2020 Volume 126 Issue 5 Pages 233-249
It has been inferred that Lake Tazawa (Tazawa-Ko or Tazawako in Japanese) is a caldera lake, but the products of the caldera-forming eruption have yet to be confirmed. Here, we report the discovery of the rhyolitic pyroclastic density current deposits (PDCs) that erupted from Tazawa-Ko between 2 and 1.8 Ma and shall propose to call this confirmed caldera the Tazawako caldera after the name of Tazawako Town (Tazawako-Machi), where Lake Tazawa (Tazawa-Ko) is located. The lower PDCs are wavy to parallel-stratified deposits that are dominated by polyhedral and platy glass shards, suggesting that the caldera-forming eruption began with phreatomagmatic explosions. The upper PDCs are poorly sorted, largely non-stratified deposits that are closely associated with the debris avalanche deposits around Tazawa-Ko, and are welded where they occur in a valley 30 km to the north of Tazawa-Ko, thereby representing the climactic eruption. These caldera-forming eruption products are limited in volume even at proximal locations, presumably due to a long-term erosion. However, the total eruption volume is estimated to be ~1011 m3 with the ~8-km-diameter of the source caldera. This volume is supported by correlative fallout ash observed up to 800 km from Tazawa-Ko. Two lava domes erupted on the caldera floor, and two lava flows were effused on the somma between 1.8 and 1.6 Ma, following caldera formation. While pre-caldera volcanism remains largely unknown, a small volume of andesite lava flow was extruded from a site on the southeastern caldera rim. Finally, we shall propose to call this confirmed caldera the Tazawako caldera after the name of Tazawako Town (Tazawako-Machi), where Lake Tazawa (Tazawa-Ko) is located.
