Morphology and growth style of a Miocene submarine dacite lava dome at Atsumi, northeast Japan

A Miocene submarine lava dome at Tate-iwa, Atsumi, Yamagata, Japan, displays well-preserved primary morphological features. The dome currently rises above a wave-cut coastal platform and is 55 m high and 90–180 m across. It comprises a massive core and a lava lobe–hyaloclastite rim, both of which are composed of compositionally uniform, feldspar–phyric dacite (SiO2=64 wt.%). The boundary between these two zones is distinct but gradational. The massive core consists of homogeneous, coherent dacite and is characterized by flow banding along the margin and by columnar joints radiating from the centre to the margin. The lava lobe–hyaloclastite rim encircles the massive core and consists of a complex of 80–90% dacitic lava lobes and 10–20% hyaloclastite. The lava lobes are 1–6 m thick, 3–12 m wide and more than 5 m long. Each lobe consists of a radially columnar-jointed core and a glassy rim 10–30 cm thick. The hyaloclastite comprises polyhedral dacite clasts 5–100 cm across in a matrix of dacite fragments up to 5 mm across. The lava lobes and hyaloclastite have gradational contacts in places. Paleobathymetric studies based on foraminifera in sediment beneath the dome, and the K–Ar age of the dome, suggest that it was extruded in a middle-bathyal environment (1500–2000 m below sea level) at 12.9±0.6 Ma. The internal structures of the dome suggest that it formed by a combination of exogenous growth involving extrusions of small dacite lobes, and endogenous growth involving a continuous magma supply and simple expansion from the interior. The presence of small lava lobes along the rim suggests that the magma had relatively low viscosity at the time of extrusion, in spite of its high silica content. Magma temperatures calculated by two-pyroxene and Fe–Ti oxide geothermometers were 999–1042 and 957–1005 °C, respectively. The inferred low viscosity may be attributed to the high temperature of the magma and/or a high confining pressure resulting from the deep-sea environment.