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
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
4.4 Andisols: Soils Dominated by Non-crystalline Inorganic Constituents
87
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
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
4.4 Andisols: Soils Dominated by Non-crystalline Inorganic Constituents
87