すஅᓥࡢᄇⅆ࡟ࡼࡿᆅᙧ࣭ᆅ㉁࣭ᄇฟ≀ࡢ≉ᚩ࡜ࡑࡢኚ໬๓㔝 ῝  ࠊྜྷᮏ඘ᏹ 

Characteristics and change of geomorphology, geology and eruptive materials in
the eruption of Nishinoshima volcano, Ogasawara
Fukashi MAENO1* & Mitsuhiro YOSHIMOTO2

1. ౨‫ښ‬୉ָஏਔ‫ॶڂݜ‬㸦ࠛ113-0032౨‫౐ښ‬ช‫ۢښ‬໽ਫ਼ 1-1-1㸧
Earthquake Research Institute, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan.
2. ࢃཨ‫ݟ‬෍࢞ࢃՌָ‫ॶڂݜ‬㸦ࠛ403-0005 ᒣ᲍┴ᐩኈྜྷ⏣ᕷୖྜྷ⏣Ꮠ๢୸ᑿ 5597-1㸧
Mount Fuji Research Institute, Yamanashi Prefectural Government, 5597-1 Kenmarubi,Kamiyoshida, Fujiyoshida, Yamanashi 403-0005, Japan.
* fmaeno@eri.u-tokyo.ac.jp (author for correspondence)せ᪨ 2019 ᖺ 9 ᭶࡟ᑠ➟ཎㅖᓥすஅᓥ࡟ୖ㝣ࡋࠊ㏆ᖺࡢᄇⅆάື࡟ࡼࡾ⏕ࡌࡓᆅᙧࡸᆅ㉁ࠊᄇฟ
≀ࡢㄪᰝ࡜ヨᩱ᥇ྲྀࢆ⾜ࡗࡓࠋ2017 ᖺᄇⅆ࡟ࡼࡾᓥࡢすഃ࠾ࡼࡧ༡すഃ࡟ὶฟࡋࠊᆅᙧࢆ኱
ࡁࡃኚ࠼ࡓ⁐ᒾὶࡣࠊ⁐ᒾ⭾ᙇ๭ࢀ┠ࡸᯈ≧⠇⌮ࡢⓎ㐩࡞࡝ࠊ⁐ᒾࡢὶື࡜෭༷࡟క࠸⏕ࡌࡓᵝࠎ࡞≉ᚩⓗ࡞ᵓ㐀ࡸᒾ┦ࢆ᭷ࡍࡿࠋࡇࢀࡽࡢᆅ㉁࣭ᆅᙧⓗ≉ᚩ࡟ຍ࠼࡚ࠊ㢧ᚤ㙾ୗ࡛ࡢᒾ▼Ꮫⓗ≉ᚩࡣ 2013−2015 ᖺᄇⅆࡢᄇฟ≀࡜ࡼࡃఝ࡚࠸ࡿࠋ໬Ꮫศᯒࡢ⤖ᯝࠊ2017 ᖺᄇⅆࡢᄇฟ≀ࡣᏳᒣᒾ࡛࠶ࡿࡶࡢࡢࠊ2013−2015 ᖺᄇⅆ࡜඲ᒾ໬Ꮫ⤌ᡂࡀࢃࡎ࠿࡟␗࡞ࡗ࡚࠸ࡿࡇ࡜ࡀࢃ࠿ࡗࡓࠋࡋ࠿ࡋࡑࡢኚ໬ࡣࢃࡎ࠿࡞ࡶࡢ࡛࠶ࡾࠊ࣐ࢢ࣐≀ᛶࢆ኱ࡁࡃኚ࠼ࡿ࡯࡝ࡢࡶࡢ࡛ࡣ࡞࠸࡜⪃࠼ࡽࢀࡿࠋ㏆ᖺࡢすஅᓥࡢᄇⅆάື࡟ࡼࡾࠊᓥࡢᆅᙧࡸᆅ㉁ࡣ኱ࡁࡃኚࢃࡾ
ࡘࡘ࠶ࡿࠋࡲࡓᄇฟࡋ࡚࠸ࡿ࣐ࢢ࣐ࡢ≉ᚩࡶάືẖ࡟ࡸࡸ␗࡞ࡿࠋ௒ᚋࡶⅆᒣᄇฟ≀ࡢ≉ᚩ࡜ࡑࡢኚ໬ࢆࣔࢽࢱࣜࣥࢢࡍࡿࡇ࡜࡟ࡼࡾࠊᄇⅆ⌧㇟ࡸ࣐ࢢ࣐౪⤥⣔ࡢ⌮ゎࢆ㐍ࡵ࡚࠸ࡃࡇ࡜ࡀ㔜せ࡛࠶ࡿࠋ− 37 −࣮࣮࢟࣡ࢻ Ᏻᒣᒾࠊⅆᒣࠊⅆᒣᒾࠊࢸࣇࣛࠊ⁐ᒾὶ㸯㸬ࡣࡌࡵ࡟ すஅᓥࡣᑠ➟ཎㅖᓥࡢす᪉⣙ 130 km ࡟఩⨨ࡍࡿⅆᒣᓥ࡛࠶ࡿࠋኴᖹὒࣉ࣮ࣞࢺࡀࣇ࢕ࣜ
ࣆࣥᾏࣉ࣮ࣞࢺୗ࡬ỿࡳ㎸ࡴࡇ࡜࡟ࡼࡾᙧᡂࡉࢀࡓఀ㇋ᑠ➟ཎᘼࡢⅆᒣࣇࣟࣥࢺࡢ୍㒊ࢆᵓᡂࡍࡿ㸦ᅗ 1㸧ࠋᾏ㠃ୖ࡟ࡣ㝣ᆅࡀࢃࡎ࠿࡟⌧ࢀ࡚࠸ࡿࡢࡳ࡛࠶ࡿࡀࠊᾏᗏ࠿ࡽࡢẚ㧗ࡣ3000 m ࡟㐩ࡍࡿ኱ᆺࡢάⅆᒣ࡛ࠊᐩኈᒣ࡟༉ᩛࡍࡿつᶍࡢᒣయࢆ᭷ࡍࡿࠋすஅᓥࡢⓎぢࡣ1702 ᖺ࡟㐳ࡿࡀࠊグ㘓࡟ṧࡿᄇⅆࡣ 1970 ᖺ௦࡜ 2010 ᖺ௦ࡢᄇⅆࡢࡳ࡛࠶ࡾࠊࡇࢀ௨እ࡟᭷ྐ࡟ᄇⅆࡀ࠶ࡗࡓ࠿࡝࠺࠿ࡶྵࡵ࡚ࠊⅆᒣάືࡢᒚṔࡣࡼࡃࢃ࠿ࡗ࡚࠸࡞࠸ࠋ㝸⤯ࡉࢀࡓሙᡤ࡟࠶ࡿࡓࡵࠊ┠ᧁグ㘓ࡀᑡ࡞࠸ࡇ࡜ࡸࠊᒣయࡢ࡯࡜ࢇ࡝ࡣᾏ㠃ୗ࡟࠶ࡿࡓࡵࠊᾏᗏᄇⅆࡀ࠶ࡗࡓ࡜ࡋ࡚ࡶㄆ㆑ࡉࢀ࡟ࡃ࠸ࡇ࡜ࡶⅆᒣάືࡢᐇែࢆࢃ࠿ࡾ࡟ࡃࡃࡋ࡚࠸ࡿࠋすஅᓥࢆᵓᡂࡍࡿⅆᒣయࡢᾏ୰㒊࡟ࡣ」ᩘࡢഃⅆᒣࡸഃⅆཱྀࡀㄆࡵࡽࢀࠊᾏ㠃ୖ࡟⌧ࢀ࡞࠸άືࡶ⧞ࡾ㏉ࡋ㉳ࡇࡗ࡚ࡁࡓ࡜⪃࠼ࡽࢀࡿࠋࡇࡢࡼ࠺࡞⫼ᬒࡀ࠶ࡾࠊすஅᓥⅆᒣࡢ࣐ࢢ࣐ࡢ≉ᚩࡸᄇⅆᒚṔ࡟ࡘ࠸࡚ࡣᮍ▱ࡢ㒊ศࡀከ࠿ࡗࡓࠋࡑࡢࡓࡵࠊ2013 ᖺ࠿ࡽጞࡲࡗࡓ㏆ᖺࡢᄇⅆάືࡣࠊすஅᓥⅆᒣࡢ࣐ࢢ࣐ࡸᄇⅆ⌧㇟ࡢ≉ᚩࢆ▱ࡿࡓࡵࡢ㈗㔜࡞ᶵ఍ࢆᥦ౪ࡋ࡚࠾ࡾࠊ୍㐃ࡢάື࡟ࡼࡿᄇฟ≀ࡢ◊✲ࡸࠊⅆᒣάືࣔࢽࢱࣜࣥࢢࡢ◊✲ࡀ㐍ᒎࡋࡘࡘ࠶ࡿ㸦Maeno et al
Sano et al., 2016Shinohara et al., 2017 Tamura et al., 2018 Ṋᑿ࡯࠿ࠊ2018 ๓㔝࡯࠿ࠊ2018㸧ࠋ
2016 ᖺ 9 ᭶࡟ࡣᮾி኱Ꮫᆅ㟈◊✲ᡤࢆ୰ᚰ࡜ࡍࡿ◊✲ࢢ࣮ࣝࣉ࡟ࡼࡿୖ㝣ㄪᰝࡀ⾜ࢃࢀࠊ2013−2015 ᖺࡢάື㸦➨ 1 ᮇ࡜࿧ࡪ㸧࡟ࡘ࠸࡚ከࡃࡢⅆᒣᏛⓗ▱ぢࡀᚓࡽࢀࡓࡀࠊࡑࡢᚋࠊ
2017 ᖺ௨㝆ࡢ᪂ࡓ࡞ᄇⅆάື㸦2017 ᖺᄇⅆࢆ➨ 2 ᮇࠊ2018 ᖺᄇⅆࢆ➨ 3 ᮇ࡜࿧ࡪ㸧࡟ࡼࡾࠊすஅᓥࡢᆅᙧࡸᆅ㉁ࡣ෌ࡧ኱ࡁࡃኚࢃࡗࡓ㸦ᅗ 2㸧ࠋᡃࠎࡢࢢ࣮ࣝࣉ࡛ࡣࠊ2019 ᖺ 9 ᭶ࡢୖ㝣ㄪᰝ࡟࠾࠸࡚ࠊᮍゎ࡛᫂࠶ࡿ➨ 2 ᮇ௨㝆ࡢⅆᒣάື࡟ὀ┠ࡋࠊ᪂ࡓ࡟ᄇฟࡋࡓᆅ㉁ࠊᆅᙧࠊ
ᄇฟ≀ࡢ≉ᚩ࡜ࡑࡢኚ໬ࢆ᫂ࡽ࠿࡟ࡍࡿࡇ࡜ࢆ┠ⓗ࡜ࡋࡓࠋᮏㄽ࡛ࡣࠊ2019 ᖺୖ㝣ㄪᰝ࠾ࡼࡧࡑࢀ࡟ࡼࡾᚓࡽࢀࡓ⤖ᯝࡢᴫせ࡟ࡘ࠸࡚㏙࡭ࡿࠋ㸰㸬ㄪᰝ᪉ἲ࣭ሙᡤ すஅᓥୖ㝣ㄪᰝࡣࠊᓥす㒊ࡢ A ࡜ B ࡢ 2 ࡘࡢᆅᇦ㸦ᅗ 1b㸧࡟࠾࠸࡚ࠊࡑࢀࡒࢀ 9 ᭶ 3−4
᪥ࠊ9 ᭶ 5 ᪥࡟ᐇ᪋ࡋ㸦ᅗ 3㸧ࠊ」ᩘࡢᆅⅬ࡛ᆅ㉁ㄪᰝ࠾ࡼࡧᒾ▼ヨᩱࡢ᥇ྲྀࢆ⾜ࡗࡓ㸦Table1㸧
ࠋArea A ࡣࠊすஅᓥࡢ୰࡛᭱ࡶᾏ὾ࡀⓎ㐩ࡋࡓሙᡤ࡛࠶ࡾࠊࢃࡎ࠿࡟ṧᏑࡋ࡚࠸ࡿᪧᓥࠊ
2014−2015 ᖺᄇฟࡢ⁐ᒾࠊ2017 ᖺᄇฟࡢ⁐ᒾ㸦໭࣮ࣟࣈ⩌㸧ࢆྵࡴࠋArea B ࡣࠊ2017 ᖺ௨㝆࡟᪂ࡓ࡟Ⓨ㐩ࡋࡓ◁♟὾࡜ࡑࢀࢆྲྀࡾᕳࡃࡼ࠺࡟ศᕸࡍࡿ 2017 ᖺᄇⅆ࡟ࡼࡿ⁐ᒾ㸦໭࣮ࣟࣈ⩌࠾ࡼࡧ༡す࣮ࣟࣈ⩌㸧ࠊࡑࡋ࡚ࢃࡎ࠿࡟ṧᏑࡋ࡚࠸ࡿ 2014 ᖺ⁐ᒾࢆྵࡴࠋ
 Area A ࡛ࡣࠊᪧᓥࢆ୰ᚰ࡜ࡋࡓᆅ㉁ㄪᰝ࡟ࡼࡾࠊ➨ 1 ᮇ⁐ᒾ࡜➨ 2 ᮇ⁐ᒾࡢቃ⏺㒊㸦ᅗ 4㸧ࡢᢕᥱࡸࠊ➨ 2 ᮇ⁐ᒾࡢෆ㒊ᵓ㐀ࡢグ㍕ࠊ⁐ᒾヨᩱࡢ᥇ྲྀࠊ࠾ࡼࡧᪧᓥୖ࡟ሁ✚ࡋࡓࢸࣇࣛ

− 38 −

ࡢグ㍕ࠊヨᩱ᥇ྲྀࢆ⾜ࡗࡓࠋࡲࡓࠊすᓊࡢ໭㒊࡛ࡣ➨ 1 ᮇ㸦2014 ᖺ 11−12 ᭶㡭㸧࡟ὶฟࡋࡓ
⁐ᒾࡢグ㍕࡜ヨᩱ᥇ྲྀࢆ⾜ࡗࡓࠋࡇࡢ໭㒊ᆅᇦࡣࠊ2016 ᖺㄪᰝࡢ㝿࡟ࡣᾏᓊ࡟ἢࡗ࡚ࡢ࢔ࢡ
ࢭࢫࡣ୙ྍ⬟࡛࠶ࡗࡓࡀࠊ➨ 2 ᮇάື࡟ࡼࡾᾏᓊᆅᙧࡀ኱ࡁࡃኚࢃࡗࡓ⤖ᯝࠊᚐṌ࡛࢔ࢡࢭࢫࡀྍ⬟࡟࡞ࡗࡓࠋࡲࡓࠊすᓊࡢ༡㒊࡛ࡣ➨ 2 ᮇ⁐ᒾὶ㸦໭࣮ࣟࣈ⩌㸧ࡢㄪᰝ࠾ࡼࡧヨᩱ᥇ྲྀࢆ⾜ࡗࡓࠋ
 Area B ࡣࠊ➨ 2 ᮇ⁐ᒾ໭࣮ࣟࣈ⩌࡜༡す࣮ࣟࣈ⩌ࡢ୰㛫࡟఩⨨ࡋࠊẚ㍑ⓗ኱ࡁ࠸◁♟὾ࡀⓎ㐩ࡋ࡚࠸ࡿࠋࡑࡇ࡛ࡣࠊ୧࣮ࣟࣈ⩌ࡢ௦⾲ⓗ࡞ሙᡤ࡛ᆅ㉁ㄪᰝࢆ⾜࠸ࠊ⁐ᒾࡢෆ㒊ᵓ㐀ࡸ⾲㠃ᙧែࡢほᐹ㸦ᅗ 5㸧ࠊヨᩱ᥇ྲྀࢆ⾜ࡗࡓࠋࡲࡓ୍㒊ࡢ⁐ᒾ࡟ࡘ࠸࡚ࡣࠊ࣮࣎ࢺ࠿ࡽࡢ┤᥋ヨᩱ᥇ྲྀ࠾ࡼࡧ⮳㏆㊥㞳࡛ࡢ⁐ᒾᵓ㐀ࡢほᐹࢆ⾜࠺ࡇ࡜࡟ࡼࡾࠊᗈᇦ࡛⁐ᒾࡢ≉ᚩࢆᢕᥱࡍࡿࡇ࡜ࡀ࡛ࡁࡓ㸦ᅗ 1b㸧ࠋ㸱㸬ゎᯒ࣭ศᯒ⤖ᯝ㸱Ѹ㸯㸬⁐ᒾࡢศᕸ࣭ᵓ㐀ࡢ≉ᚩ すஅᓥࡢᆅᙧࡢ≉ᚩ࡜ࡋ࡚ࠊ⁐ᒾὶ࡟ࡼࡾᙧᡂࡉࢀࡓ↓ᩘࡢ⁐ᒾ࣮ࣟࣈᆅᙧࡀᣲࡆࡽࢀࡿࠋ࣮ࣟࣈ࡜ࡣࠊ⁐ᒾὶඛ➃ࡢ⯉≧࣭⿄≧ࡢᵓ㐀ࢆព࿡ࡋࠊ⢓ᛶࢆࡶࡗࡓ⁐⼥యࡢὶື࡟క࠸ᙧᡂࡉࢀࡿ≉ᚩⓗ࡞ᵓ㐀࡛࠶ࡿࠋすஅᓥࡢ➨ 1 ᮇάື࡛ࡣࠊ⁐ᒾὶࡢඛ➃ࡸഃ᪉࡛ఱᗘࡶศᒱࡀ㉳ࡁࠊࡑࢀࡒࢀࡢᨭὶࡢඛ➃࡛࣮ࣟࣈࡀᙧᡂࡉࢀࠊࡇࢀࢆ⧞ࡾ㏉ࡍࡇ࡜࡟ࡼࡾ㉳అ࡟ᐩࢇࡔ≉ᚩⓗ࡞⁐ᒾᆅᙧࢆᙧᡂࡋࡓ㸦ᅗ 1㸧ࠋ⁐ᒾὶࡣ෭༷࡟క࠸⾲㠃࡟⓶Ẇࡀᙧᡂࡉࢀࠊඛ➃࡯࡝ὶືࡋ࡟ࡃࡃ࡞ࡿࠋࡇࢀ࡟ࡼࡾ⁐ᒾࡢෆᅽࡀ㧗ࡲࡾࠊ᭱⤊ⓗ࡟࣮ࣟࣈ⾲㠃࡟ࡣட⿣ࡀ⏕ࡌࠊࣃ࢘ࣥࢻࢣ࣮࢟ࡢࡼ࠺࡞๭ࢀ┠㸦⁐ᒾ⭾ᙇ๭┠㸧ࡀᙧᡂࡉࢀࡿࠋࡇ࠺ࡋࡓ⭾ᙇ๭ࢀ┠ࡢⓎ㐩ࡢᵝᏊࡣ 2013−2015 ᖺᄇⅆࡢ㝿࡟ほᐹࡉࢀ࡚࠸ࡓ㸦Maeno et al., 2016㸧
ࠋ2017 ᖺࡢάືࡢ㝿࡟ࡶ⾨ᫍ෗┿ࡢゎᯒ࡟ࡼࡾࠊከࡃࡢ⁐ᒾὶ࡟࠾࠸࡚ከᩘࡢ⣽࠿࠸ᨭὶࡸ⁐ᒾ࣮ࣟࣈࡀほᐹࡉࢀ࡚࠸ࡓࡀ㸦Kaneko et al., 2019㸧ࠊ⁐ᒾ⾲㠃ࡸෆ㒊ᵓ㐀ࡢ≉ᚩࡢヲ⣽࡞ゎᯒࡣゎീᗘⓗ࡟㞴ࡋ࠿ࡗࡓࠋୖ㝣ㄪᰝ࡛ࡣࠊ᪂⁐ᒾࡢศᕸࢆ☜ㄆࡍࡿ࡜࡜ࡶ࡟ࠊࡇࢀࡽࡢ⁐ᒾ࣮ࣟࣈࢆ」ᩘࡢ⟠ᡤ࡛ほᐹࡋࠊ⾲㠃ࡸෆ㒊ᵓ㐀ࡢヲ⣽ࡸ⭾ᙇ๭ࢀ┠ࡢ≉ᚩࢆほᐹࡋࡓࠋ➨ 1 ᮇࠊ➨ 2 ᮇάື࡜ࡶ࡟ࠊ⁐ᒾ⾲㠃ࡢ኱ࡁ࡞ட⿣ࡣⰼᘚࡢࡼ࠺࡟ᕥྑ࡟኱ࡁࡃᣑࡀࡾࠊࡼࡃఝࡓ≉ᚩࢆ᭷ࡋ࡚࠸ࡓ㸦ᅗ 6㸧ࠋࡲࡓࠊᾏᓊ௜㏆ࡢ࠸ࡃࡘ࠿ࡢ඾ᆺⓗ࡞⁐ᒾ࣮ࣟࣈ࡛ࡣࠊࡑࡢ୰ᚰ㍈࠿ࡽྠᚰ෇≧࡟ᯈ≧⠇⌮ࡀⓎ㐩ࡋ࡚࠾ࡾࠊ⋢ࡡࡂࡢ᩿㠃ࡢࡼ࠺࡞ᵓ㐀ࢆᙧᡂࡋ࡚࠸ࡿࡇ࡜ࡀࢃ࠿ࡗࡓ㸦ᅗ 7㸧ࠋࡇࡢࡼ࠺࡞⠇⌮ࡸᵓ㐀ࡢ≉ᚩࡣࠊ෭༷᫬ࡢ཰⦰࡟ࡼࡾྠᚰ≧ࡢட⿣ࡀ⏕ࡌࡿࠊ࠶ࡿ࠸ࡣ⇕࠸⁐ᒾࡀእഃ࠿ࡽ෭༷࣭ᅛ⤖ࡋ࡚⓶Ẇࢆᙧᡂࡍࡿ୍᪉࡛ࠊෆ㒊ࡣࡲࡔὶືᛶࢆಖࡕ๓㐍ࡍࡿࡓࡵࠊ⓶Ẇ࡟ࡏࢇ᩿ᛂຊࡀࡣࡓࡽࡁࠊࡑࡢ⤖ᯝὶື࡜ྠࡌ᪉ྥ࡟ட⿣ࢆ⏕ࡌࡿࠊ࡞࡝ࡢࣉࣟࢭࢫ࡟ࡼࡾⓎ㐩ࡋࡓྍ⬟ᛶࡀ࠶ࡿࠋࡲࡓࠊ⁐ᒾὶ⾲㠃ࡢࢡ࣮ࣜࣥ࢝㒊ศࡶ➨ 1 ᮇ⁐ᒾ࡜ྠᵝࡢ≉ᚩࢆ᭷ࡋ࡚࠾ࡾࠊⓎἻᗘࡀⰋ࠸ሢ≧ࡢࡶࡢ࠿ࡽࠊᖹᯈ≧࡛⣽࠿࠸Ჲ≧ࡢ✺㉳ࡀ⾲㠃࡟Ⓨ㐩ࡋ࡚࠸ࡿࡶࡢ࡞࡝ࠊᵝࠎ࡞ࢱ࢖ࣉࡀほᐹࡉࢀࡓ㸦ᅗ 8㸧ࠋ ⁐ᒾࡢ໬Ꮫ⤌ᡂࡣᚋ㏙ࡍࡿࡼ࠺࡟࡯ࡰ୍ᵝ࡛࠶ࡿࡀࠊⓎἻᗘࡸẼἻᙧែ࡞࡝ࡢ⣽࠿࠸⤌⧊
ࡣᅗ 9 ࡢࡼ࠺࡟ከᵝ࡛࠶ࡿࠋ࡜ࡃ࡟ᯈ≧⠇⌮ࡀⓎ㐩ࡋࡓ⟠ᡤࡢ⁐ᒾ࡟ࡣࠊட⿣࡜ྠ᪉ྥࡢࣇ

− 39 −࢛࢚࣮ࣜࢩࣙࣥ㸦㠃ᵓ㐀㸧ࡀⓎ㐩ࡋ࡚࠸ࡿࡼ࠺࡛࠶ࡾࠊࡑࡢᵓ㐀ࡸᚤ⣽⤌⧊ࡣ≉ᚩⓗ࡛࠶ࡿࠋ㸱Ѹ㸰㸬ᪧᓥୖࢸࣇࣛ
 2013 ᖺ௨๓࠿ࡽ၏୍ṧᏑࡋ࡚࠸ࡿᪧᓥୖ࡛ࡣࠊᅵተᒙࢆᇶᗏ࡜ࡋࠊࢫࢥࣜ࢔㉁㝆ୗࢸࣇࣛࡀ⾲ᒙ࡟ㄆࡵࡽࢀࡓࠋࢸࣇࣛᒙཌࡣሙᡤ࡟ࡼࡗ࡚␗࡞ࡿࡀࠊ10−30 cm ⛬ᗘ࡛࠶ࡿࠋሁ✚≀ࡣᑡ࡞ࡃ࡜ࡶ 6 ᯛࡢࣘࢽࢵࢺ࡟༊ศ࡛ࡁࡿ㸦ᅗ 10㸧ࠋୗ఩ࣘࢽࢵࢺࡣࠊ୺࡟ሢ≧↓ᒙ⌮࡛ẚ㍑ⓗῈửࡢⰋ࠸⢒⢏ⅆᒣ⅊࠿ࡽ࡞ࡿࡀࠊ┤ᚄ 1 cm ࢆ㉸࠼ࡿ㧗ⓎἻᗘࡢࢫࢥࣜ࢔㉁ⅆᒣ♟ࡶྵࡲࢀࡿ㸦Layer 1−4㸧ࠋࡑࡢୖ఩࡟ࠊᩘ mm ࡢ〓Ⰽᒙࢆᣳࡳࠊୖ఩ࡣᒙཌᩘ mm ࠿ࡽᩘ cm ࡢⷧ࠸⣽⢏ⅆᒣ⅊ᒙࡢ✚ࡳ㔜࡞ࡾ࠿ࡽ࡞ࡿᡂᒙᵓ㐀ࢆ᭷ࡍࡿࠋ2016 ᖺୖ㝣ㄪᰝࡢ㝿࡟ࡶᪧᓥୖ
ࡢࢸࣇࣛࡢㄪᰝࡀ⾜ࢃࢀ࡚࠾ࡾࠊୗ఩ᒙ࡜ྠᵝࡢሁ✚≀ࡢ≉ᚩࡀグ㍕ࡉࢀࠊ2013−2015 ᖺᄇⅆࡢሁ✚≀࡜ゎ㔘ࡉࢀ࡚࠸ࡿ㸦๓㔝࡯࠿ࠊ2017㸧ࠋ௒ᅇࡢㄪᰝ࡛ࡣࠊࡇࢀࡽࡢሁ✚≀ࡢୖ఩࡟᪂ࡓ࡞ሁ✚ࣘࢽࢵࢺ㸦Layer 5㸪Layer 6㸧ࢆ☜ㄆࡋࡓࠋࡇࡢሁ✚≀ࡣࠊ➨ 2 ᮇ௨㝆ࡢᄇⅆάື࡟ࡼࡿሁ✚≀࡜⪃࠼ࡽࢀࡿࡀࠊ2018 ᖺࡢάືࡣ▷ᮇ㛫࡛つᶍࡶ࠿࡞ࡾᑠࡉ࠸ࡇ࡜࠿ࡽࠊ኱
㒊ศࡣ2017 ᖺࡢ➨2 ᮇάື࠾ࡼࡧࡑࡢᚋࡢ㢼ᡂస⏝࡟ࡼࡾᙧᡂࡉࢀࡓሁ✚≀࡜⪃࠼ࡽࢀࡿࠋ
㸱Ѹ㸱㸬ᄇฟ≀ࡢᒾ▼Ꮫⓗ≉ᚩ 2017 ᖺᄇฟ≀ࡢ࠺ࡕ௦⾲ⓗ࡞⁐ᒾヨᩱ࡟ࡘ࠸࡚ᒾ▼ⷧ∦ࢆసᡂࡋࠊ೫ග㢧ᚤ㙾ほᐹࢆ⾜ࡗࡓ࡜ࡇࢁࠊᩬᬗ㖔≀ࡣᩳ㛗▼ࠊ༢ᩳ㍤▼ࠊ┤᪉㍤▼࠾ࡼࡧ୙㏱᫂㖔≀࡛ࠊࡑࡢ๭ྜࡣ 10 vol.%௨ୗ࡜ᚤ㔞࡛࠶ࡗࡓ㸦ᅗ 11㸧ࠋ▼㉁㒊ศࡢ⤖ᬗᗘࡣ㧗ࡃࠊ୺࡟ᩳ㛗▼ࡀᬗฟࡋ࡚࠸ࡿࠋࡇࡢ
≉ᚩࡣ➨ 1 ᮇ⁐ᒾ࡜ࡼࡃఝ࡚࠸ࡿࠋࡲࡓࠊ2017 ᖺ⁐ᒾ 12 ヨᩱࢆྵࡴࠊୖ㝣ㄪᰝ࡛᥇ྲྀࡋࡓ௦⾲ⓗヨᩱࡢ඲ᒾ໬Ꮫ⤌ᡂࢆࠊᮾி኱Ꮫᆅ㟈◊✲ᡤࡢ⺯ග X ⥺ศᯒ⿦⨨࡟ࡼࡾศᯒࡋࡓࠋࡇࡢ⤖ᯝࢆ㋃ࡲ࠼࡚ࠊࡇࢀࡲ࡛ࡢすஅᓥࡢάື࡟ࡼࡿᄇฟ≀ࡢ໬Ꮫ⤌ᡂ࡟ࡘ࠸࡚ᩚ⌮ࡋࡓ⤖ᯝ
ࢆᅗ 12 ࡟♧ࡍࠋ࡞࠾ࠊ⏝࠸ࡓࢹ࣮ࢱࡣࠊୖグࡢ 2017 ᖺ⁐ᒾヨᩱ࡟ຍ࠼࡚ࠊ➨ 1 ᮇ 2013−2015ᖺᄇฟ≀㸦2016 ᖺ 10 ᭶࠾ࡼࡧ 2019 ᖺ 9 ᭶ୖ㝣ㄪᰝ᫬࡟すஅᓥすᓊ໭㒊࡛᥇ྲྀࡋࡓ⁐ᒾ࠾ࡼࡧࢫࢥࣜ࢔㸧ࠊ➨ 2 ᮇ 2017 ࡲࡓࡣ 2018 ᖺᄇฟ≀㸦2019 ᖺ 6 ᭶࡟ࢻ࣮ࣟࣥㄪᰝ᫬࡟࠾࠸࡚ 2018
ᖺⅆཱྀ௜㏆࠿ࡽ᥇ྲྀࡋࡓࢫࢥࣜ࢔ 1 ヨᩱ㸧࡛࠶ࡿࠋ ໬Ꮫศᯒࡢ⤖ᯝࡢヲ⣽ࡸゎ㔘࡟ࡘ࠸࡚ࡣูㄽᩥ࡟࡚㏙࡭ࡿணᐃ࡛࠶ࡿࡀࠊ௒ᅇࡢศᯒ࠾ࡼ
ࡧᚑ᮶ࡢࢹ࣮ࢱ࡜ࡢẚ㍑ࡢ⤖ᯝࠊ➨ 2 ᮇ௨㝆ࡢᄇฟ≀ࡢ඲ᒾ໬Ꮫ⤌ᡂࡣࢃࡎ࠿࡛ࡣ࠶ࡿࡀࠊ
ࡇࢀࡲ࡛ࡢᄇⅆ࡜␗࡞ࡗ࡚࠸ࡿࡇ࡜ࡀࢃ࠿ࡗࡓࠋ➨ 2 ᮇ௨㝆ࡢᄇฟ≀ࡣࠊ➨ 1 ᮇᄇฟ≀ࡼࡾ
MgO ࡞࡝࡟ࡸࡸᐩࡴ୍᪉ࠊSiO2 ࡞࡝࡟ࡸࡸஈࡋ࠸ࠋࡇࢀࡲ࡛ 2013−2015 ᖺᄇฟ≀ࡢࡳࡢศᯒ࡟࠾࠸࡚ࠊ໬Ꮫ⤌ᡂࡢᙅ࠸᫬⣔ิኚ໬ࡀ࠶ࡿࡇ࡜ࡣㄆ㆑ࡉࢀ࡚࠸ࡓࡀ㸦๓㔝࡯࠿ࠊ2016㸧ࠊ
2017−2018 ᖺ࡟࠾ࡅࡿ໬Ꮫ⤌ᡂࡢኚ໬ࡣࠊࡑࡢ᫬⣔ิኚ໬࡟ᑐᛂࡋࡓኚື࡛࠶ࡿྍ⬟ᛶࡀ࠶ࡾࠊ௒ᚋࡉࡽ࡟ヲࡋࡃ᳨ウࡍࡿᚲせࡀ࠶ࡿࠋ㸲㸬⪃ᐹ࣭ࡲ࡜ࡵ ᮏㄪᰝ࡟ࡼࡾすஅᓥ➨ 2 ᮇάືࡢᆅᙧࡸᆅ㉁ࠊᄇฟ≀ࡢᴫせࢆᢕᥱࡍࡿࡇ࡜ࡀ࡛ࡁࡓࠋ㐲− 40 −㝸࠿ࡽࡢほ ࡟ࡼࡾ➨ 2 ᮇࡢ⁐ᒾᆅᙧࡢ≉ᚩࡣࠊ➨ 1 ᮇࡢ⁐ᒾ࡜ࡼࡃఝ࡚࠸ࡿࡇ࡜ࡀ♧၀ࡉࢀ࡚࠸ࡓࡀ㸦Kaneko et al., 2019㸧ࠊୖ㝣ㄪᰝ࡟ࡼࡿ┤᥋ࡢほᐹ࡟ࡼࡗ࡚ࡶࠊ⁐ᒾ⭾ᙇ๭ࢀ┠ࡸᯈ≧ࢡ࣮ࣜࣥ࢝ࡢⓎ㐩࡞࡝ࠊ➨ 1 ᮇ⁐ᒾ࡜ᴟࡵ࡚ࡼࡃఝࡓ≉ᚩࡀほᐹࡉࢀࡓࠋࡇࡢẚ㍑⤖ᯝࡣࠊ୧άື࡛ࡣ࣐ࢢ࣐⤌ᡂࡀኚ໬ࡋ࡚࠸࡞࠸ࡇ࡜ࢆ᝿㉳ࡉࡏࡿࡀࠊ໬Ꮫศᯒࡢ⤖ᯝࠊ➨ 2 ᮇάື࡛ࡣ඲ᒾ໬Ꮫ⤌ᡂࡀࢃࡎ࠿࡟␗࡞ࡗ࡚࠸ࡿࡇ࡜ࡀࢃ࠿ࡗࡓࠋࡋ࠿ࡋࡇࡢ໬Ꮫ⤌ᡂࡢ㐪࠸ࡣࢃࡎ࠿࡞ࡶࡢ࡛࠶ࡾࠊ࣐ࢢ࣐ࡢ≀ᛶࢆ኱ࡁࡃኚ࠼ࡿ࡯࡝ࡢࡶࡢ࡛ࡣ࡞࠸࡜⪃࠼ࡽࢀࡿࠋࡑࡢࡓࡵࠊୖ✵࠿ࡽほᐹࡉࢀࡓ⁐ᒾࡢὶືࡸᐃ⨨ࠊᵓ㐀࡞࡝ࡢᕧどⓗ≉ᚩࡣࠊ➨ 1 ᮇ࡜➨ 2 ᮇ࡛㢮ఝࡋ࡚࠸ࡿ࡜⪃࠼ࡽࢀࡿࠋ ࡇࢀࡲ࡛ࡢ 2013−2015 ᖺᄇฟ≀୰ࡢᩬᬗ㖔≀ࡢศᯒ࠿ࡽࠊ㏆ᖺࡢᄇⅆ࡟ࡼࡿ࣐ࢢ࣐ࡣࠊὸ㒊ప ࣐ࢢ࣐⁀ࡾ࡜῝㒊㧗 ࣐ࢢ࣐⁀ࡾ࡟⏤᮶ࡋࠊὸ㒊࣐ࢢ࣐⁀ࡾࡣ῝㒊࣐ࢢ࣐ࡢ」ᩘᅇࡢ
ὀධࣉࣟࢭࢫࢆཷࡅ࡚࠸ࡿࡇ࡜ࡀ᥎ᐃࡉࢀ࡚࠸ࡿ㸦๓㔝࡯࠿ࠊ2018㸧ࠋࡇࡢ࣐ࢢ࣐౪⤥⣔ࡢᥥീࢆ⪃៖ࡍࡿ࡜ࠊ඲ᒾ໬Ꮫ⤌ᡂࡢ᫬㛫ኚ໬ࡢ≉ᚩࡣࠊ᫬㛫⤒㐣࡜࡜ࡶ࡟ࡼࡾ῝㒊࡟⏤᮶ࡍࡿⱞ㕲㉁࣐ࢢ࣐ࡢᐤ୚ࡀ኱ࡁࡃ࡞ࡗ࡚࠸ࡿࡇ࡜ࢆ♧၀ࡍࡿࠋࡇࡢኚ໬ഴྥࡀ⥅⥆ࡋ࡚࠸ࡃ࠿
࡝࠺࠿ࡶྵࡵࠊ2019 ᖺ 12 ᭶࡟άືࢆ෌㛤ࡋࡓすஅᓥࡢ࣐ࢢ࣐ࡢ≉ᚩࢆヲࡋࡃㄪ࡭ࡿࡇ࡜ࡀ
௒ᚋࡢㄢ㢟ࡢ୍ࡘ࡛࠶ࡿࠋ
 ㏆ᖺࡢすஅᓥࡢᄇⅆάື࡟ࡼࡾࠊᓥࡢᆅᙧࡸᆅ㉁ࡣ኱ࡁࡃኚࢃࡾࡘࡘ࠶ࡿࠋࡲࡓࠊᄇฟࡋ
࡚࠸ࡿ࣐ࢢ࣐ࡶάືẖ࡟ࡸࡸ␗࡞ࡿ࡜࠸࠺≉ᚩࡀ࠶ࡿࠋ௒ᚋࡶⅆᒣᄇฟ≀࡜ࡑࡢኚ໬ࢆࣔࢽ
ࢱࣜࣥࢢࡍࡿࡇ࡜࡟ࡼࡾࠊすஅᓥࡢᄇⅆάື࡟ࡼࡿ㝣ᇦࡢኚ໬ࡔࡅ࡛࡞ࡃࠊᆅୗࡢ࣐ࢢ࣐౪
⤥⣔ࡢ⌮ゎࡶ㐍ࡵ࡚࠸ࡃࡇ࡜ࡀࠊすஅᓥⅆᒣࡢ඲యീࢆ᫂ࡽ࠿࡟ࡍࡿୖ࡛㔜せ࡛࠶ࢁ࠺ࠋ㸳㸬ㅰ㎡
 すஅᓥୖ㝣ㄪᰝ࠾ࡼࡧࡑࢀ࡟ࡶ࡜࡙ࡃᮏ◊✲ࡣࠊ⎔ቃ┬࡟ࡼࡿ௧࿴ඖᖺᗘすஅᓥ⥲ྜᏛ⾡ㄪᰝ࠾ࡼࡧᮾி኱Ꮫᆅ㟈◊✲ᡤᡤ㛗⿢㔞⤒㈝࡟ࡼࡿࢧ࣏࣮ࢺࢆཷࡅ࡚ᐇ᪋ࡉࢀࡓࠋᒾ▼ヨᩱࡢ඲ᒾ໬Ꮫศᯒ࡟㛵ࡋ࡚ࡣࠊᮾி኱Ꮫᆅ㟈◊✲ᡤᏳ⏣ᩔ෸ᩍᤵࠊእすዉὠ⨾ᢏᐁࡢ༠ຊ㡬࠸ࡓࠋグࡋ࡚ឤㅰࡋࡲࡍࠋ
㸴㸬ᘬ⏝ᩥ⊩
Kaneko T, Maeno F, Yasuda A, Takeo M & Takasaki K (2019) The 2017 Nishinoshima eruption:
combined analysis using Himawari-8 and multiple high-resolution satellite images. Earth, Planets
and Space 71: 140.
Maeno F, Nakada S & Kaneko T (2016) Morphological evolution of a new volcanic islet sustained bycompound lava flows. Geology 44: 259–262.
๓㔝 ῝࣭୰㔝 ಇ࣭ྜྷᮏ ඘ᏹ࣭኱‖ 㝯㞝࣭Ώ㑓 ⠜ᚿ࣭ᕝୖ ࿴ே࣭༓⏣ ᬛᇶ࣭Ṋᑿ ᐇ (2017)᪂ⅆᒣᓥࡢึୖ㝣ㄪᰝ ņすஅᓥ㸦ᮾி㒔ᑠ➟ཎᮧ㸧ņ㸦᪥ᮏࡢ㟢㢌࣭ᬒほ 100 㑅㸧. ᆅᏛ㞧ㄅ 126: N1–N13.
๓㔝 ῝࣭Ᏻ⏣ ᩔ࣭୰㔝 ಇ࣭ྜྷᮏ ඘ᏹ࣭኱‖ 㝯㞝࣭Ώ㑓 ⠜ᚿ࣭㔠Ꮚ 㝯அ࣭୰⏣ ⠇ஓ࣭− 41 −Ṋᑿ ᐇ (2018) ᄇฟ≀࠿ࡽ᥈ࡿすஅᓥࡢ᪂ⅆᒣᓥᙧᡂࣉࣟࢭࢫ. ᾏὒ⌮ᕤᏛ఍ㄅ24(1): 35–44. ᑠᆏ ୔ண (1975) すஅᓥⅆᒣࡢάື࡜ࡑࡢほ 㸦⥆㸧. ᆅ㉁ࢽ࣮ࣗࢫ 246: 1–9.
ᑠᆏ ୔ண࣭኱ᖹ ὒᏊ࣭‖ ୍㑻 (1974) すஅᓥ௜㏆ᾏᗏᄇⅆ࡟ࡘ࠸࡚㸦ࡑࡢ 3㸧. ⅆᒣ 19: 37–38.
Sano T, Shirao M, Tani K, Tsutsumi Y, Kiyokawa S & Fujii T (2016) Progressive enrichment of arc
magmas caused by the subduction of seamounts under Nishinoshima volcano, Izu-Bonin Arc,
Japan. Journal of Volcanology and Geothermal Research 319: 52–65.
Shinohara M, Ichihara M, Sakai S, Yamada T, Takeo M, Sugioka H, Nagaoka Y, Takagi A, Morishita T,Ono T & Nishizawa A (2017) Continuous seismic monitoring of Nishinoshima volcano,Izu-Ogasawara, by using long-term ocean bottom seismometers. Earth, Planets and Space 69: 159.
Ṋᑿ ᐇ࣭኱‖ 㝯㞝࣭๓㔝 ῝࣭⠛ཎ 㞞ᑦ࣭㤿ሙ ⪷⮳࣭Ώ㑔 ⠜ᚿ࣭ᕷཎ ⨾ᜨ㺃す⏣ ✲࣭㔠Ꮚ 㝯அ࣭Ᏻ⏣ ᩔ࣭ᮡᒸ ⿱Ꮚ࣭὾㔝 ὒ୕࣭ከ⏣ カᏊ࣭୰㔝 ಇ࣭ྜྷᮏ ඘ᏹ࣭㧗ᮌ ᮁ඘࣭㛗ᒸ ඃ (2018) すஅᓥࡢᆅ⌫≀⌮ほ ࡜ୖ㝣ㄪᰝ. ᾏὒ⌮ᕤᏛ఍ㄅ 24(1): 45–56.
Tamura Y, Ishizuka O, Sato T & Nichols ARL (2018) Nishinoshima volcano in the Ogasawara Arc: newcontinent from the ocean? Island Arc H doi: 10.1111/iar.12285.
ᾏ㔝 㐍࣭୰㔝 ಇ (2007) ࠗ∗ᓥิᓥᆅᇦࡢᆅ㉁. ᆅᇦᆅ㉁◊✲ሗ࿌㸦5 ୓ศࡢ 1 ᆅ㉁ᅗᖜ㸧࠘⏘⥲◊ᆅ㉁ㄪᰝ⥲ྜ◊✲ࢭࣥࢱ࣮, 71p.

− 42 −

ᅗ 1 (a) すஅᓥࡢ఩⨨࠾ࡼࡧࢸࢡࢺࢽࢵࢡࢭࢵࢸ࢕ࣥࢢ (b) ᅜᅵᆅ⌮㝔࡟ࡼࡿ✵୰෗┿㸦2018 ᖺ 12 ᭶ 1 ᪥᧜ᙳ㸧
ࠋFigure 1. (a) Location and tectonic setting of Nishinoshima volcano. (b) Aerial photo of Nishinoshima,taken on 1 December 2018 by Geospatial Information Authority.

ᅗ 2. 2019 ᖺ 9 ᭶ୖ㝣᫬Ⅼ࡛ࡢすஅᓥࡢᆅ㉁ᅗ
Figure 2. Geological map of Nishinoshima at the time of landing survey in September 2019

− 43 −


ᅗ 3. すஅᓥす㒊ࡢᣑ኱࡜すᓊ࠾ࡼࡧ༡すᓊࡢㄪᰝ࣮ࣝࢺ࣐ࢵࣉ࠾ࡼࡧヨᩱ᥇ྲྀ఩⨨Figure 3. Magnification of the western part of Nishinoshima. Route map and sampling locations at thewest and southwest coasts of the island.

− 44 −

ᅗ 4 ᪧᓥୖࡢ⁐ᒾࡢศᕸࡢᵝᏊ㸦2014 ᖺ࡜ 2017 ᖺ⁐ᒾࡢቃ⏺㒊 GM1-7 ௜㏆㸧
Figure 4. Lava flows on the old Nishinoshima (The boundary between the 2014 and 2017 lava flows,near GM1-7)

ᅗ 5. ༡すᓊB ᆅᇦ࡟࠾ࡅࡿ2017 ᖺ⁐ᒾࡢ᩿㠃㸦ᇶᗏ㒊࡟ᯈ≧⠇⌮ࡀⓎ㐩ࡍࡿGM22 ௜㏆㸧
Figure 5. A cross section of the 2017 lava in the southwestern coast (Area B. Platy joints are developingin the basal part of the lava. Near GM22)− 45 −

ᅗ 6. (a) 2014 ᖺ⁐ᒾ࡟Ⓨ㐩ࡋࡓ⁐ᒾ⭾ᙇ๭ࢀ┠㸦GM15 ௜㏆㸧ࢫࢣ࣮ࣝࡣ 1 mࠋ(b) 2017 ᖺ⁐ᒾࡢ⁐ᒾ⭾ᙇ๭ࢀ┠ࠋGM10 ௜㏆ࠋ⁐ᒾࡢཌࡉࡣ⣙ 3 mࠋFigure 6. (a) A lava inflation cleft seen in the 2014 lava (near GM15)
A scale is 1 m. (b) A lava inflation cleft seen in the 2017 lava (near GM10). The thickness of lava isabout 3 m.

ᅗ 7. ༡すᾏᓊ࡟ぢࡽࢀࡿ⁐ᒾὶࡢ᩿㠃ᯈ≧⠇⌮ࡀⓎ㐩ࡋࠊࡓࡲࡡࡂ≧ᵓ㐀ࢆ࿊ࡍࡿࠋ
Figure 7. A cross section of lava flow seen in the southwestern coast
Platy joints are developing concentrically, making an onion-like structure.

− 46 −ᅗ 8. 2017 ᖺ⁐ᒾ⾲ᒙࢆᵓᡂࡍࡿࢡ࣮ࣜࣥ࢝ (a) GM24 ௜㏆ࠊ(b) GM10 ௜㏆Figure 8. Clinker consisting the surface of the 2017 lava, (a) near GM24 and (b) near GM10

ᅗ 9. ᵝࠎ࡞ᒾ┦ࢆ♧ࡍすஅᓥ⁐ᒾ(a) GM8: 2017 ᖺ⁐ᒾࡢࢡ࣮ࣜࣥ࢝㒊ࠋ(b) GM12: すᓊ༡㒊࡟࠾ࡅࡿ 2017 ᖺ⁐ᒾ㸦໭࣮ࣟࣈ⩌㸧ࡢ▼㉁㒊ࠋ(c) GM13: すᓊ༡㒊࡟࠾ࡅࡿ 2017 ᖺ⁐ᒾ㸦໭࣮ࣟࣈ⩌㸧ࡢ࢞ࣛࢫ㉁㒊ศࠋ(d)GM15: すᓊ໭㒊࡟࠾ࡅࡿ 2014 ᖺ⁐ᒾࡢ▼㉁㒊ࠋ(e) GM22: ༡すᓊ࡟࠾ࡅࡿ 2017 ᖺ⁐ᒾ㸦໭࣮ࣟࣈ⩌㸧ࡢᯈ≧⠇⌮㒊ศࠋ(f) GM27: ༡すᓊ࡟࠾ࡅࡿ 2017 ᖺ⁐ᒾ㸦༡す࣮ࣟࣈ⩌㸧ࡢ⦓ᐦ
࡞࢞ࣛࢫ㉁㒊ศࠋFigure 9. Various lithofacies of Nishinoshima lava(a) GM8: A clinker part of the 2017 lava (b) GM12: A lithified part of the 2017 lava (N-lobes) in thesouth of the west coast (c) GM13: A glassy part of the 2017 lava (N-lobes) in the south of the westcoast (d) GM15: A lithified part of the 2014 lava in the north of the west coast (e) GM22: A platy joint
part of the 2017 lava (SW-OREHV LQWKHVRXWKZHVWFRDVW (f) GM27: A dense glassy part of the 2017 lava(SW-lobes) in the southwest coast.

− 47 −

ᅗ 10. すஅᓥᪧᓥୖࡢࢸࣇࣛࡢᵝᏊ
Figure 10. Tephra layers consisting the surface of the old Nishinoshima.

ᅗ 11. すஅᓥ⁐ᒾࡢⷧ∦㢧ᚤ㙾෗┿ୖ࠿ࡽ㡰࡟ GM11ࠊGM27ࠊGM22 ࡢᒾ▼ヨᩱࠋ୰ኸࡣ࣮࢜ࣉࣥࢽࢥࣝࠊྑࡣࢡࣟࢫࢽࢥࣝ࡟ࡼࡿ᧜ᙳࠋ
Figure 11. Microscopic photos of Nishinoshima lavas
GM11, GM27 and GM22 from upper. Center is one nicol, and right is crossed nicol.

− 48 −

ᅗ 12. すஅᓥᄇฟ≀ࡢ඲ᒾ໬Ꮫ⤌ᡂ2017–2018 ᄇฟ≀ࡣࠊ2013–2015 ᄇฟ≀ࡼࡾ MgO ࡟ࡸࡸᐩࡳࠊ໬Ꮫ⤌ᡂ࡛༊ู࡛ࡁࡿࠋ
Figure 12. Whole rock chemical composition of eruptive materials of Nishinoshima.
The 2017–2018 eruptive materials are richer in MgO than the 2013–2015 ones, and can bediscriminated from others in chemistry.

− 49 −

⾲ 1 2019 ᖺすஅᓥୖ㝣ㄪᰝ࡛᥇ྲྀࡋࡓ௦⾲ⓗᒾ▼ヨᩱ
Table 1 Representative rock samples collected during the 2019 landing survey at Nishinoshima
Sample No.

Loc. No.

LocationType

LatitudeLongitude

Eruption time

NSH2019090301

GM01On old island

Scoria27°14'45.14"N

140°52'24.37"E

Early 2014

NSH2019090302

GM03

On old island

Scoria27°14'45.28"N

140°52'23.92"E

Early 2014

NSH2019090303

GM08

On old island

Lava

27°14'44.30"N

140°52'23.96"E

2017/4/17-4/28

NSH2019090304

GM09

Old island

Lava

27°14'49.16"N

140°52'22.46"E

Unknown

NSH2019090401

GM10South of W coast

Lava

27°14'43.48"N

140°52'15.23"E

2017/4/17-4/28

NSH2019090402

GM11South of W coast

Lava

27°14'40.67"N

140°52'12.89"E

2017/6/16-7/31

NSH2019090403

GM12South of W coast

Lava

27°14'44.55"N

140°52'14.73"E

2017/4/17-4/28

NSH2019090405

GM14On new flat

Lava

27°14'54.31"N

140°52'18.03"E

2014/11/9-12/23

NSH2019090406

GM15

North of W coast

Lava

27°14'59.02"N

140°52'19.17"E

2014/11/9-12/23

NSH2019090407

GM16

North of W coast

Lava

27°14'58.61"N

140°52'16.42"E

2014/11/9-12/23

NSH2019090501

GM18

SW coast

Lava

27°14'17.80"N

140°52'25.46"E

2017/5/1-6/5

NSH2019090502

GM19

SW coast

Lava

27°14'15.00"N

140°52'34.81"E

2017/5/1-6/5

NSH2019090503

GM20

SW coast

Lava

27°14'16.11"N

140°52'28.45"E

2017/5/1-6/5

NSH2019090504

GM21

SW coast

Lava

27°14'31.94"N

140°52'11.05"E

2017/6/6-6/15

NSH2019090505

GM22

SW coast

Lava

27°14'29.68"N

140°52'16.22"E

2017/6/6-6/15

NSH2019090506

GM23

SW coast

Lava

27°14'29.16"N

140°52'16.76"E

2017/6/6-6/15

NSH2019090508

GM25

SW coast

Lava

27°14'28.23"N

140°52'21.13"E

2017/6/6-6/15

NSH2019090509

GM26

SW coast

Lava

27°14'24.72"N

140°52'21.22"E

2017/5/1-6/5

NSH2019090510

GM27

SW coast

Lava

27°14'23.83"N

140°52'22.33"E

2017/5/1-6/5− 50 −

OGASAWARA RESEARCHSUMMARY

Characteristics and change of geomorphology, geology and eruptive materials in
the eruption of Nishinoshima volcano, Ogasawara
Fukashi MAENO1* & Mitsuhiro YOSHIMOTO2
1. Earthquake Research Institute, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan.
2. Mount Fuji Research Institute, Yamanashi Prefectural Government, 5597-1 Kenmarubi, Kamiyoshida,Fujiyoshida, Yamanachi 403-0005, Japan.
* fmaeno@eri.u-tokyo.ac.jp (author for correspondence)
Landing survey at Nishinoshima was carried out in September 2019, and geomorphology, geology
and eruptive materials from the recent eruptions at this volcano were investigated. Lava flows that
effused to the western and southwestern sides of the island in 2017 have characteristic structures such aslava inflation cleft and platy joints. In addition to these structures, lithofacies and microscopic features arealso similar to the 2013–2015 lavas. Petrological observation and geochemical data suggest that the2017 products are andesite as in the 2013–2015 eruption. Although the whole rock chemicalcomposition of the 2017 products is slightly different from that of 2013–2015, the difference is too little
to make a significant change on physical properties of magma. The eruptive activity of Nishinoshima
has changed geomorphology and geology drastically, and the magma erupted is also not the same
throughout the eruption time. Monitoring eruptive materials and their change will be important inunderstanding eruptive phenomena and magma supply system of Nishinoshima volcano.
Key words
Andesite, Lava flow, Tephra, Volcano, Volcanic rock− 51 −