Sequence of the 1895 eruption of the Zao volcano, Tohoku Japan

Abstract

The most recent major eruption event of the Zao volcano comprised a series of phreatic eruption episodes on 15 and 19 February, 22 August, and 27–28 September 1895, with several precursory vulcanian eruptions during February–July 1894. All were generated at the Okama crater lake located inside the Umanose caldera. The eruption products consist mainly of hydrothermally altered ash with altered blocks, except for ash from 1984. The eruption deposits of 1895 are divided lithologically into six layers (1–6). Comparison of the document with the lithofacies of deposits shows that layers 1, 2, 3–4, and 5–6 were correlated respectively with eruption episodes of 15 February (episode 1), 19 February (episode 2), 22 August (episode3), and 27–28 September (episode 4). During these four episodes, ca. 0.5%, 0.5%, 1.5%, and 98% of the total mass of the products had been discharged. Based on lithologic, stratigraphic, granulometric, and component analyses and on distributional features for these layers, the following depositional mechanisms were inferred. Layers 1, 3, and 4 were formed mainly from their related small pyroclastic density currents, whereas layer 2 resulted mainly from a small pyroclastic fall. In contrast, layers 5 and 6 are larger-scale near-vent pyroclastic fall deposits from ash clouds and eruption clouds, which might have included some juvenile fragments. The three early episodes in 1985 led to the climactic episode of 27–28 September. Furthermore, the andesitic magma chamber at <3kb depth, which caused the 1894 vulcanian eruptions, became a hydrothermal alteration source for the 1895 erupted materials. The chamber was re-activated before 1895 eruption by injection of basaltic magmas from greater depth. The injection reached maximum at the climactic event. The inferred course of that series of eruption episodes provides useful information to predict future volcanic phreatic-type eruptions at this volcano.