Dozen volcano is located in the Japan Sea off the Shimane Peninsula, SW Japan, and is known as a caldera composed of volcanic rocks evolved from alkali olivine basalt magma. Many isotopic ages reported so far indicate the volcano is Pliocene in age, but the onset time of volcanic activity has remained uncertain, as the subsequent eruption products cover the initial products of the volcano. Dating was, therefore, made this time on the basal trachybasalt lava collected by drilling into a part of the somma, Nakanoshima Island. The plateau Ar-Ar age obtained for the groundmass is 6.382±0.018Ma, consistent with the underground stratigraphy in the drilling hole. The time span from this age of the basal lava to the K-Ar age of the central Takuhiyama pyroclastic cone is approximately 1 million years. A model calculation suggests that this long activity could have been sustained by a mantle diapir of ca. 5000km3 in volume and ca. 20km in diameter. Provided 10% of the melt fraction in the diapir and the estimated total eruption volume of 100-300km3, a large amount of the melt may have remained and solidified at depths beneath Dozen volcano. This can account for the high-gravity anomaly centering Dozen volcano, which suggests that a large mass ～20km across and denser than granitic rocks exists below the volcano.
