The Explosive Activity of Zavaritsky Volcano (Simushir, Kuril Islands) during Holocene Time

Abstract

The present paper reports first data on the Holocene explosive activity of Zavaritsky Volcano—a major caldera center on Simushir Island, Central Kurils. We have managed, for the first time ever, to reconstruct the chronology of explosive eruptions in this volcanic center for the last 10 thousand years, as well as to estimate the parameters of its larger eruptions. Overall, more than 40 tephra layers have been identified in pyroclastic soil sections, enabling us to estimate the frequency of eruptions, one event per every 250 years. The age model based on radiocarbon dates that we determined for proximal deposits and on published data for the tephra of Zavaritsky Volcano and for the CKr key tephra layer have enabled us to find the ages of most eruptions. It was found that the volcanic glasses of the Holocene pyroclastics have compositions corresponding to low-K basaltic andesites and rhyolites; the very low concentration of K₂O is a reliable indicator to distinguish the Zavaritsky tephra, not only from that of adjacent medium-K volcanoes, but also from the tephra of the other low-K volcanoes in the Kuril–Kamchatka island arc. The Holocene activity of Zavaritsky Volcano began with two major eruptions whose conservative magnitude estimates (М) were 6.4 and 5.6. These events occurred about 9.5 ka and 9.2 ka ago. The tephra of the earlier eruption (ZV-1) propagated northeast, and was found as far as the northwestern North America. The tephra of the second large eruption (ZV-3) has traveled north, and was found in sediments of the Sea of Okhotsk. The ZV-1 pyroclastics is characterized by volcanic glass of rhyolitic composition with the highest concentration of SiO₂ (72.5–74.0 wt %). The glasses in the ZV-3 eruption products varied between dacites and rhyodacites (65.0–71.9 wt % SiO₂). The products of later eruptions were cinders with glasses of the dacite–andesite and basaltic andesite compositions. Dacite glasses appeared again only in the tephra of the latest large explosive eruption which occurred a short time before the mid-19th century. Our study has revealed a catastrophic character of the Early Holocene explosive eruptions of Zavaritsky Volcano and its practically constant activity throughout the Holocene. The fact that the tephra of the last large eruption of this volcano (ZV-40) contained high silica glasses tells us that a large eruption is likely to occur in the near future.