Abstract
On Ioto Island (Iwo-jima), 44 phreatic eruptions have been recorded since 1889, when people began to settle there. Four of these eruptions, after the beginning of continuous observation by seismometers in 1976, were accompanied by intense seismic activity and rapid crustal deformation beforehand. Other eruptions on Ioto were without obvious crustal activities. In this paper, we discuss the mechanisms of phreatic eruptions on Ioto. Regular geodetic surveys and continuous GNSS observations show that Ioto intermittently uplifts at an abnormally high rate. All of the four eruptions accompanied by the precursors took place during intermittent uplifts. The crustal deformation before and after one of these eruptions revealed that a sill-like deformation source in the shallow part of Motoyama rapidly inflated before and deflated after the beginning of the eruption. From the results of a seismic array and a borehole survey, it is estimated that there is a layer of lava at a depth of about 100–200 m, and there is a tuff layer about 200–500 m beneath it. The eruptions accompanied by the precursors probably occurred due to abrupt boiling of hot water in hydrothermal reservoirs in the tuff layer, sealed by the lava layer and triggered by intermittent uplift. For the eruptions without precursors, the hydrothermal systems are weakly sealed by clay or probably occurred on the same principle as a geyser because phreatic eruptions had occurred beforehand and hydrostatic pressure is applied to the hydrothermal reservoirs.
Highlights
Ioto Island (Iwo-jima) is a volcanic island located about 1200 km south of Tokyo, Japan (Fig. 1)
Summary In Ioto, phreatic eruptions occur with intense seismic activity and crustal deformation beforehand as well as with no obvious precursors
Crustal deformation before and after the four eruptions accompanied by precursors indicates that a sillshaped deformation source beneath the shallow part of Motoyama rapidly expands before an eruption and contracts with an eruption
Summary
Ioto Island (Iwo-jima) is a volcanic island located about 1200 km south of Tokyo, Japan (Fig. 1). It is one of the volcanoes of the Izu-Bonin-Mariana island arc accompanying the subduction of the Pacific plate beneath the Philippine Sea plate. With an altitude of about 110 m, on Motoyama is covered with pyroclastic flow deposits and lava flows from a large-scale magmatic eruption (the Motoyama eruption) dated to 2.7 cal kBP (Nagai and Kobayashi 2015). The drilling core shows alternating layers consisting of lava flows and pyroclastic rocks. Lava layers found on the east coast (Nagai and Kobayashi 2015) at depths of 24–97.9 and 106.6–130.5 m were presumed to be Motoyama lava and Hanareiwa lava, respectively.
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