Mode of Occurrence and Eruption Age of the Tatsuko-Tai and Shinko-Tai Lava Domes in the Tazawako Caldera, NE Japan

Tatsuko-Tai and Shinko-Tai are lava domes located under Lake Tazawa(Tazawa-Ko or Tazawako in Japanese), Akita Prefecture, NE Japan as inferred from observed high magnetic anomalies and dacite and andesite (or dacite) samples, respectively, collected from the domal masses. Recently obtained acoustic sounding data and underwater video images were used to characterize the surface morphology. Shinko-Tai consists of a symmetric lava dome and a flow of lava branching westwards from the dome, whereas Tatsuko-Tai is a slightly asymmetric and smaller single lava dome that shows coarse columnar joints on its surface. The dacite sample collected from Tatsuko-Tai yields a Ar/Ar plateau age of 1.73±0.08 (1σ) Ma. This age is consistent with the ages of lava flows along the ridge of the caldera rim, suggesting that the Tazawako caldera formed earlier than 1.7 Ma.