5:15 PM - 7:15 PM
[SVC34-P10] Magma processes in the Pliocene Hitsusaki volcanic conduit with an outcrop-scale cone sheet in Nagasaki Prefecture
Keywords:volcanic conduit, intrusion, stoping, magma flow, columnar jointing
An outcrop-scale ring structure is exposed on a wave-cut platform at Cape Hitsuzaki, Nagasaki Prefecture. The inferred magmatic processes based primarily on outcrop observations are addressed in this poster using numerous photographs. (As the site is a natural monument, destructive sampling was not possible, so the study mainly relied on field observations.) The intrusions are composed of olivine basalt, which contains abundant xenoliths of peridotite and hornblende gabbro. Based on this lithology, they are correlated with the Pliocene Higashi-Matsuura Basalt, which covers the surrounding hills at an elevation of 140 meters. Furthermore, for reasons discussed later, it is considered to have been one of the conduits for this volcanic system.
The cone sheet has a major axis of 16.0 m, a minor axis of 10.6 m, and a thickness of approximately 1 m. It surrounds an assemblage of lingulate-shaped intrusive bodies and intrusion breccia. Since chilled margins remain in some parts of the boundary with the host rock, the relationship with the host rock is intrusive rather than fault-related. Furthermore, the absence of arcuate faults in the surrounding area indicates that, although it exhibits a ring structure, it is not a collapse structure.
The cone sheet exhibits well-developed flow banding. The banding is almost parallel to the strike of the cone sheet, but in some areas, they have "unconformable" relations with each other, and they are also truncated by intrusive breccia at the outer margin of the cone sheet. Therefore, this cone sheet is considered to have formed through repeated intrusion and destruction between the conduit plug and the country rock. Additionally, the inner surface of the cone sheet and the "unconformity" surfaces are smooth, suggesting that the walls of the arc-shaped voids, several tens of centimeters thick, were smoothed by gust of pyroclastic granular flow. In the next stage, magma intruded into these voids, forming a part of the cone sheet.
Surrounded by the cone sheet, lingulate-shaped intrusive bodies and intrusive breccia remain on a scale of approximately 6 m in horizontal extent and about 2 m in height. The breccia occupies the basal part. The lingulate bodies range in width from several tens of centimeters to 2 m and mostly dip at high angles toward the center of the ring structure. Some of the lingulate ones become fragmented at their ends, gradually transform into breccia.
At the top of the remnant lies a sheet-like intrusive body with flow banding and columnar joints. The characteristics of the flow structures and joints indicate that the upper part of this body has been lost by erosion. This body remains about 1 meter thick near the center of the ring structure, where it exhibits nearly horizontal flow structures and almost vertical columnar joints. As it approaches the cone sheet, this body branches into several, gently lying, lingulate lobes, each several tens of centimeters thick, reaching a total thickness of about 2 meters near the cone sheet. The lowermost lobe further divides into multiple fingers near the sheet, which then become fragmented and transition into breccia. The surfaces of these fingers feature a breadcrust-textured chilled margin. Flow banding of the uppermost lobe stand nearly vertical in the range of 2-3 meters from the inner wall of the cone sheet. Additionally, the columnar joints cutting across the banding are nearly perpendicular to the inner wall.
Thus, the magma that formed this intrusion likely flowed from near the center of the ring complex toward the pre-existing cone sheet, splitting into multiple lobes. As these lobes overlapped each other as it approached the cone sheet, the uppermost ones ascended along the cone sheet while being cooled by them. Such a magma intrusion event probably caused the conduit plug above them to tilt and uplift, having further led a crater uplift.
Near the cone sheets, the arrangement of columnar joints in the uppermost lobe is disordered along a prominent crack; however, flow banding is not involved in this disturbance. Therefore, after the magma had fully crystallized, fluids likely infiltrated the crack, disturbing the temperature gradient and leading to the disordered columns directions.
The cone sheet has a major axis of 16.0 m, a minor axis of 10.6 m, and a thickness of approximately 1 m. It surrounds an assemblage of lingulate-shaped intrusive bodies and intrusion breccia. Since chilled margins remain in some parts of the boundary with the host rock, the relationship with the host rock is intrusive rather than fault-related. Furthermore, the absence of arcuate faults in the surrounding area indicates that, although it exhibits a ring structure, it is not a collapse structure.
The cone sheet exhibits well-developed flow banding. The banding is almost parallel to the strike of the cone sheet, but in some areas, they have "unconformable" relations with each other, and they are also truncated by intrusive breccia at the outer margin of the cone sheet. Therefore, this cone sheet is considered to have formed through repeated intrusion and destruction between the conduit plug and the country rock. Additionally, the inner surface of the cone sheet and the "unconformity" surfaces are smooth, suggesting that the walls of the arc-shaped voids, several tens of centimeters thick, were smoothed by gust of pyroclastic granular flow. In the next stage, magma intruded into these voids, forming a part of the cone sheet.
Surrounded by the cone sheet, lingulate-shaped intrusive bodies and intrusive breccia remain on a scale of approximately 6 m in horizontal extent and about 2 m in height. The breccia occupies the basal part. The lingulate bodies range in width from several tens of centimeters to 2 m and mostly dip at high angles toward the center of the ring structure. Some of the lingulate ones become fragmented at their ends, gradually transform into breccia.
At the top of the remnant lies a sheet-like intrusive body with flow banding and columnar joints. The characteristics of the flow structures and joints indicate that the upper part of this body has been lost by erosion. This body remains about 1 meter thick near the center of the ring structure, where it exhibits nearly horizontal flow structures and almost vertical columnar joints. As it approaches the cone sheet, this body branches into several, gently lying, lingulate lobes, each several tens of centimeters thick, reaching a total thickness of about 2 meters near the cone sheet. The lowermost lobe further divides into multiple fingers near the sheet, which then become fragmented and transition into breccia. The surfaces of these fingers feature a breadcrust-textured chilled margin. Flow banding of the uppermost lobe stand nearly vertical in the range of 2-3 meters from the inner wall of the cone sheet. Additionally, the columnar joints cutting across the banding are nearly perpendicular to the inner wall.
Thus, the magma that formed this intrusion likely flowed from near the center of the ring complex toward the pre-existing cone sheet, splitting into multiple lobes. As these lobes overlapped each other as it approached the cone sheet, the uppermost ones ascended along the cone sheet while being cooled by them. Such a magma intrusion event probably caused the conduit plug above them to tilt and uplift, having further led a crater uplift.
Near the cone sheets, the arrangement of columnar joints in the uppermost lobe is disordered along a prominent crack; however, flow banding is not involved in this disturbance. Therefore, after the magma had fully crystallized, fluids likely infiltrated the crack, disturbing the temperature gradient and leading to the disordered columns directions.