Growth of phreatomagmatic explosion craters: A model inferred from Suoana craterin Miyakejima Volcano, Japan

摘要

A subvertical cross section of a maar-diatreme volcano is exposed in the wall of the A.D. 2000 caldera on Miyakejima Volcano. The maar, Suoana, is one of the lateral vents of this volcano and it was inferred to be formed in the 7th century. The subvertical wall of the A.D. 2000 caldera truncated the Suoana maar crater at its center revealing the near comlete cross section of this small maar-diatreme volcano. Exposed in the cross section are a 400 m wide maar crater, an associated tuff ring with a maximum thickness of 20 m, a diatreme extending vertically to a depth of about 220 m from the floor of the maar crater, and a feeder dike connected to the base of the diatreme. The depth of the diatreme structure is about 260 m from the original ground surface. The outline of the diatreme resembles an upward-opening funnel with an almost vertical wall below 560 m asl and an upward flaring wall above 560 m asl. Coarse grained volcanic breccia fills the diatreme, the deposits of which can be divided into 6 units based on lithological and structural characteristics. The upper half of the diatreme is filled with landslide deposits, mainly derived from the surrounding crater wall. The bottom of the diatreme is occupied by massive explosion breccia. Some coherent blocks were detached from the wall of the diatreme and preserved in the diatreme fill. The Y-shaped cross-sectional geometry of the Suoana diatreme is the result of a combination of an underground subsidence in the lower part of the diatreme and the surface landslide in its upper part. The inwardly-inclined stratification of pyroclastic rock units and development of many small faults in the diatreme-filling breccia indicate successive collapse and deformation of these materials within the diatreme during the eruption. The upper part of the diatreme was formed by the landslides of the crater wall induced by the subsidence of the crater floor. Discharge of tephra from the bottom of the diatreme caused infill subsidence, which induced sliding of the inner wall of the crater. As a result, the topographic diameter of the crater became much larger than that of the diatreme itself. The tephra ring surrounding the crater consists mainly of pyroclastic fall deposits.