Nature of the 1977-1978 Volcanic Ash from Usu Volcano, Hokaoido, Japan

The ejected ashes were collected from the points 0 to 4.5 km from the sources during the whole period of the pumice and phreatic eruptions in the 1977-1978 activity of Usu Volcano, Hokkaido, Japan. The dry ash samples (＜2 mm) of pumice eruptions (Aug. 7-14, 1977) were weakly alkaline (pH=7.3-8.2), and rich in water-soluble and exchangeable Ca²⁺ and Na⁺, but poor in water-soluble Cl^- (69-580 ppm) and SO₄^2- (282-1231 ppm). By contrast, the ejected ashes of phreatic-phreatomagmatic eruptions (at the Craters B, H and I) were strongly acidic (pH=3.4-5.9), and rich in water-soluble Cl^- (495-17260 ppm) and SO₄^2- (160-4830 ppm), as shown in Table 1 and Fig.4. Whereas the ejected ashes from the Craters K, L and M were neutral or weakly alkaline (pH=6.8-8.5), and showed a slight decrease in the concentration of the water-soluble Cl^- (421-1064 ppm) and SO₄^2- (256-1031 ppm). When the Cl/S molar ratios of the water-soluble components of the ash (1.9-19.2) ejected from the Craters A, B, H and I were also taken into consideration, it was suggested that the temperatures of the coexisting gas phase with these ashes were very high. As shown in Table 1, the ash samples contained 2.5-6.2% under 2μ clay. The clay fractions (either under 2μ or under 0.2μ) separated from the samples were characterized by the presence of abundant montmorillonite and subordinate chlorite and metahalloysite (Figs. 6, 7, 11 and 12). These clay minerals are considered to have derived from the atrio-deposits on the summit of Usu Volcano. In this connection, late in August, 1977, it was confirmed that two clay flows consisting mainly of montmorillonite were squeezed out on the surface of the atrio due to crustal movement (Fig. 3). The ejected ash had a high adsorptivity and viscosity, and also showed high dispersion in air and water. Such properties are probably due to the presence of the montmorillonitic clay in the ash. The presence of such clay minerals in the ash gave rise to severe disaster to the urban, forest and agricultural lands near the volcano during the eruptions on rainy days. The silica contents of the 1977-1978 ashes, pumices and essential blocks are plotted in a variation diagram together with those of the 1944 tephra (Showa-Shinzan) (Fig. 13). The silica contents of the 1977-1978 ashes showed a wide range from 53.91 (Crater A) to 68.00 wt% (Crater K). The difference in the color of dry ash may depend on the chemical and mineralogical compositions. In this connection an interesting relationship was found between the variation in silica contents of the ash and the sequence of the Second Stage eruption. As shown in Table 1 and Fig. 13, the ashes ejected in the Substage I (typical phreatic eruptions) were greyish brown or grey in color and poor in silica (SiO₂=53.91-57.55 wt%), whereas in the Substages II and III (phreatomagmatic-magmatic eruptions) the grey or greyish white and silica-rich ashes (SiO₂=61.63-68.00 wt%) became abundant. It was noticed that the ashes produced by the magmatic eruptions were mostly consisted of the essential dacite, such as the greyish white ash (SiO₂=68.00 wt%) which was ejected from the Crater K on August 24, 1978.