Post-caldera geology of Gora Region in Hakone Volcano Group, Japan

Abstract

This study examined the geological development of the Gora region in the NW quadrant of the caldera of the Hakone Volcano Group in Japan, covering the period since the first caldera-forming stage (< ka). Previous studies on borehole samples in this region have documented subsidence during MIS and MIS , probably as a result of large pumice flow eruptions; these ages are supported by the pollen assemblage in tuffaceous sand-mud of the caldera fill. A new borehole drilled in by the Japan Meteorological Agency (JMA-V) yields a continuous succession from the second calderaforming stage during MIS to the present surface. The lake that formed in this area during MIS is considered to have been saline to some extent, as indicated by brackish diatom species in lacustrine deposits. The source of salt was probably volcanic fluid, which also feeds the present-day NaCl-rich hot springs in this area. The lake was then filled with pumice, which is a secondary deposit of Plinian eruptions after the caldera-forming stage (tephra group named Hk-CC). Analysis of the lacustrine deposit indicates that the ancient lake level of MIS was > m above sea level, which is higher than the present-day terrace formed at the caldera outlet. The pumice is overlain by the Hayakawa Debris Flow (HDF), which forms the basal part of the Younger Central Cones (YCC) of this region. A blockand-ash flow deposit overlies the HDF. The juvenile andesite clasts in the block-and-ash flow are almost identical to other YCC lavas, although with a much higher Rb content (> ppm). The uppermost part of the section comprises a talus deposit that is ~ m thick and that formed at ca. ka.

Imaging the Source Region of the 2015 Phreatic Eruption at Owakudani, Hakone Volcano, Japan, Using High‐Density Audio‐Frequency Magnetotellurics

“…2 of 8 (Mannen, 2014). Yellow stars and black triangles indicate the major geothermal areas and mountain peaks in the caldera, respectively.…”

“…There are several geothermal areas that discharge fumarolic gases and hot spring waters within the caldera: the most active geothermal area is called Owakudani and is located north of Mt. Kamiyama, one of the youngest central volcanic cones (Mannen, 2014). A phreatic eruption occurred at Owakudani from June 29 to July 1, 2015 and formed four new craterlets and dozens of fumaroles (Mannen et al., 2018).…”

“…The area enclosed by the black rectangle represents the area shown in (b). The pink line represents the morphological rim of the Hakone caldera (Mannen, 2014). Yellow stars and black triangles indicate the major geothermal areas and mountain peaks in the caldera, respectively.…”

“…Yellow stars and black triangles indicate the major geothermal areas and mountain peaks in the caldera, respectively. The black hachure area with dots represents the Younger central cones formed since 40 ka (e.g., Mannen, 2014). (b) Map showing the observation sites (red circles) around Owakudani.…”