Cone-building block-and-ash flows: the Senyama volcanic products of O’e Takayama volcano, SW Japan

Abstract

The Senyama volcanic products of the late Pliocene to early Pleistocene O’e Takayama volcano overlie a 100-m-thick, late Pliocene coastal quartz-sandstone and are intruded by an early Pleistocene dacite dome. The Senyama volcanic products are the remains of a cone that retains a basal part 1.5 km across and 150–250 m high from the substrate. The cone comprises dacite block-and-ash flow deposits and minor base-surge deposits occur at the base. Single beds of the block-and-ash flow deposits are 1–16 m thick and dip inward 20–40° at the base of the cone and inward or outward 10–20° at the summit. Juvenile fragments in the block-and-ash flow deposits are non- to poorly vesicular and commonly have curviplanar surfaces and prismatic joints extending inward from the surfaces, which imply quenching and brittle fracturing of dacite lava. They are variably hydrothermally altered. Nevertheless, juvenile blocks appear to retain a uniform direction of the magnetization vector residual during thermal demagnetization between 280°C and 625°C. At the time of the eruption, the well-sorted sand of the substrate was at the coast and a good aquifer that facilitated explosive interaction of water and the ascending dacite lava. The mechanism of the explosion perhaps involved thermal contraction cracking of the dacite lava, water-inflow into the interior of the lava, and explosive expansion of the water. Initial phreatomagmatic explosions opened the vent. Succeeding phreatomagmatic or phreatomagmatic–vulcanian explosions produced block-and-ash flow deposits around the vent. Hydrothermal silver-ore deposits and manganese-oxide deposits occur in the Senyama volcanic products and the underlying sandstone, respectively. They could represent post-eruptive activity of the hydrothermal system developed in and around the cone.