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The 3400 BC caldera-forming eruption of Numazawa Volcano in NE Japan started with a cataclysmic pyroclastic flow eruption (∼ 2 km3 dense rock equivalent), which accounted for more than 90% of the total volume of the eruption products. This was followed by a Plinian eruption, phreatomagmatic eruptions, and a second Plinian eruption. The juvenile pyroclasts produced during the first two eruption phases are mostly dacitic, whereas those in the two subsequent phases are essentially andesitic. We recognized two distinct types of dacitic pumices: white and gray pumices. The two pumices are virtually identical in terms of both whole-rock composition (∼ 64.8-66.1 wt% SiO2) and crystal content (∼ 40-46 vol%), but are very different in terms of texture. The white pumice, which typically contains abundant euhedral phenocrysts, is the dominant variety and occurs throughout the pyroclastic flow deposits and in the overlying Plinian fall deposit. The gray pumice, which typically has bread-crusted surfaces and contains ubiquitous crystal fragments and xenoliths, occurs in the upper flow units of the pyroclastic flow deposits. We suggest that the textural differences between the two types of pumices reflect the difference in local stress conditions within the volcanic conduit below the magma fragmentation level. The abundance of euhedral phenocrysts in the white pumice suggests that it is the vesicular equivalent of the dacite that chamber ascended through the central part of the conduit that suffered low shear stress. In contrast, the magma that generated the gray pumice is thought to have originated from similar dacitic magma; the gray pumice magma was probably produced by the mechanical breakage of phenocrysts during magma ascent along the region of high shear stress near the conduit walls. The limited accumulation of the gray pumice in the upper units of the pyroclastic flow deposits suggests that crystal size reduction mainly occurred during the late stage of the pyroclastic flow eruption prior to the first Plinian eruption. It is likely that an abrupt conduit blockage caused by the waning of the eruption activity and the resulting temporal high shear stress at the conduit walls promoted crystal fragmentation, thus forming the gray-pumice magma and resulting in the subsequent eruption being a Plinian eruption.

Two types of dacitic pumices from the caldera-forming eruption of Numazawa Volcano, NE Japan