Surface Weathering in Zao Tephra Dominated by Mafic Glass

We studied the process of surface weathering in young Zao tephra dominated by colored glass and compared it with that observed for young Towada tephra dominated by noncolored glass. Zao-soils formed from Zao-a tephra (basaltic andesite, <1,000 BP), are distributed over a wide altitudinal range (130–1,800 m) under different climatic conditions and vegetation. The Zao-soils showed higher pH(H2O) values (4.78–5.80) than the Towada soils (3.96–5.77) even when compared at the same elevation or the same pH(H2O) of the F-Jayer. Colored glass contributed significantly to the retardation of soil acidification and influenced remarkably the weathering of tephra through the change of soil pH(H2O). The clay fractions of the soils with a pH(H2O) <5.0, were dominated by presence of smectite, opaline silica, and volcanic glass with a small amount of illite and kaolin. On the other hand, those of the soils with a pH(H2O) >5.0, showed an abundance of allophane, imogolite, and volcanic glass accompanied by a small amount of chloritized 2 : 1 minerals and trace amount of illite, kaolin, and opaline silica. The formation of Al- and Fe-humus complexes and opaline silica was favored by decreasing pH(H2O) of the soil, whereas the formation of allophane, imogolite, and iron oxides, proceeded remarkably with increasing soil pH(H2O) above 5. These results indicate that soil acidity as well as soil organic matter play an important role in the formation of some major minerals in the humus horizons of soils derived from tephra dominated by colored glass. The weathering process of this tephra is basically similar to that observed for tephra dominated by noncolored glass.