| Abstract |
Momo-iwa, Rebun Island, Hokkaido, Japan, is a dacite cryptodome (intrusive dome) 200-300 m across and 190 m high. The dome displays well preserved, primary internal structures in a cross-section and provides an excellent opportunity to describe the internal structures and fracture networks of a shallow-level intrusion. The internal structure of the dome is concentric, with a massive core, banded rim, and narrow brecciated border, all of which are composed of compositionally uniform feldspar-phyric dacite. Boundaries between each of the zones are distinct but gradational. The massive core consists of homogeneous coherent dacite and is characterized by radial columnar joints 60-200 cm across. The banded rim encircles the massive core and is 40 m wide. It is characterized by large-scale flow banding parallel to the dome surface. The flow banding comprises alternating partly crystalline and more glassy bands 80-150 cm thick. The banding is overprinted by radial columnar joints which are continuous with the radial columnar joints in the massive core. The columns in the banded rim are hexagonal and occasionally pentagonal in cross-section, and gradually decrease in diameter outward from 60-80 cm near the massive core to 40-60 cm at the margin of the banded rim. The outermost brecciated border is up to 80 cm thick, and consists of in situ breccia and blocky peperite. The in situ breccia comprises polyhedral dacite clasts 5-20 cm across and a cogenetic granular matrix. The blocky peperite consists of polyhedral dacite clasts 0.5-2 cm across separated by the host sediment (mudstone). The internal structures of the dome suggest endogenous growth involving a continuous magma supply during a single intrusive phase and simple expansion from the interior, and simple cooling history. Although much larger, the internal structures and fractures of Momo-iwa closely resemble those of smaller lobes (tens of metres across) in subaqueous felsic lobe-hyaloclastite lavas. Larger intrusions for HDR (Hot Dry Rock) geothermal electric power plants (such as granite intrusion) would have similar internal structures and fracture networks, if the intrusion had similar mode of growth and simple cooling history. |