Non-crystalline Inorganic Constituents of Soil

Abstract

Non-crystalline inorganic constituents of soil, such as volcanic glasses, phytoliths, laminar opaline silica, allophane, and imogolite are introduced using optical and electron microscope images and energy dispersive X-ray (EDX) analysis. The Al-humus complex and Al-rich Sclerotia grains are also introduced. The volcanic glasses are formed from magma and can be categorized as primary. All of these non-crystalline inorganic constituents are found in volcanic ash soils. Among these, phytoliths can be found under vegetation in many other soils than volcanic ash soils. Formation of allophanic materials from fresh pumice is shown stepwise using polished sections to demonstrate microscopic distribution of elements and inorganic constituents. Allophane and imogolite are rich in Al whereas their parent material, volcanic ash, is silica-rich. Changes in morphological property and element concentration of volcanic ash or volcanic glass during the formation of these secondary non-crystalline constituents are discussed.

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

4.1 Introduction

The non-crystalline constituents present in soil depend strongly on soil environmental conditions. The non-crystalline silicate and silica constituents in soil are volcanic glass , allophane , imogolite , laminar opaline silica , and phytoliths (Table 4.1). All these are present in volcanic ash soils. Among these materials, allophane and imogolite are also found in spodic horizons, such as Bs and Bhs horizons, and they confer unique properties upon Andisols and Spodosols . Volcanic ash soils are found in volcanic areas worldwide. Spodosols are found mainly in the cold regions of the world, and can also be formed in volcanic ash deposits. Phytoliths form in plant cells and occur commonly in many A and buried A horizon soils.

Table 4.1 Non-crystalline silicate and silica constituents in soil

The Al-humus complex is also covered in this chapter despite its partially organic structure because it is one of the major constituents of active Al. Active Al is important to the formation of Andisols and Spodosols , classification of these soils, and the phosphate sorption reaction.

Other non-crystalline materials which may be present in soils include non-crystalline phases of iron sulfide , iron phosphate, and aluminum phosphate. Non-crystalline iron sulfide and iron phosphate were covered in Chap. 5, and non-crystalline aluminum phosphate were covered in Sect. 6.4. The term “volcanic ash soils” encompasses all soils derived from volcanic ash, whereas Andisols are soils defined by Soil Taxonomy of United States Department of Agriculutre (USDA).

4.2 Volcanic Glass

Volcanic ash, in this monograph, refers collectively to volcanic ejecta or tephra , including pyroclastic fall and flow materials such as volcanic ash, cinders , lapilli , scoria , and pumice (Dahlgren et al. 1993). Volcanic glass is a non-crystalline silica-alumina material and is a major constituent of the volcanic ash ejected from volcanoes (Yamada and Shoji 1975; Heiken and Wohletz 1985). The morphological types of volcanic glasses include sponge-like, bubble-wall type (curved platy), fibrous, and berry-like (Shoji et al. 1993). The diameter of sponge-like and berry-like volcanic glasses can exceed 2 mm. The diameter of volcanic glass ranges below 2 μm. The color of volcanic glasses is related to the rock type of volcanic ash. Sponge-like, bubble-wall, or fibrous glasses are non-colored. Coloured volcanic glass is mostly berry-like with crystallites.

4.2.1 Chemical Composition of Volcanic Glasses

Shoji et al. (1975) classified volcanic ashes into five rock types based on the total SiO₂ content. The five rock types are rhyolite , dacite , andesite , basaltic andesite , and basalt . Their total SiO₂ content ranges are 100–70, 70–62, 62–58, 58–53.5, and 53.5–45%, respectively. Reclassification of 26 tephra reported by Shoji et al. (1975) using the updated classification of Le Maitre (2002), known as the total alkali silica classification (TAS diagram ), yielded the same results for 22 tephra. Two of the remaining tephra were close to the boundary between andesite and dacite, and the other two lay between dacite and rhyolite , because the Na₂O + K₂O contents of all 26 tephra were below the boundaries between the four rock types (basalt, basaltic andesite, andesite, and dacite) and the corresponding trachy-types of the TAS diagram .

Volcanic ash is sorted by its particle size and the specific gravity of its constituent mineral particles during transportation in air. In the case of Tarmae-a (Ta-a) tephra (AD1739), Hokkaido, Japan, the heavy mineral content decreased with distance from source volcano. As the heavy mineral content decreased, the Fe and Mg content of Ta-a tephra also decreased while the Si content increased correspondingly (Mizuno et al. 2008).

The color of volcanic glasses is related to the rock type of the tephra. Volcanic ash of rhyolite , dacite , and andesite rock types is dominated by non-colored volcanic glasses including a slightly purplish one, whereas basalt and basaltic andesite rock type is dominated by colored volcanic glasses (Shoji 1986). The chemical composition of volcanic ash is also related to rock types. The Al₂O₃, FeO, MgO, and CaO content of 26 volcanic ash samples significantly, and Na₂O, K₂O and TiO₂ also correlated with SiO₂ content (Shoji et al. 1975). In nine non-colored volcanic glass samples, SiO₂ content ranged between 74.24 and 77.89%, and Al₂O₃ content ranged between 12.65 and 14.71%. The variation in the concentrations of SiO₂ and Al₂O₃ of the non-colored volcanic glasses are minor compared to that for the total SiO₂ and Al₂O₃ concentrations of the volcanic ashes. In contrast, in glasses present in basaltic andesite and basalt volcanic ashes, the Al₂O₃, Fe₂O₃, MgO, and CaO contents increase with decreasing SiO₂ content.

Figure 4.1 shows the elemental composition of volcanic glasses in the form of EDX spectra. The vertical axis (y-axis) shows the relative amount of each atom as a percentage of the total. Although peak height in EDX spectra does not directly yield the actual percentage of each atom in the sample, the EDX spectrum-mimic graphs are useful proxies to compare overall elemental composition quickly.

Fig. 4.1

Chemical analyses of volcanic glass collected from volcanic ash of different rock types (Shoji et al. (1975)). (a) Towada-a tephra with rhyolite type, (b) Chuseri tephra with dacite type, (c) Tarumae-a tephra with andesite type, (d) Zao-b tephra with basaltic andesite type, (e) Hoei tephra with basalt type. The vertical axis (y-axis) shows the percentage (the number of atom) of each element. The horizontal axis (x-axis) shows energy of characteristic X-ray as in an EDX spectrum (EDX spectrum-mimic graphs)

4.2.2 Sponge-Like Volcanic Glass

Figure 4.2 shows an example of sponge-like volcanic glass . Among the three types of noncolored volcanic glasses , the sponge-like volcanic glass is the most common (Shoji 1986). As shown in the scanning electron microscope (SEM) image (Fig. 4.2b), sponge-like volcanic glass is highly vesicular, suggesting strong bubbling in the viscous magma during the eruption. The EDX spectrum in Fig. 4.2c indicates that the Si concentration is high and the Mg and Fe concentrations are low. The sponge-like volcanic glass includes various amounts of phenocrysts. Large sponge-like volcanic glass is synonymous with pumice .

Fig. 4.2

Sponge-like glass particle. (a) Optical micrograph , (b) SEM image of the dashed area of the sponge-like glass particle, (c) EDX spectrum of the dashed area in (b). The sponge-like glass particles were collected from the sample shown in Fig. 2.7c (the 2–0.5 mm fraction of Mt. Pinatubo 1991 volcanic ash)

4.2.3 Bubble-Wall Type Volcanic Glass

The bubble-wall type glasses shown in Fig. 4.3 were separated from Kikai-Akahoya (K-Ah) tephra , one of the widely distributed tephra in Japan (Machida 2002b), in Miyazaki Prefecture, Japan. At this site, the K-Ah tephra resides at a depth of several tens of centimetres, shown with the label “K-Ah” in Fig. 4.3a, b. After H₂O₂ digestion and sieving, bubble-wall type glasses (Fig. 4.3c, d) were handpicked from the sand fraction. The bubble-wall type glasses appear to be formed by pulverization of much large vesicles or bubbles from highly viscous magma. The EDX spectrum (Fig. 4.3e) shows high Si and low Mg and Fe concentrations, and is similar to that shown in Fig. 4.2c.

Fig. 4.3

Bubble-wall type volcanic glass . (a) Soil profile having Kikai-Akahoya (K-Ah) tephra , (b) close-up of the K-Ah deposit, (c) optical micrograph, (d) SEM image of bubble-wall type volcanic glass separated from the K-Ah deposit, (e) EDX spectrum of a glass particle of dashed square in (d)

4.2.4 Fibrous Volcanic Glass

The huge eruption that created the Aira Caldera (northern part of Kagoshima bay, Japan) produced the Ito pyroclastic flow deposit (Aramaki 1984; Baer et al. 1997) (Fig. 4.4a). Fibrous glasses (Fig. 4.4b, c) were handpicked from the pyroclastic flow deposit (Fig. 4.4d) after ultrasonic treatment (Busacca et al. 2001) to remove fine particles. The fibrous glass contains many elongated vesicular pores as shown in Fig. 4.4c. The fibrous glass is a minor type of non-colored volcanic glass . Major constituents of the Ito pyroclastic flow deposit are sponge-like glass, bubble-wall type glass , plagioclase , and quartz , among others (Sonehara 2016). The EDX spectrum of the fibrous volcanic glass is close to those shown in Fig. 4.2c and Fig. 4.3e.

Fig. 4.4

Fibrous volcanic glass . (a) The Ito pyroclastic flow deposit (29 ka), (b) fibrous volcanic glass particles separated from the air-dried Ito pyroclastic flow deposit, (c) the SEM image of an edge of fibrous volcanic glass, and (d) close-up of the sampled pyroclastic flow deposit of the EDX spectrum of the dashed square

4.2.5 Berry-Like Volcanic Glass

Mt. Fuji emits scoriaceous tephra (Kobayashi et al. 2007; Miyaji 2002). Soil horizons of A1 to C were formed from Hoei, and horizons of 2A1 from Jogan tephra (Fig. 4.5a). This soil is used as farmland to grow vegetables (Fig. 4.5b). Figure 4.5c shows the 0.5–0.2 mm fraction of the scoriaceous Jogan tephra (2A1 horizon of Fig. 4.5a). This particle-size fraction is colorful, displaying dark-colored scoria (Fig. 4.6), red-colored scoria (Fig. 4.7), olivine (Fig. 2.4), and others.

Fig. 4.5

Berry-like volcanic glass . (a) Scoria deposits from Mt. Fuji, (b) landscape of the pedon site, and (c) 0.5–0.2 mm fraction of the 2A1 horizon of the scoria deposit

Fig. 4.6

A scoria particle separated from Fig. 4.5c. (a) Optical photograph, (b) SEM image, (c) EDX spectrum of the large dashed area shown in (b), (d) a magnified SEM image of the small dashed area in (b)

Fig. 4.7

A red scoria particle separated from the sample shown in Fig. 4.5c. (a) Optical micrograph (b), SEM image, (c) EDX spectrum of the large dashed area shown in (b), (d) a magnified SEM image of the small dashed area in (b)

Figure 4.6 shows an example of a scoria particle or colored volcanic glass . The particle is dark and has many concavities (Fig. 4.6a, b). The EDX spectrum (Fig. 4.6c) of the dashed area (Fig. 4.6b) shows that Mg, Al, Ca, and Fe concentrations are high, and that the Si concentration is low compared with non-colored volcanic glasses (Figs. 4.2c, 4.3e, and 4.4c). Many plagioclase crystallites are visible in the magnified SEM image (Fig. 4.6d).

Brownish red particles can be found in Fig. 4.5c, and they are also scoriaceous. Figure 4.7a is a red scoriaceous particle and Fig. 4.7b is the SEM image of Fig. 4.7a that resembles Fig. 4.6b in that it has round concavities. The EDX spectrum (Fig. 4.7c) of the dashed square in Fig. 4.7b is similar to that of the dark-colored scoria particle (Fig. 4.6c) in terms of the high Mg, Al, Ca, and Fe concentration compared with those for Figs. 4.2c, 4.3e, and 4.4c. In the magnified SEM image (Fig. 4.7d), small plagioclases are identified by EDX analyses. The brownish red color of the particle shown in Fig. 4.7a can be explained by high-temperature oxidation of iron.

4.3 Secondary Non-crystalline Inorganic Constituents

Non-crystalline inorganic constituents in this section are newly formed in soil or plants whereas volcanic glasses are of magmatic origin.

4.3.1 Allophane and Imogolite

Allophane and imogolite (Yoshinaga 1986; Wada 1989; Harsh et al. 2002) are typical weathering products of volcanic glass under a humid climate and well-drained conditions; they are also found in the spodic horizon soils. Allophane has a spherical structure and is hollow inside with some holes in its wall (Fig. 4.8a). Allophane is a non-crystalline aluminosilicate with a Si:Al atomic ratio of 1:2〜1:1. The range of diameters of allophane spherules is 3–5 nm. Imogolite has a tubular structure and a Si:Al atomic ratio of 1:1 (Fig. 4.8b). The diameter of an imogolite tube is 2 nm.

Fig. 4.8

Cross sections of (a) allophane and (b) imogolite , (c) plane view from the inside of a unit particle model, and (d) its side view

The outside of allophane and imogolite resembles a gibbsite sheet (Fig. 4.8c). The dehydration of three H₂O molecules from the three front OH groups and the three OH groups of Si(OH)₄ yields the aluminosilicate wall of allophane (Fig. 4.8c). Because there is no Si-O-Si bond, allophane and imogolite are nesosilicates . The Si-OH groups point towards the inside of the spherules and tubes, as shown in Fig. 4.8a, b, respectively. Allophane having a Si:Al ratio of 1:2 is called Al-rich allophane and is the major type found in Andisols. When the atomic Si:Al ratio of allophane approaches 1, it is considered Si-rich allophane, the Si dimer is thought to increase. The Si-rich allophane is sometimes found in the weathered pumice layer of the deeper part of Andisol profiles.

Imogolite gel films can be found in field-moist weathered pumice (Fig. 4.9a) taken from the 2Bw1 horizon of the pedon shown in Fig. 4.19. After separation of the film from soil clods and suspending it in pure water followed by ultrasonic treatment, a thin imogolite suspension can be prepared. After spotting the suspension on the micro grid attached copper mesh, a photograph of allophane and imogolite can be obtained using transmission electron microscope (TEM) (Fig. 4.9b). The thin fibrous material visible in Fig. 4.9b is imogolite, and the small distorted circles are allophane. Similar imogolite gel film (Fig. 4.9c) was also found in the 3Bw5 horizon (Fig. 4.19).

Fig. 4.9

Allophane and imogolite . (a) A gel film (white arrow) in the 2Bw1 horizon (Fig. 4.19a), (b) TEM image of allophane and imogolite in the gel film, (c) similar gel film (white arrow) found in the 3Bw5 horizon (Fig. 4.19a)

As shown in Fig. 4.9a and c, the imogolite gel film exists at the voids among pumice particles, and it contains not only imogolite but also allophane, suggesting that these films are formed through precipitation reactions. Allophanic materials form as altermorphs of vesicle walls inside the pumice particle as shown later in Figs. 4.26 and 4.29. Thus, two types of occurrences are possible for allophane in the weathered pumice .

The allophane and imogolite content can be estimated by oxalate extraction as 7.1 times Si_o, where Si_o denotes the amount of oxalate-extractable Si (Parfitt and Henmi 1982; Parfitt and Wilson 1985). The occurrence of allophane and imogolite is further discussed in Sect. 4.4.

4.3.2 Laminar Opaline Silica

Laminar opaline silica or pedogenic opal is a thin, disk-like, or elliptical form of silica (Fig. 4.10b), with diameters ranging from several micrometers to submicrons. Laminar opaline silica is often found in the upper horizons of young volcanic ash within a few thousand years (Shoji and Masui 1969, 1971; Shoji and Saigusa 1978; Henmi and Parfitt 1980). Chemically, laminar opaline silica is non-crystalline silica. Laminar opaline silica is more common in A horizons where Al is complexed with humus and Al-O-Si bonding is inhibited. For example, in the A and Bw horizons of volcanic ash soil, the Si concentration in soil solution is 2–6 mg Si L⁻¹ in a humid climate (Ugolini et al. 1988), and Si can become sufficiently concentrated to precipitate silica when evaporation of water from the surface horizon to air is high or, alternately, the surface soil is frozen. Wada and Nagasato (1983) reported formation of silica microplates by freezing a silica solution in the laboratory.

Fig. 4.10

Occurrence of laminar opaline silica . (a) Soil profile of Udivitrand which includes opaline silica at the A horizon, (b) SEM image of the clay fraction containing thin disc-like opaline silica, (c) EDX spectrum of laminar opaline silica

Figure 4.10a shows a profile of Udivitrand in Hokkaido, Japan. In the clay fraction of the uppermost A horizon, laminar opaline silica particles can be easily found. The EDX spectrum of laminar opaline silica is highly dominated by Si (Fig. 4.10c).

4.3.3 Phytoliths

Phytoliths , or plant opals , are a type of biogenic opals (Wilding et al. 1977; Drees et al. 1989). Phytoliths are typically found in the particle size fraction of 5–50 μm and are larger than laminar opaline silica (Fig. 4.10b). Phytoliths are abundant in A or buried A horizons. Phytoliths are non-crystalline silica, formed in plant cells. As the morphological form of a phytolith is specific to plant species to some extent, the form of phytoliths in a buried A horizon can be used to estimate the paleovegetation of the buried A horizon (Kondo et al. 1988). Phytoliths possibly benefit plants by contributing to Si nutrient cycling.

Figure 4.11a shows an example of the Udand profile, which has abundant phytoliths in the A1 horizon. Figure 4.11c is an optical micrograph of the 0.05–0.02 mm fraction, and particles indicated by white arrows can be judged as phyoliths according to their morphological characteristics.

Fig. 4.11

Occurrence of phytoliths . (a) Udand profile displaying abundant phytoliths in the 0.05–0.02 mm fraction of the A1 horizon, (b) present-day vegetation, (c) optical micrograph of the 0.05–0.02 mm fraction

Biogenic opals, laminar opaline silica , and volcanic glasses are non-crystalline. Morphological characteristics are used to identify non-crystalline inorganic constituents. Although XRD is a powerful tool for the identification of crystalline constituents, it is not effective for non-crystalline inorganic constituents. Elemental composition is an useful tool for identification of non-crystalline inorganic constituents. EDX is effective for the determination of the approximate elemental composition of particles in soil.

In order to examine the andic soil properties in USDA Soil Taxonomy and andic properties in World Reference Base for Soil Resources, glass counting is needed for the young soils (Eden 1992). In glass counting, the 2–0.02mm fraction is used and a distinction between volcanic glass and phytoliths is necessary. Subdivision of the 2–0.02mm fraction into 2–0.2, 0.2–0.05 and 0.05–0.02mm fractions is effective in allowing each particle to be identified. Fractionation of these particle-size fractions should be done after H₂O₂ digestion, dithionite-citrate-bicarbonate treatment, and wet sieving. If data on the smaller particle-size fraction are needed, the 0.02–0.005-mm fraction can be prepared, removing the fraction less than 0.005-mm fraction by a dispersion –sedimentation and siphoning procedure. For quantitative purpose, eight repetitions of this procedure must be performed.

A few characteristics of the 2–0.005mm particle-size fraction can be noted. Phytoliths are not included in the 2–0.2mm fraction, and are not prevalent in the B and C horizon soils. Volcanic glasses are present in all size fractions, although the abundance of volcanic glass in each particle-size fraction changes depending on the individual sample (Yamada and Shoji 1975).

Figure 4.11c shows an optical micrograph of the 0.05–0.02mm fraction sampled from A1 horizon of Fig. 4.11a. From the morphological characteristics of the particles, the three particles indicated by white arrows were identified as phytoliths . However, identification may be rendered difficult owing to weathering or the presence of composite particles containing different materials. In that case, SEM image (Fig. 4.12a), element maps (Fig. 4.12b, c), and micrographs (Fig. 4.12d) are effective tools for distinguishing among volcanic glass , phytoliths, and crystalline minerals. An important point is that the same sample must be used for both polarizing microscope and SEM -EDX observations. For a detailed procedure, see footnote¹.

Fig. 4.12

Distinction of inorganic particles. (a) SEM image, (b) Al and (c) Si element maps, (d) photograph using crossed polarizers of the lower right area of Fig. 4.11c

Only the wholly visible particles in Fig. 4.12 (numbered particles 1 through 15) were targeted for interpretation. Figure 4.12b, c are element maps of Al and Si, respectively. Particles 7, 13, and 14 are seen to contain Al. All the particles contain Si. Particles 1 to 6, 8 to 12, and 15 contain few elements other than Si, suggesting that they are silicas. Among these silica particles, a micrograph with crossed polarizers (Lynn et al. 2008) (Fig. 4.12d) suggests that particles 3, 5, 6, 8, 9, 14, and 15 are anisotropic, which indicates that they are crystalline silica minerals. The same results were obtained after turning the rotating stage of the polarizing microscope. Other silica particles (1, 2, 4, 10, 11, and 12) are isotropic, suggesting that they are phytoliths . Particle 13 is identified as volcanic glass from its vesicular morphological properties in addition to its elemental composition (Fig. 4.12b, c) containing both Si and Al. Particles 7 and 14 appear to be aluminosilicates. Although particle 7 is isotropic, its Al content is too high to allow for identification as a volcanic glass.

Diatoms, which are also biogenic opals, are also found in the clay or silt fractions of A horizon soils. Diatoms can be identified by their characteristic frustule. As laminar opaline silica is almost always smaller than 5 μm, it does not affect the glass counting of the 2–0.02 mm fraction.

Thus, by using both polarizing microscope and SEM -EDX, the ability to identify non-crystalline soil constituents such as volcanic glass and phytoliths is greatly enhanced. Remaining issues include a few phytoliths and diatoms that are weakly anisotropic, and crystalline particles completely covered by volcanic glass. Although some iron minerals are not transparent under optical microscope, they can be identified using an Fe element map.

4.3.4 Al-Humus Complex

Al-humus complex (Al-humus) is one of the active Al materials in Andisols and Spodosols . Al-humus was recently reviewed by Takahashi and Dahlgren (2016). This section is focused on Al-humus in Andisols , because in these soils Al-humus is much less mobile, whereas that in Spodosols appears more mobile and is related to eluviation–illuviation processes (Ito et al. 1991). The justification for covering Al-humus despite its partially organic structure is that (i) it is highly reactive with phosphate, (ii) Al-humus formation appears to be competitive with allophane and imogolite in Andisols depending on the pH and organic matter content, and (iii) it is the major active Al form in the A horizons of non-allophanic Andisols .

The amount of Al in Al-humus is typically measured as the Al extractable by pyrophosphate extraction (Al_p) (Dahlgren 1994). Al-humus cannot be chemically isolated from Andisols, but its structure is roughly estimated from cation exchange capacity (CEC), Al_p, functional group analysis of humic acid (Fig. 4.13), and OH⁻ release through the reaction with F⁻. The presence of hydroxyl groups in Al-humus is suggested by the high pH (NaF) of non-allophanic Andisols (Shoji et al. 1985). Figure 4.13 was constructed using data from A horizon soils of typical non-allophanic Andisols excluding the uppermost A horizons with fresh organic matter, which cannot yet be Al-humus .

Fig. 4.13

The approximate formula of an Al-humus complex at pH 7. Of every 40 organic carbon atoms, four belong to carboxyl groups in A-type humic acid (Yonebayashi and Hattori 1988), three of which are complexed with Al (Shoji et al. 1993), and one of which will dissociate to have a negative charge. The amount of negative charge was derived from the slope of the regression line between organic C content and CEC

4.4 Andisols: Soils Dominated by Non-crystalline Inorganic Constituents

Non-crystalline inorganic constituents are important to both the parent materials and soil formation products of Andisols . The major parent material of Andisols is volcanic ash, and the major constituent of the volcanic ash is volcanic glass (Fig. 4.14 and Table 4.2). Major soil formation products in Andisols are allophane , imogolite , and Al-humus complexes, especially under humid climate and good drainage conditions. Kaolin minerals are also present in the soil formation products in the lower horizons of Udands, in Udands under poor drainage, and in Andisol under semi-dry climate. In this section, the processes of Andisol formation are introduced (Shoji et al. 1993; Arnalds et al. 2007; McDaniel et al. 2012).

Fig. 4.14

Schematic of Andisol formation from volcanic ash. Different profiles (a, b, and c) are formed depending on the soil forming factors

Table 4.2 The Si/Al atomic ratio of major constituents in volcanic ash soils

The distribution of Andisols is largely determined by the locations of volcanic regions. The major volcanic regions in the world are the Circum-Pacific volcanic zone, the Hawaiian Islands, the Sumatra-Java-Lesser Sunda Islands, Eastern Africa, the Mediterranean area, Iceland, the Azores, and the Canary Islands (Sievert et al. 2010; Machida 2002a). In the case of huge volcanic eruptions, which occur intermittently, interspersed with dormant periods of hundreds or thousands of years, a large amount of volcanic ash affects wide areas (Machida 2002a). Volcanic ash deposits in these areas are important parent materials of soils.

Figure 4.14 shows a schematic of Andisol formation from volcanic ash. Highly explosive eruptions are caused by viscous, Si-rich magma, which form non-colored volcanic glass . In the mid-latitudes, volcanic ash is mainly distributed on the eastern side of volcanoes owing to the influence of the strong westerly winds, whereas at low latitudes the ash is found either all around the volcano or slightly to the western side owing to the influence of the weak trade winds.

Volcanic ash deposits on the slopes of volcanoes are further transported by water as volcanic mudflows (lahar). In the case of the Mt. Pinatubo ash deposit in 1991, a large amount of volcanic ash was transported to lower elevations along large rivers during the rainy season (Fig. 2.7). Mudflows continued to occur for several years after the eruption.

The occurrence of intermittent huge eruptions accompanied by large ash falls is evident from the thaptic property in the vicinity of a volcano. The thaptic property is the existence of a buried Andisol profile under a new volcanic ash soil profile , forming a multisequum profile (Fig. 4.14a, b). Within the volcanic ash-affected area, the amount of ash fall and the size of the ash particles decreases as the distance from the volcano increases. As a result of thin and cumulative deposition of volcanic ash, a thick humus-rich A horizon, which is a characteristic of the Pachic subgroup in USDA Soil Taxonomy, is formed (Fig. 4.14c). Andisols are typically formed on hills and uplands with good drainage. Leaching losses of Si from volcanic ash cause higher Si concentrations in the rivers in the vicinity of volcanoes than in those in non-volcanic areas. Although volcanic ash also falls in the lowlands, marshes, or basins with restricted drainage areas, the weathering of volcanic glass is slow and halloysite is formed, possibly because high Si concentrations are maintained in the soil water. The formation of smectite from volcanic glass is also reported under hydrothermal conditions in the laboratory (Tomita et al. 1993; Cuadros et al. 1999).

Although humus-rich horizons are generally dark-colored, highly black colored A horizons are often found beneath grass vegetation. The grass vegetation was maintained even under humid climates through burning by ancient people. Under forest vegetation, the color of A horizons is dark brown. The difference in color between these two types of A horizons is due to the differences in their humic acid types. The humic acid, under grass vegetation is A-type, which is highly humified and rich in aromatic groups whereas that under forest vegetation is either P-type or B-type. The A-type humic acid is separated from other types of humic acid by having a melanic index lower than 1.7, whereas the other types of humic acid have melanic indices of 1.7 or higher (Shoji et al. 1993).

Andisols are characterized by high active Al and Fe contents. The active Al materials are allophane , imogolite , and Al-humus , and the active Fe material is ferrihydrite . These materials are mainly formed from volcanic glass . The typical Si:Al atomic ratios of non-colored and colored volcanic glasses are 5.0 and 2.4, respectively. These values are significantly higher than those for soil formation products such as kaolin minerals (often halloysite ), allophane , imogolite, and Al-humus, as shown in Table 4.2. Morphological changes of volcanic glasses accompanying these changes in elemental composition were examined using polished sections of new pumice particles, partially weathered pumice particles and soil clods from Udands.

4.4.1 Fresh Pumice Particle

A sample of fresh pumice was obtained from the 1991 Mt. Pinatubo tephra. Figure 4.15a shows an optical micrograph of the polished section. The white part is sponge-like volcanic glass with inclusion of feldspar , quartz , and other particles. Figure 4.15b shows an EDX spectrum of a glassy area of the particle shown by a dashed square (b) in Fig. 4.15a, and it is close to the typical non-colored volcanic glass (Fig. 4.2c). Magnifying the rim of the pumice, fine particles of 10 μm or less are found in the open cavities, and these are also non-colored volcanic glasses as shown by the similar EDX spectrum (Fig. 4.15e) to that for Fig. 4.15b. Figure 4.15e may correspond to the start of the micromass coating formation (Stoops 2007). Highly vesicular characteristics similar to Fig. 4.2b are seen inside the pumice .

Fig. 4.15

Fresh pumice . (a) Polished section of a pumice particle obtained from the 1991 Mt. Pinatubo volcanic ash deposit, (b) EDX spectra of the selected areas (dashed square (b) in (a)), (c) very small particles, (d) SEM image of the selected area (c) of the pumice particle, (e) EDX spectrum of a fine particle from the area (e), (f and g) Al and Si element maps of the pumice particle, respectively

Figure 4.15f, g are the element maps of Al and Si, respectively. The cyan color used to indicate Si displays its second highest intensity for the major and glassy part of the pumice particle. The highest color intensity of Si may indicate that some quartz was included in the pumice. The magenta color chosen for Al is strongest for feldspar particles, and the intensity of the magenta color for the volcanic glass is lower than the cyan color intensity as suggested from the EDX spectra shown in Fig. 4.15b, e.

4.4.2 Partially Weathered Pumice Particle

In the Central Plain of Luzon , the ash from the 1991 eruption (C horizon) deposited on the previous volcanic ash soil (2A1 or below) (Fig. 4.16a). A partially weathered pumice from the 2Bw horizon was sampled and a polished section (Fig. 4.16b) was prepared. The particle is brownish all the way through, suggesting oxidation and weathering of iron within the pumice. The primary land use type around the pedon is pasture (Fig. 4.16c), and the grass vegetation on the new ash had recovered by 1993, because the depth of the new ash at this site was only 13 cm.

Fig. 4.16

Partially weathered pumice . (a) Soil profile , (b) polished section of a weathered pumice obtained from 2Bw horizon of (a), (c) a landscape around the soil profile

Figure 4.17a, b shows element maps for Fig. 4.16b. The Al concentration, indicated by the magenta color, appears to be high for the phenocrysts, suggesting that these may be feldspars . Phenocrysts aside, the Al concentration at the rim of the pumice particle appears to be higher than the sponge-like inside of the pumice particle. The intensity of the cyan color, which indicates Si, appears nearly identical to the fresh pumice particle. Figure 4.17c shows a magnified SEM image of the upper part of the pumice. Sponge-like or highly vesicular structures can be found inside the pumice. At the rim of the pumice, small particles, similar to those seen in Fig. 4.15e, are found. Several large pores display vacant parts where the penetration of resin was insufficient. The area shown by the dashed square (Fig. 4.17d) was further magnified in Fig. 4.18.

Fig. 4.17

The SEM -EDX analyses of partially weathered pumice . (a and b) Al and Si element maps, respectively, of the polished section in Fig. 4.16b, (c) magnified SEM image of the upper selected part of the pumice particle

Fig. 4.18

Magnified SEM -EDX analyses of partially weathered pumice . (a and b) Si and Al element maps, respectively, (c) SEM image of the selected area (d) of Fig. 4.17c, (d and e) EDX spectra (lower right) of the spots (d) and (e) indicated by white arrows in (c)

Figure 4.18a, b shows element maps for Si and Al, respectively. The intensity of the cyan color, which indicates Si concentration, is somewhat weak at the rim, although similar low intensities are found in parts of the interior. The intensity of the magenta color, which indicates Al concentration, is especially high at the rim, for example at the site indicated by the white arrow labeled (d) in the SEM image (Fig. 4.18c), where Si concentration appears low. In contrast, the Si concentration still appears to be quite high at the site indicated by the white arrow labeled (e) in Fig. 4.18c. These observations can be confirmed by the EDX spectra (lower right of Fig. 4.18) of the small parts shown by white arrows labeled (d) and (e) in Fig. 4.18c. The EDX spectrum (d) shows high Al and low Si concentrations, suggesting the existence of allophane and imogolite , whereas the EDX spectrum (e) is close to that of the non-colored volcanic glass (Fig. 4.15e). Thus, consistent with the brownish color of the pumice in Fig. 4.16b, partial weathering of the pumice particle is shown by the SEM-EDX analyses. In this example, weathering appears most intensive at the rim sites of the pumice particle.

4.4.3 A Horizon Soil with Andic Soil Properties

The next example is a matured Andisol including a humus-rich A horizon and highly weathered Bw horizons. Figure 4.19a shows a Pachic Melanudand profile. The profile has thick black A horizons, yellow 2Bw horizons (Nantai-Shichihonzakura (Nt-S) tephra ), and reddish brown 3Bw horizons (Nantai-Imaichi (Nt-I) tephra , Ishizaki et al. 2017). Both Nt-I and Nt-S were ejected from Mt. Nantai, Japan. As no other layer is found between the two, the interval between Nt-I and Nt-S deposition is estimated to be short. The age of Na-I is estimated to be ca. 17 ka (Ishizaki et al. 2017 and references therein). By definition, the bulk density of matured Andisols is 0.90 Mg m⁻³ or less. Bulk density values of matured Andisols are as low as 0.5 or 0.6 Mg m⁻³.

Fig. 4.19

Thin sections of a matured Andisol . (a) Pachic melanudand profile from Kiwadashima, Tochigi, Japan, (b) landscape around the soil profile , (c, d, and e) images of the thin section of soil clod from the A3 horizon, 2Bw1 horizon, 3Bw5 horizon in plain-polarized light, respectively

The micromorphological and chemical properties of the A3, 2Bw1, and 3Bw5 horizons were examined using thin sections and polished sections. The vegetation of the sampled site is Quercus Serrata and undergrowth (Otowa and Shoji 1987) (Fig. 4.19b). Thin sections of the A3 horizon (Fig. 4.19c) showed porous and granular microstructure (Stoops 2007) similar to those reported by Kawai (1969). In the 2Bw1 horizon (Fig. 4.19d), a coating of fine, light brown material is probably a more weathered part of pumice particles. Vesicular structure remains inside the pumice. The light gray parts of Fig. 4.19d are voids. In the 3Bw5 horizon (4.19e), small light brown and orange parts are found within the weathered pumice . These parts are similar in appearance to the orange-reddish altermorph reported for the Bw3 horizon of the Andosol of Tenerife by Stoops (2007). The light gray parts are also voids in Fig. 4.19e. The micromass of these three thin sections, Fig. 4.19c–e, showed undifferentiated b-fabric between crossed polarizers, indicating that isotropic or non-crystalline materials are dominant in the fine materials of these horizons.

SEM -EDX analyses of polished sections further reveal the properties of the A3, 2Bw1, and 3Bw5 horizons. Figure 4.20a shows an optical microscope photograph of the polished section prepared from a clod of the A3 horizon (Fig. 4.19a). The dashed square (Fig. 4.20b) in the upper left corner of Fig. 4.20a was magnified in Fig. 4.20c. Weathered minerals and other particles appear in the dark-colored fine materials with the humus. Figure 4.20d, e, the same area as in Fig. 4.20c, show element maps for Al (magenta) and Si (cyan), respectively. Comparing the color intensity, Al concentration is higher than Si for all of the fine materials except for the coarse particles. This result is highly contrasting with Fig. 4.15f, g where the Si concentration is higher than the Al concentration. The element maps of Fig. 4.20d and e are the result of Andisol formation, consisting of changes in the Si-rich parent material of volcanic glass to form Al-rich allophane , imogolite and Al-humus (Table 4.2). The concentrations of oxalate-extractable Al (Al_o), Al_p, and Si_o in horizon are 7.6, 2.2, and 2.5%, respectively, indicating that the A3 horizon is highly weathered and allophanic, and that it contains Al-humus complexes. Nevertheless, there are still coarse silica particles that are probably cristobalite , quartz , or phytoliths . In Fig. 4.20c, a few reticulated patterns were observed, and one of them (highlighted by dashed square (f)) was magnified in Fig. 4.21a.

Fig. 4.20

Polished section of A horizon soil. (a) Polished section of a clod obtained from A3 horizon of Fig. 4.19a, (c) magnification of the dashed square (b) of (a), (d and e) Al and Si element maps of (c), respectively

Fig. 4.21

SEM -EDX analyses of a reticulated pattern found in the polished section (Fig. 4.20f). (a) SEM image of Fig. 4.20f, (b and c) EDX spectra of the selected areas (b) and (c) in (a), respectively. (d and e) Al and Si element maps of (a), respectively

Figure 4.21a shows a reticulated pattern of Fig. 4.20f with magnification, and the EDX spectrum of the dashed area is highly dominated by Al (Fig. 4.21b), whereas the EDX spectrum of the fine materials near the reticulated pattern suggests allophanic material (Fig. 4.21c). The Al element map of the reticulated area (Fig. 4.21d) shows that the Al concentration is nearly homogeneous with little Si present (Fig. 4.21e). These results suggest that the reticulated material is a sclerotia grain .

Sclerotia grains separated from the A3 horizon soil were examined. Water was added to the air-dried soil and ultrasonic treatment was carried out for 10 min. After allowing the suspension to stand for several minutes, dark spherical particles floating on the surface of water were ladled out on a filter paper and air-dried. Figure 4.22a shows the spherical particles separated from the A3 horizon soil, and Fig. 4.22b is the magnified SEM image of the particle. Further magnification of the dashed square in Fig. 4.22b gives Fig. 4.22c, where small holes appear. The hole is one of the characteristics of the sclerotia grain (Watanabe et al. 2001, 2002, 2004, 2007). The EDX spectrum of the dashed square in Fig. 4.22d shows that the outside of the sclerotia grain is Al-rich (Fig. 4.22e)

Fig. 4.22

Sclerotia grains separated from the A3 horizon (Fig. 4.19a). (a) Optical micrograph of sclerotia grains, (b) SEM image of the smallest grain in (a), (c) magnified SEM image of the dashed area of (b), (e) EDX spectrum of the dashed area (d) in (c)

The largest particle shown in Fig. 4.22a was broken to examine the inside of the particle, of which the SEM image is shown in Fig. 4.23a. The inside of the particle displays a reticulated structure. A magnified SEM image (Fig. 4.23b) of the dashed square in Fig. 4.23a shows that each concavity has a few small holes, which is characteristic of sclerotia grains (Watanabe et al. 2002). The EDX spectrum obtained from the whole of Fig. 4.23b shows that the inside of the particle is also Al-rich. Hence, the Al-rich reticulated structure observed in Fig. 4.21 is the sclerotia grain. Sclerotia grains contains green pigment compounds related to perylene quinone (Kumada and Hurst 1967). These green pigment compounds are typically found in soils containing P-type humic acid. Except the sclerotia grains , a special distribution pattern for Al is not found in Fig. 4.20a.

Fig. 4.23

Inside of the largest sclerotia granule shown in Fig. 4.22a. (a) SEM image, (b) magnified SEM image of the dashed area of (a), (c) EDX spectrum of (b)

The formation of Al-humus , phytoliths , sclerotia grains, and diatoms results from biological activities in the A horizons. The formation of laminar opaline silica may be facilitated by the formation of Al-humus. Allophane , imogolite , and ferrihydrite are formed from inorganic parent materials not only in the A horizons but also in the Bw horizons of Andisols .

4.4.4 B Horizon Soil with Andic Soil Properties

The 2Bw1 horizon consists of weathered Nt-S tephra . Figure 4.24a shows part of a polished section prepared from a clod consisting of weathered pumice . Although the color is yellowish brown, the inside of the pumice retains its highly vesicular structure . The outside of pumice is browner than the inside. The Al element map (Fig. 4.24d) shows that the Al concentration is high along the rim of weathered pumice. The EDX spectrum (Fig. 4.24e) suggests that the dashed square e in the Fig. 4.24d is an allophanic material. Regarding the inside of the weathered pumice, the Al concentration also appears to be increasing. Al and Si element maps were also examined for the dashed square (c) in Fig. 4.24a.

Fig. 4.24

Polished section of a clod from the 2Bw1 horizon shown in Fig. 4.19a. (a) Optical micrograph, (c) selected area for Fig. 4.25, (d) Al element map of the dashed area (b) in (a), (e) EDX spectrum of the dashed area (e) in (d)

Figure 4.25 shows a magnified SEM image of the dashed square Fig. 4.24c. Weathered phenocrysts and many small bubbles, probably owing to inadequate penetration of resin to the small vesicular pores , are observed. Nevertheless, comparing the color intensity of the Al and Si element maps, Fig. 4.25b, c, respectively, the concentration of Al appears to be higher in the vesicular or sponge-like part than that of Si. To further examine the distribution of Al and Si, the dashed squares (d) and (e) in Fig. 4.25a were magnified and are shown in Figs. 4.26 and 4.27, respectively.

Fig. 4.25

SEM -EDX analyses of the selected area (c) in Fig. 4.24a. (a) SEM image, (b and c) Al, and Si maps of (a), respectively

Fig. 4.26

Weathering of volcanic glass inside the Nt-S pumice . (a) Magnified SEM image of the selected area Fig. 4.25d, (b, c, and d) EDX spectra of the selected areas (b), (c) and (d) of (a), respectively, (e and f) Al and Si element maps of (a), respectively

Fig. 4.27

Allophanic materials in the cleavage opening of a phenocryst. (a) SEM image of the selected area in Fig. 4.25e, (d and e) Al and Si element maps of (a)

In Fig. 4.26a, which is the magnified SEM image of dashed square Fig. 4.25d, vesicular structures remain inside the sponge-like volcanic glass . However, two types of vesicle walls can be distinguished: thick and thin. Two thin walls (indicated by red boxes labeled c and d in Fig. 4.26a) were found to be Al-rich according to their EDX spectra (Fig. 4.26c, d). These EDX spectra are close to that for allophanic material. In contrast, a thick wall (indicated by the blue box labelled b in Fig. 4.26a), has an EDX spectrum similar to that of non-colored volcanic glass like Fig. 4.1a–c. Examining the Al and Si element maps of Fig. 4.26a shown in Fig. 4.26e, f, respectively, thin walls in the Fig. 4.26a are Al-rich and Si-poor, whereas thick walls are Al-poor and Si-rich. These observations suggest that the thin walls of vesicles are allophanic altermorphs described by Stoops (2007) and Gerard et al. (2007). Comparing the element maps of Al and Si, Fig. 4.26e, f, respectively, it can be seen that allophanic altermorphs also surround the thick volcanic glass. Hence, the allophanic altermorph formed by releasing Si, Ca, Na, and other soluble elements from the surface of volcanic glass , and remains at the original site.

Figure 4.25e shows a phenocryst that displays cleavage. Examining the cleavage reveals (Fig. 4.27a) thin stripes along the edge of the remaining phenocryst. Examining EDX spectra of the thin stripe (dashed square (b) in Fig. 4.27a) and phenocryst (dashed square (c) in Fig. 4.27a) reveals that they are allophanic material and plagioclase , respectively. Examining Al (Fig. 4.27d) and Si (Fig. 4.27e) element maps, thin stripes of the plagioclase phenocrysts are evident. The Al concentration is higher than the Si concentration in these thin stripes as shown by the EDX spectrum (Fig. 4.27b). Two possibilities exist for the formation of the allophanic thin stripes. One is the formation of altermorphs of plagioclase and the other is precipitation of allophanic materials in the cleavage opening.

The 3Bw5 horizon of Fig. 4.19a is a highly weathered example formed from Na-I tephra . The weathered particle is almost wholly reddish yellow to yellow (Fig. 4.28a). The many small vacant bubbles visible in the SEM image of Fig. 4.28b are probably due to inadequate penetration of resin into the small vesicular pores . The Al (Fig. 4.28c) and Si (Fig. 4.28d) element maps show that the majority is Al-rich and Si-poor, except for several small spots in the Si element map. To examine the fine structure of the inside of the particle, a small red square (Fig. 4.28b) was selected and magnified in Fig. 4.29.

Fig. 4.28

Polished section of a clod from the 3Bw5 horizon of Fig. 4.19a. (a) Optical micrograph , (b) SEM image, (c and d) Al and Si element maps, respectively

Fig. 4.29

Weathering of volcanic glass inside the Nt-I pumice . (a) SEM image of the selected area of Fig. 4.28b, (b and c) EDX spectra of the selected areas (b) and (c), respectively, (d and e) Al and Si element maps of (a), respectively

The magnified SEM image (Fig. 4.29a) shows that an altermorph of the vesicle walls remains and that materials with diffuse boundaries are filling part of the pores. The EDX spectra of the red squares (b) and (c) in Fig. 4.29a show that these are both Al-rich materials, possibly allophanic in nature. The Al (Fig. 4.29d) and Si (Fig. 4.29e) element maps show that no glassy parts remain. Although the peaks are small, the iron content is higher than that in Fig. 4.26c, d. The reddish yellow color of Fig. 4.28a and the 3Bw5 horizon of Fig. 4.19a is probably owing to iron or ferrihydrite in the allophanic aggregate. The oxalate-extractable Fe (Fe_o) content of the 3Bw5 horizon is 4.78%, and is even higher than the 2Bw1 horizon (Fig. 4.19a, 0.46%). The vesicle walls of the Fig. 4.29a are thin compared to the glass walls remaining in the Fig. 4.26a. Weathering is more intensive in Nt-I than in Nt-S even though the difference in age of the Nt-I and Nt-S is estimated to be small. Possible reasons are that (i) the glass wall was thin, and (ii) the iron content of the volcanic glass was high. Another difference between the 3Bw5 horizon and the scoria particles in Figs. 4.6 and 4.7 is the lack of phenocrysts in the former.

4.4.5 Changes in Elemental Composition with Andisol Formation

The changes in element concentrations are large during the process of Andisol formation (Fig. 4.14) as shown by the differences in Si:Al atomic ratio (Table 4.2) between the parent materials and weathering products. In short, Al-rich products form from Si-rich parent materials. Changes in the concentrations of 57 elements during Andisol formation were reported as a function of Al_o and Si_o (Nanzyo et al. 2002). To discuss the mechanism of the changes in element concentration during the process of Andisol formation, we constructed Fig. 4.30 (Nanzyo et al. 2007). Horizontal axes show an index of the weight ratio (W_p−W_s)/W_s = W_p/W_s−1, where W_p is the weight of parent volcanic ash, and W_s the weight of Andisol. Vertical axes show the concentrations of the elements, although the absolute concentration range depends on the individual element. The element concentration (E_j) at W_p/W_s−1 = 0, with rearrangement W_p = W_s, is the element concentration present in the parent volcanic ash (E_j,p). The element concentration at W_p/W_s−1 = 2, which is the same as W_p = 3W_s, is the element concentration when the weight of the Andisol has been reduced to one-third of the weight of the parent volcanic ash. If an element j is immobile during Andisol formation, Ej at W_p/W_s−1 = 2 is 3E_j,p.

Fig. 4.30

Changes in element concentration of andesitic tephra during Andisol formation. Red lines show changes in concentration of elements with weight loss of soils if the elements are immobile

Two analyses were performed before constructing Fig. 4.30. One was a principal component analysis of element concentration data of 46 elements using 95 Andisol samples from 18 pedons with different rock types. The first principal component was suggested to be depletion and enrichment of elements, and the second principal component (PC2) was suggested to be related to rock type. Then, andesitic Andisols , for which the variation in PC2 scores was relatively small, was used for Fig. 4.30.

The other preliminary analysis was to estimate W_p/W_s value for each sample. Assuming that volcanic glass quantitatively weathered to form Al_o materials, and the average Al content of volcanic glass in the andesitic volcanic ash is 69.6 g kg⁻¹ (Kobayashi et al. 1976); W_p/W_s was calculated as follows on an ignition residue basis:

$$ {\mathrm{W}}_{\mathrm{p}}/{\mathrm{W}}_{\mathrm{s}}=\mathrm{OIIR}+{\mathrm{Al}}_{\mathrm{o}}/69.6 $$

(4.1)

where OIIR (kg kg⁻¹) is the oxalate-insoluble residue, and was determined experimentally.

In Fig. 4.30, the ideal concentration changes of immobile elements were also drawn as a red line. The use of the red lines in this figure is based on the open-system mass transport that yields the chemical gain and losses of elements in a soil sample compared with the parent material after Brimhall et al. (1991) and Nieuwenhuyse and van Breemen (1997). For immobile elements, their equation can be simplified by multiplying the volume by the bulk density to give Eq. (4.2) (Kurtz et al. 2000):

$$ {\mathrm{W}}_{\mathrm{p}}{\mathrm{E}}_{\mathrm{j},\mathrm{p}}={\mathrm{W}}_{\mathrm{s}}{\mathrm{E}}_{\mathrm{j},\mathrm{s}} $$

(4.2)

Equation (4.2) is further converted to Eq. (4.3):

$$ {\mathrm{E}}_{\mathrm{j},\mathrm{s}}=\left({\mathrm{W}}_{\mathrm{p}}/{\mathrm{W}}_{\mathrm{s}}\right){\mathrm{E}}_{\mathrm{j},\mathrm{p}} $$

(4.3)

As W_p/W_s is calculated using Eq. (4.1), the red lines in Fig. 4.30 can be drawn so as to pass through the average concentration E_j,s and the average weight change W_p/W_s. If an element is immobile during Andisol formation, its concentration is plotted along this line. The theoretical E_j,p can also be calculated using E_j,s and W_p/W_s so long as an element is immobile. If a parent tephra has higher E_j,p than average, the plot of E_j,s will appear above the solid line, and if it has a lower E_j,p, it will appear below the solid line. If different E_j,p values of elements are scaled similarly, the slopes of their E_j,s plots will also be similar as seen in Fig. 4.30.

Among 54 elements, at least 27 (Be, Al, Ti, Fe, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Tl, Pb, Th, and U) were enriched in the Andisols , and the increase in these concentrations was related to total weight loss owing to the soil formation processes. Of the major elements in soil, concentrations of Si, Ca, and Na clearly decrease with the weight loss as shown in Fig. 4.30. However, the slope of the decrease in the element concentration is steeper for Ca and Na than for Si (Fig. 4.30). One possible reason is that Ca and Na are not the major constituents of Andisol formation products, but Si is the main structural constituent of allophane and imogolite . Other possible explanations are that Si can be sorbed by ferrihydrite , and that Si is also partly retained in Andisols as opaline silica or phytoliths . The weight-loss of tephra , maintaining the altermorphs (Figs. 4.26 and 4.29), may be the major reason for the reduction of bulk density in the Bw horizon of Andisols (Nanzyo et al. 2007).

4.4.6 Volcanic Ash Soils Under Various Drainage Conditions

The weathering of volcanic ash depends on moisture and drainage conditions in soil. Under moist and well-drained conditions, active Al and Fe materials form as summarized in Fig. 4.14. In contrast, halloysite formation prevails in volcanic ash soils with restricted drainage or under semi-dry weather conditions. Figure 4.31 shows changes in the weathering of dacitic volcanic ash Numazawa-Numazawako (Nm-NK) (4.6 ka, Ishizaki et al. 2009; Yamamoto 2003, 2007) with topography and drainage conditions at Aizu basin, Japan. In this area, tephra from the Numazawa caldera is distributed on the buried A horizon soil. At the well-drained hilly sites (Jotohara and Urushihara in Fig. 4.31a, b), the color of Nm-NK tephra is brownish, the range of Al_o + Fe_o/2 values is 12–19 g kg⁻¹, and the clay fractions are mainly composed of allophane and imogolite with small amounts of halloysite. On the other hand, at the Yukawa and Amanuma sites (Fig. 4.31d, e) with restricted drainage, the color of the Nm-NK is grayish, the range of Al_o + Fe_o/2 values is 1.2–5.3 g kg⁻¹, and the major clay mineral is halloysite. Although the Nm-NK tephra is thick at the Yukawa site, the reason may be secondary deposition from sounding areas due to transportation by water. At Sobanome (Figs. 3.6a, 4.31c), which is also inside the basin and poorly drained, the color of the Nm-NK is orange-gray, the range of Al_o + Fe_o/2 values is 4.8–6.6 g kg⁻¹, and the major clay mineral is halloysite. Changes in weathering of volcanic glass with drainage conditions were also observed by Stoops (2007).

Fig. 4.31

Major weathering products, and examples of soil profile including Nm-NK tephra in Aizu, Japan. Jotohara is the nearest study site to Numazawa caldera lake (a, b: good drainage, c, d, e: restricted drainage). The shaded position of (a) to (e) shows Nm-NK tephra at each site, and major weathering products “Allo.”, “Im.” and “Ht.” denote allophane , imogolite , and halloysite , respectively