2:30 PM - 2:45 PM
[SVC29-04] High resolution bathymetric and structural survey along the coastal area of the Izu-Oshima volcano
Keywords:Izu-Oshima volcano, coastal area, bathymetry, seismic reflection survey
Izu-Oshima volcano is a basaltic volcano in the northern Izu-Bonin arc, and is characterized by numerous, subparallel NW-SE trending submarine ridges extending > 20km to the NW and the SE from the central vent. (Ishizuka et al., 2014, 2015). Submarine ridges of Izu-Oshima show geochemical differences between ridges. Systematic dating and geochemical analysis of subaerial effusives revealed that Izu-Oshima experienced systematic temporal variation of magma chemistry during the last 12,000 years (Ishizuka et al., 2015). Combined with geochemical correlation between the submarine and subaerial vents, each ridge is supposed to record lateral magma transport of different ages. Pairs of subaerial cones and submarine ridges represent the locus of magma transport events away from the reservoir beneath Izu-Oshima. Hence, magma from this crustal reservoir, after being affected by crystal fractionation and crystal accumulation, moved upwards to feed the on-edifice subaerial cones, as well as laterally to supply submarine fissures and dykes.
Izu-Oshima volcano has multiple locations where products of phreatomagmatic eruption are exposed. This indicates that this type of eruption caused by satellite vents in shallow water is common for this volcano, possibly caused by lateral magma transport from the central magma reservoir. However, since research vessels usually avoid entering such shallow, near shore area, geological and geophysical data in this area is almost nonexistent. Volcanic activity along the coastal area could cause significant hazard because major towns are often located near or on the coast with ports.
We conducted hydroacoustic survey in the coastal area of the Izu-Oshima volcano to reveal volcanic and structural characteristics of near shore area of this volcano. This survey was also accompanied by on shore geological survey to attempt to correlate subaerial exposures to submarine features.
Bathymetric data was collected by multibeam echosounder Seabat T-20 system using 512 beams with mainly 300 kHz, and gridded with a 1- 1.5 m gridcell size where water depth is less than 200 m. This data clearly reveals distribution of series of fault, satellite cones and numerous lava flows. This data also provide detailed information about surface morphology of lava flows and stratigraphic relationship.
To the northwest of the Izu-Oshima Island, several small cones with basal diameter of 200 m and relative height of 20 m were found. These cones are generally aligned in NW-SE direction, which implies that these cones formed as a consequence of lateral magma transport from the central reservoir of the Izu-Oshima volcano. In addition to these cones, multiple circular structures with larger relief than the cones were observed with variable basal diameters. These structures appear to be larger and older volcanic edifices possibly eroded when the sea level was lower. These older edifices are dissected by NW-SE trending scarps, and overlapped with smaller cones. Seismic reflection survey revealed that these NW-SE trending scarps are surface manifestation of a series of normal fault. These geographic relationships clearly demonstrate occurrence of multiple stages of satellite cone formation under regional extension.
To the southwest of the Senba-misaki, southwestern part of the island, an eruption fissure with a different strike was discovered. This eruption fissure is also observed on shore at the tip of the Senba-misaki, and continues about 1.4 km downslope into the sea. This fissure is recognised as a NE-SW trending ridge with a relative height of 15-20 m and a width of 70–80 m, and mainly composed of agglutinate. Based on geological and geochemical characteristics, this fissure was active during the pre-caldera stage of the Izu-Oshima volcano. The distinct tend of this eruption fissure from those of other younger chains of satellite cones may imply temporal change in regional or local stress regime affecting the volcano.
Izu-Oshima volcano has multiple locations where products of phreatomagmatic eruption are exposed. This indicates that this type of eruption caused by satellite vents in shallow water is common for this volcano, possibly caused by lateral magma transport from the central magma reservoir. However, since research vessels usually avoid entering such shallow, near shore area, geological and geophysical data in this area is almost nonexistent. Volcanic activity along the coastal area could cause significant hazard because major towns are often located near or on the coast with ports.
We conducted hydroacoustic survey in the coastal area of the Izu-Oshima volcano to reveal volcanic and structural characteristics of near shore area of this volcano. This survey was also accompanied by on shore geological survey to attempt to correlate subaerial exposures to submarine features.
Bathymetric data was collected by multibeam echosounder Seabat T-20 system using 512 beams with mainly 300 kHz, and gridded with a 1- 1.5 m gridcell size where water depth is less than 200 m. This data clearly reveals distribution of series of fault, satellite cones and numerous lava flows. This data also provide detailed information about surface morphology of lava flows and stratigraphic relationship.
To the northwest of the Izu-Oshima Island, several small cones with basal diameter of 200 m and relative height of 20 m were found. These cones are generally aligned in NW-SE direction, which implies that these cones formed as a consequence of lateral magma transport from the central reservoir of the Izu-Oshima volcano. In addition to these cones, multiple circular structures with larger relief than the cones were observed with variable basal diameters. These structures appear to be larger and older volcanic edifices possibly eroded when the sea level was lower. These older edifices are dissected by NW-SE trending scarps, and overlapped with smaller cones. Seismic reflection survey revealed that these NW-SE trending scarps are surface manifestation of a series of normal fault. These geographic relationships clearly demonstrate occurrence of multiple stages of satellite cone formation under regional extension.
To the southwest of the Senba-misaki, southwestern part of the island, an eruption fissure with a different strike was discovered. This eruption fissure is also observed on shore at the tip of the Senba-misaki, and continues about 1.4 km downslope into the sea. This fissure is recognised as a NE-SW trending ridge with a relative height of 15-20 m and a width of 70–80 m, and mainly composed of agglutinate. Based on geological and geochemical characteristics, this fissure was active during the pre-caldera stage of the Izu-Oshima volcano. The distinct tend of this eruption fissure from those of other younger chains of satellite cones may imply temporal change in regional or local stress regime affecting the volcano.