron microscope data. Results The results of the clay mineral analysis are given in Table I. The ash beds may be considered in t w o groups, those that contain halloysite and those that do not. Beds w i t h o u t halloysite. The clay fraction of the beds that do n o t contain halloysite all contain clay minerals showing broad bands in their infrared spectra in the 3000--3600 cm -1 and 900--1200 cm -1 regions. These spectra are typical of allophane where allophane is defined as a hydrated aluminosilicate material which is amorphous to X-rays (Fieldes, 1955; Mitchell et al., 1964). The spectrum of the clay from P l - - l a shows t w o broad bands centred at 3400 and 1100 cm -1 and a small band at 800 cm -1 (Fig.6a). This is the spectrum of amorphous silica (Fieldes and Furkert, 1966; Shoji and Masui, 1969a, b, 1971). It is similar to the spectra obtained by Fieldes (1955) for allophane B which was originally found in peroxidized A horizons in New Zealand. Amorphous silica (allophane B) was also found in the A horizons of P4 and P28. Fig.4. Profile section of the Silopy road area. For legend see Fig.2. Fig.5. Bedded airfall deposits exposed in the Silopy Road area. (See Fig.4 for details. )

130

¢

Si allophane

i

:

Frequency (crn-I)

Fig.6. I n f r a r e d s p e c t r a o f t h e 2 p fraction, a = pit 1, s a m p l e l a ; b = pit 16, s a m p l e 1; c = pit 16, s a m p l e 2 ; d = pit 16, s a m p l e 5.

The remaining allophane samples showed broad bands at 3400 -~ and near 1000 cm - ' . The latter band can be used to distinguish between a silica-rich allophane and an alumina-rich aUophane. Where the band is above 1000 cm -~ (e.g., Fig.6b), the allophane is silica-rich, and where below 1000 cm -1, it is alumina-rich (e.g., Fig.6c) (Mitchell et al., 1964; Lai and Swindale, 1969; Wada and Greenland, 1970). These allophanes probably correspond to allophane AB and allophane A, respectively, since the spectra are similar to those of Fieldes (1955). In samples P l - - l b , P4--2, P28--1 and P16--1 several broad bands and shoulders were noted between 950 and 1100 cm -~, indicating that the aUophane has a range of compositions in these beds. A band at 800 cm -~ due to amorphous silica (ailophane B) was noted in samples of P l - - l b and P4--2. The other bands and shoulders were assigned to silica-rich allophane (AB) and alumina-rich allophane (A) according to their frequency with respect to 1000 c m - '

Halloysite beds. Halloysite is a crystalline clay mineral and can be identified using X-ray diffraction powder photographs. A 10-& line was observed that collapsed to 7 & on heating to 100°C. HaUoysite appeared as tubes and spherules in the electron micr0graphs (Fig.7A) and was similar to the halloysite reported by Sudo and Takahusti (1956). The electron micrographs showed that allophane occurs in close association with the halloysite and is probably a silica-rich allophane (Parfitt, 1972). The infrared spectra of these clays showed the bands expected for halloysite at 3600 cm -~ and in the 600-1200 cm -~ region (Fig.6d). The bands were surprisingly sharp considering Fig.7. E l e c t r o n m i c r o g r a p h s o f t h e 2 u fraction. A. Pit 1, s a m p l e 2, halloysite, x 3 0 , 0 0 0 . B. Pit 1, s a m p l e l a , a m o r p h o u s silica ( a l l o p h a n e B) × 12,000.

p

-

4b

' ~t

132

the amount of allophane observed in the electron micrographs. The electron micrographs of samples 3, 4, 5 and 6 of the clays from P16 revealed that thin hexagonal-shaped crystals are also present. These were not detected using X-ray diffraction because allophane masks their presence. However, when allophane and halloysite were removed using 0.5 M NaOH for 15 min at 90°C, weak lines appeared corresponding to other clay minerals which are listed in Table I. The clays from the A horizons from P1, P4 and P28 all contained large amounts of amorphous silica (allophane B). The electron micrographs of P1 and P4 showed that this material largely consisted of thin pla.te-like particles with a number of small circular holes (Fig.7B). They were similar to the electron micrographs of opaline silica found in young Japanese ash soils (Shoji and Masui, 1971), except that their shape was not as regular, suggesting that the fragments had been weathered and/or broken by the ultrasonic probe. X-ray diffraction revealed weak but characteristic lines due to quartz and cristobalite in some samples. These are given as trace amounts in Table I. Fibres similar in appearance to imogolite were observed under the electron microscope in several beds, but they could not be extracted by the usual dissolution techniques. DISCUSSION

Samples P l - - l a and P4--1 of the Lavege area are dominated by amorphous silica (allophane B). Samples P l - - l b and P4--2 contain some amorphous silica (allophane B), but silica-rich allophane (AB) is also present. In the Rikau area both amorphous silica (allophane B) and silica-rich allophane (AB) occur together in the A horizon of P28. These results agree with the earlier findings for temperate regions that allophane B occurs in the youngest volcanic-ash soils (Fieldes, 1955; Kanno, 1959). Fieldes (1955) proposed that allophane B derives from glass and feldspar in a weathering sequence that advances with increasing age through the stages (1) volcanic glass and feldspar, (2) allophane B, (3) allophane AB, (4) allophane A and (5) halloysite. No amorphous silica (allophane B) was found in the Silopy Road exposure. Here, silica-rich allophane (AB) occurred in the A horizon together with alumina-rich allophane (A). If it is assumed that allophane AB follows allophane B in the weathering sequence this would suggest a more advanced stage of weathering in the A horizon in comparison with the two other locations. This could be explained by the addition of some fresh ash from recent Pago eruptions to the Lavege and Rikau areas which are very close to the Pago volcano. The eight major lava flows on Pago each represent a major eruption that took place between the formation of Pago and 1914--17, when the last eruption is known to have occurred (Blake and Bleeker, 1970). During the last eruption very little emission of ash took place as told by Boas, a local chieftain who was a young boy a t the time of this eruption. However, gardens in

133

the vicinity of Pago were destroyed. The other Pago eruptions were also thought to have caused little deposition of ash, the emission of lava being the major phase of all eruptions. However, some ash m a y have been deposited. For instance, a clearly recognizable dark medium sandy-ash layer, 6--10 cm thick, occurring close to the surface in the Rikau area probably represents ash deposited during one of the earlier Pago eruptions. This layer was dated less than 320 + 170 years B.P.* by charcoal fragments found in another, deeper ash bed in P28 at a b o u t 65 cm below the surface**. This dark sandy-ash layer was n o t found on any o f the eight lava flows, indicating that they are less than 300 years old. However, a little deposition of ash with each eruption could have caused regular rejuvenation in nearby areas explaining the dominance of allophane B in the u p p e r m o s t A horizons of the Rikau and Lavege areas, this being absent in the Silopy R o a d area. In the first buried soil of the Silopy Road area (sample 2 at 76--84 cm below the surface), silica-rich allophane (AB) was absent (Table I) and has presumably weathered to alumina-rich allophane (A). The indications in Cape Hoskins, Japan and New Zealand are that halloysite is present in old beds, allophane in y o u n g beds. It is therefore assumed that halloysite is probably formed from allophane. Charcoal fragments from sample 2 of P1 in the Lavege area have a date of 830 + 110 years B.P. This bed also contains halloysite, and even sample 2 of P28 in the Rikau area (dated a b o u t 300 years) had halloysite in the clay fraction. Similarly the deepest layer (sample 6) of the Silopy Road area located 7.5 m below the surface gave a date of 1990 + 90 years B.P., whereas three of the four overlying buried soils also showed halloysite in the clay fraction. All these data indicate that halloysite can form very rapidly under humid tropical lowland conditions. These data compare with a subtropical soil containing halloysite in Japan and having a radiocarbon date of 8000--9000 years (Aomine and Miyauchi, 1963). Data from a 4000-year old volcanic-ash soil on St. Vincent (13°15'N 61°12'W) (Hay, 1960) show that halloysite and allophane were the dominant clay minerals formed under humid tropical conditions. A date of 2590 + 300 years B.P. from a buried soil horizon underlying a thick pumice layer (named Galilo pumice) near Galilo village is interpreted to represent the last paroxysmal eruptiori of Witori volcano before it became extinct and during which the caldera was formed. This date closely corresponds to a date obtained from charcoal fragments of a horizon underlying a welded tuff layer south-west of the Witori caldera (2590 + 250 years B.P.; site D in Fig.l). However, charcoal collected beneath a thick pumice layer from a cliff exposure at a head of a gully near the caldera rim gave a date of 1530 + 100 years B.P. and appears anomalous, the charcoal possibly being tree roots charred during a later eruption. It is therefore assumed that the Galilo and Witori pumice beds of the Lavege * C h a r c o a l s a m p l e s w e r e d a t e d b y t h e ~4C m e t h o d b y Dr. Kogoshi o f G a s k u s h u i n U n i v e r s i t y , Tokyo. ** This date r e p r e s e n t s t h e m i n i m u m age o f t h e l a y e r w h e n it was b u r i e d b y a n e r u p t i o n .

134

and Rikau areas, which are very similar in appearance and granulometric composition (Bleeker, unpublished data), are of the same age. This would mean that sample 2 of P27 represents the oldest bed sampled; this is not contradicted nor supported by the clay mineral data since these change little beyond a minimum age of 300 years and a m a x i m u m age of 2500 years. Correlation of the Lavege and Rikau ash beds with those of the Silopy Road area is very difficult because of the lack of charcoal dates and the distance separating the areas. Charcoal collected from sample 6 at 730--760 cm below the surface gave a date of approximately 2000 years. Four other fossil horizons occur within this sequence representing relatively long periods during which no eruptions took place. Of these, samples 3 and 4 show a clay mineral composition similar to P1--2 and P28--2 and might therefore be of approximately the same age. Sample 5 could possibly represent the eruption 1500 years ago as indicated by the charcoal fragments found in a gully near the caldera rim. Since the last Witori eruption, 2500 years ago, more than 2 m of ash has been deposited in the Lavege and Rikau area. Except for some rejuvenation on the surface, very little material from Pago volcano has been added to the soils. In the Silopy Road exposure more than 7 m of ash has been deposited in 2000 years. It is clear that most of the ash in the Cape Hoskins area must therefore have been derived from a source other than Witori and Pago. Since the ash deposits tend to become thicker to the west, Garbuna volcano (Fisher, 1957) seems the most likely source (see Fig.l). This volcano is located between 40 and 50 km away from the Hoskins area. Although this seems relatively far away, very large areas at considerable distances from volcanoes in other parts of the world are known to have been covered by thick layers of ash. The New Zealand Taupo shower, for instance, is more than 60 cm thick over an area of upwards of 2500 km 2 and extends to a distance of up to 60 km from the crater (Cotton, 1952). ACKNOWLEDGEMENTS

The authors are grateful to Dr. D.H. Blake for the valuable discussions and comments during and after the field work and acknowledge the assistance of Dr. D.G. Lewis and Mr. T. Sherwin of the University of Adelaide in obtaining the electron micrographs. REFERENCES Aomine, S. and Miyauchi, N., 1963. Age of the youngest hydrated halloysite in Kyushu. Nature (London), 199: 1311--1312. Blake, D.H. and Bleeker, P., 1970. Volcanoes of the Cape Hoskins area, New Britain, Territory of Papua and New Guinea. Bull. Volcanol., 34(2): 385--405. Cotton, C.A., 1952. Volcanoes as Landscape Forms. Whitcombe and Tombs, Christchurch, 2nd ed., 416 pp. Fieldes, M., 1955. Clay mineralogy of New Zealand soils. II. Allophane and related mineral colloids. N. Z. J. Sci. Technol., Sect. B, 37: 336--350.

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Fieldes, M. and Furkert, R.J., 1966. The nature of allophane in soils. II. Differences in composition. N. Z. J. Sci., 9: 608--622. Fisher, N.H., 1957. Catalogue of the Active Volcanoes of the World. Part V. Catalogue of the Active Volcanoes and Solfatara Fields of Melanesia. Int. Volc. Assoc., Naples, 105 pp. Hay, R.L., 1960. Rate of clay formation and mineral alteration in a 4000-year-old volcanic ash soil on St. Vincent, B.W.I. Am. J. Sci., 258" 354--368. Kanno, I., 1959. Clay minerals of volcanic-ash soils and pumices from Japan. Adv. Clay Sci., 1: 213--233. Lai, Sung-Ho and Swindale, L.D., 1969. Chemical properties of allophane from Hawaiian and Japanese soils. Soil Sci. Soc. Am. Proc., 33: 804--808. Mitchell, B.D., Farmer, V.C. and McHardy, W.J., 1964. Amorphous inorganic materials in soils. Adv. Agron., 16: 327--383. Paijmans, K., 1973. Plant succession on Pago and Witori volcanoes, New Britain. Pac. Sci., 27 : 260--268. Parfitt, R.L., 1972. Amorphous material in some Papua New Guinea soils. Soil Sci. Soc. Am. Proc., 36: 683--686. Ruxton, B.P., 1966. Correlation and stratigraphy of dacitic ash-fall layers in northeastern Papua. J. Geol. Soc. Aust., 13: 41--67. Shoji, S. and Masui, J., 1969a. Amorphous clay minerals of recent volcanic ash soils in Hokkaido. 1. Soil Sci. Plant Nutr. (Tokyo), 15: 161--168. Shoji, S. and Masui, J., 1969b. Amorphous clay minerals of recent volcanic ash soils in Hokkaido. 2. Soil Sci. Plant Nutr. (Tokyo), 15: 191--201. Shoji, S. and Masui, J., 1971. Opaline silica of recent volcanic ash soils in Japan. J. Soil Sci., 22: 101--108. Sudo, T. and Takahashi, H., 1956. Shapes of halloysitic particles in Japanese clays. Clays Clay Min., Proc. Natl. Conf. Clays Clay Min., 4: 67--79. United States Soil Conservation Service, 1960. Soil Classification; A Comprehensive System. U.S. Government Printing Office, Washington, D.C., 265 pp. Wada, K. and Greenland, D.J., 1970. Selective dissolution and differential infrared spectroscopy for characterization of "amorphous" constituents in soil clays. Clay Min. Bull., 8: 241--254. Ward, W.T., 1967. Volcanic ash beds of the lower Waikato Basin, North Island, New Zealand. N. Z. J. Geol. Geophys., 10: 1109--1135.