Abstract
A pyroclastic density current (PDC) was observed with the phreatomagmatic eruption of the Kuchinoerabujima volcano, southern Japan, on May 29, 2015. The PDC flowed down in all direction from the source crater, forming three major branches. The PDC reached the western coast of the island and the edge of the habitation area, which is ~ 2.4 km away from the source crater. The average speed of PDC along the Mukaehama River was estimated as 42 m/s. The PDC involved both surge and block-and-ash flow. Pyroclastic surge left thin layer of volcanic ash, though no remarkable signature of the lateral transportation was found in the surge deposit. The surge deposit occupies ~ 85% of the area covered with the PDC. Block-and-ash flow deposits were also recognized, but their distribution was limited mainly at the foot of the steep slope of the edifice of Shindake. The erupted materials from the May 29, 2015 eruption were estimated to be ~ 1.3 × 109 kg, including 2.4 × 108 kg of the PDC deposit and ~ 1.1 × 109 kg of the fallout deposit in proximal and distal area. The PDC deposit consisted of rock fragments with various degrees of hydrothermal alteration. The least-altered glassy blocks of andesite, which was the candidate for the juvenile materials, occupied less than 10% of the deposit. The area covered by the PDC was damaged by both mechanical and thermal effects. The dynamic pressure of the PDC damaged the forest in the inner portion of the area covered with the PDC. The thermal influence of the PDC caused dieback of the vegetation and deformation of some plastic materials. The absence of carbonization of wood in PDC indicates that the temperature was below the ignition temperature of wood. These observations suggest that the temperature of the PDC was between 100 and 240–270 °C.
Highlights
Phreatic eruption is characterized by explosive extrusion of pressurized geothermal fluid to the ground surface due to instability of the hydrothermal system
Active geothermal areas at shallow depth have a potential of phreatic eruptions because of the instabilities in hydrothermal activities
We describe the pyroclastic density current (PDC) of the Kuchinoerabujima volcano, which occurred on May 29, 2015, as an example of a small but locally violent PDC associated with the phreatomagmatic eruption
Summary
Phreatic eruption is characterized by explosive extrusion of pressurized geothermal fluid to the ground surface due to instability of the hydrothermal system. Zone c is the area covered with coarse-grained pyroclastic deposit (Fig. 4C). Zone d is the area covered with thin pyroclastic deposit, which mainly consists of lapilli and volcanic ash with coarse-grained sand size. The first type, which characterizes most of the PDC deposit, is a thin layer of coarse-grained volcanic ash which was formed by the pyroclastic surge. Though Zone a is covered with coarse-grained thick deposit, their detailed lithofacies and structure are still unknown because of the lack of field observation in the proximal area of the active crater. These observations indicate that the PDC deposits contain abundant blocky rock fragments with various degrees of hydrothermal alterations. The PDC deposit, originally grayish color just after the eruption, turned brownish in color owing to the precipitation of the secondary ferrihydrite ~ 1.5 years after the eruption
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