Outcrop‐Scale, Ring Structures Discovered in Northwestern Kyushu, Japan

ABSTRACT Two, outcrop‐scale, basaltic, ring structures were discovered in Oligocene sandstone on an abrasion platform in northwestern Kyushu, Japan. This paper aims to report their occurrence and discuss their formation mechanisms. When viewed from above, they were shaped like a teardrop and an oval with diameters of ~16 and ~6 m, respectively; and were emplaced 5.5 m apart. There were neither radial nor concentric fractures in the country rock. The smaller structure had a ring dike and is underwater for most of the time, so its observational data are much less than those of the larger structure. The latter had a cone sheet with an intrusive contact with the host rock. The structures were probably feeder pipes of the Pliocene Higashi‐Matsuura Basalts which cap hills around the structures at an altitude of ~140 m. The structures provide a rare opportunity with their small sizes to gain a panoramic view of volcanic conduits in relation to their host rocks, offering valuable insights into magma processes. Basaltic breccia and intrusions with lingulate shapes were surrounded by the cone sheet. Flow banding of this sheet indicated that the sheet was formed by repeated intrusion and destruction events, and that magma's ascent through a few‐decimeter‐thick annular spaces formed the cone sheet. The angular projections of the outer wall of this sheet and fractures in the country rock suggest that the structures expanded their diameters by stoping. Gusts of granular flow above the level of magma fragmentation likely smoothed the originally rugged wall of arcuate openings, in which parts of the sheet were formed. There were systematic joints around the structures, and the curvatures of some of the joints suggest that the ring structures were formed simultaneously with the jointing under weak, far‐field, extensional stress with roughly north–south trending, minimum horizontal stress.