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Volcanic rocks from the Z-To5 tephra layer of Zao volcano, NE Japan, preserve petrological information that reflects the magmatic processes under the volcano. The Z-To5 rocks were formed by the mixing of three magmas that differed in composition and phenocryst assemblage: basalt (1150-1200 8C), with high Mg (Fo c. 81 ) olivine; basaltic andesite (1020-1100 8C), with Mg-rich orthopyroxene (Mg# ¼ c. 78) and clinopyroxene (Mg# ¼ c. 78), lower Mg olivine (Mg# ¼ c. 78), and calcic (An c. 85 ) plagioclase; and andesite (900-1000 8C) with Mg-poor orthopyroxene (Mg# ¼ 61-66) and clinopyroxene (Mg# ¼ 64-68), and An-poor plagioclase. The basaltic magma was formed through fractionation of Fo c. 85 olivine from a less differentiated basaltic magma during its fast ascent from the depths. The andesitic magma, which occupied a shallow magma chamber, was heated by underplating of the basaltic magma, resulting in dissolution of some minerals. Subsequently, the basaltic andesite magma was formed by mixing of the basaltic and andesitic magmas in the chamber. Petrological evidence for the rapid growth of phenocrysts in the basaltic andesite magma suggests that the magma residence time was short. The basaltic andesite magma, mixing with a small portion of the andesitic magma, was withdrawn upon eruption. The rates of these processes are inferred to be rapid based on petrological considerations.