11:00 〜 11:15
[SVC34-07] 深海底におけるプチスポット火山の噴火様式
★招待講演
キーワード：プチスポット、枕状溶岩、アルカリ玄武岩、洪水溶岩
Petit-spot volcanoes on the old Pacific plate prior to the subduction were first recognized as monogenetic eruptions owing to their melt ascent along the zone of concave flexure of the outer rise. Monogenetic petit-spot volcanoes are aligned perpendicular to the hinge line of the concavely flexed lithosphere, which are visualized as positive gravity anomalies. The petit-spot volcanoes can be recognized as the deepest volcanoes on Earth (−4000- to −6000-m water depth). Because the exploration of the small volcanic edifices (a few kilometers in diameter) of these volcanoes requires a research vessel equipped with a shipboard multibeam echosounder, the global distribution of submarine petit-spot volcanoes remains constrained. However, examples have been reported for the presence of such volcanoes on several oceanic plates prior to the subduction of Japan, Chile, Sunda, Tonga, and Mariana trenches. In this study, we discuss the morphology of volcanic edifices and occurrence of the eruption products on the deepest submarine environment on Earth via submersible dive observations using the human-operated vehicle SHINKAI 6500.
SHINKAI 6500 has observed 16 volcanoes in >100 petit spots recognized by the shipboard acoustic multibeam surveys in the NW Pacific off the Japan Trench. The petit-spot volcanic fields mainly comprise some monogenetic volcanic clusters that are a few kilometers in diameter and a few hundred meters high. The volcanic edifices are unstructured in shape, with younger and older lava flows overlapping in an edifice. The volcanic edifices predominantly comprise peperite, minor amount of pillow lava, and pillow breccia. The most important feature of petit-spot basalts is that they show high vesicularity (up to 60 vol.%) in spite of the eruption under submarine hydrostatic pressure encountered at 4000 and 6000 mbsl, caused by high levels of CO2 of magmas. The melt fractionations should be deeper than the mid-depth of the lithosphere, given that bulk compositions (mainly shoshonite on the NW Pacific plate and hawaiite on the western Pacific plate) clearly indicate the fractionation trends of olivine in the absence of phenocrysts despite the presence of lherzolitic xenoliths and/or xenocrystic olivines from the upper part of the Pacific lithosphere (shallower than 42-km depth). Two examples of petit-spot volcanoes that erupted strongly alkaline foidite lavas were recently found off the south of the Japan Trench, erupting flood lavas on the abyssal plain without any volcanic edifices. These lavas show considerably less vesicularity (<10 vol.%) in spite of higher volatile content than the majority of petit-spot lavas. Common presence of phenocrystic olivine distinctively represents the magma ascending with less stagnation through the lithosphere. As for the western Pacific plate volcanic field south of the Minamitorishima Island off the Mariana Trench, various scales of volcanic edifices are observed from a few meters to 5 km in diameter. The large edifices have some ridges that extend ~5 km along N–S or NNE–SSW trends, which are perpendicular to the hinge line of the outer-rise flexure owing to the Mariana Trench subduction. Pillow lavas mainly comprising large edifices show more differentiated hawaiite than those of the NW Pacific plate. During the dives, highly elongated pillow tubes enclosing pelagic muds were mainly observed. The terminations of the lava tubes were inflated, mainly on the flat seafloor, thereby producing spherical features. A sheet flow with ropy structures was also observed to cover an inclined plane. The occurrences of western Pacific plate petit-spots could show a larger eruption rate than those on the NW Pacific plate based on their edifice, morphology, and major element compositions. The petit-spot volcanic knoll both on NW and western Pacific plates comprises highly vesicular pillow lavas in contrast to the flood lava flows constituting nonvesicular lavas.
SHINKAI 6500 has observed 16 volcanoes in >100 petit spots recognized by the shipboard acoustic multibeam surveys in the NW Pacific off the Japan Trench. The petit-spot volcanic fields mainly comprise some monogenetic volcanic clusters that are a few kilometers in diameter and a few hundred meters high. The volcanic edifices are unstructured in shape, with younger and older lava flows overlapping in an edifice. The volcanic edifices predominantly comprise peperite, minor amount of pillow lava, and pillow breccia. The most important feature of petit-spot basalts is that they show high vesicularity (up to 60 vol.%) in spite of the eruption under submarine hydrostatic pressure encountered at 4000 and 6000 mbsl, caused by high levels of CO2 of magmas. The melt fractionations should be deeper than the mid-depth of the lithosphere, given that bulk compositions (mainly shoshonite on the NW Pacific plate and hawaiite on the western Pacific plate) clearly indicate the fractionation trends of olivine in the absence of phenocrysts despite the presence of lherzolitic xenoliths and/or xenocrystic olivines from the upper part of the Pacific lithosphere (shallower than 42-km depth). Two examples of petit-spot volcanoes that erupted strongly alkaline foidite lavas were recently found off the south of the Japan Trench, erupting flood lavas on the abyssal plain without any volcanic edifices. These lavas show considerably less vesicularity (<10 vol.%) in spite of higher volatile content than the majority of petit-spot lavas. Common presence of phenocrystic olivine distinctively represents the magma ascending with less stagnation through the lithosphere. As for the western Pacific plate volcanic field south of the Minamitorishima Island off the Mariana Trench, various scales of volcanic edifices are observed from a few meters to 5 km in diameter. The large edifices have some ridges that extend ~5 km along N–S or NNE–SSW trends, which are perpendicular to the hinge line of the outer-rise flexure owing to the Mariana Trench subduction. Pillow lavas mainly comprising large edifices show more differentiated hawaiite than those of the NW Pacific plate. During the dives, highly elongated pillow tubes enclosing pelagic muds were mainly observed. The terminations of the lava tubes were inflated, mainly on the flat seafloor, thereby producing spherical features. A sheet flow with ropy structures was also observed to cover an inclined plane. The occurrences of western Pacific plate petit-spots could show a larger eruption rate than those on the NW Pacific plate based on their edifice, morphology, and major element compositions. The petit-spot volcanic knoll both on NW and western Pacific plates comprises highly vesicular pillow lavas in contrast to the flood lava flows constituting nonvesicular lavas.