ハワイホットスポット火山の研究

２００１−２００２年度調査航海申請書

申請代表者： 高橋栄一 （東京工業大学 理学部 地球惑星科学科 教授）

乗船研究者：金松敏也　　（海洋科学技術センター）

　　　　　　仲二郎　　　（海洋科学技術センター）

　　　　　　久保雄介　　（海洋科学技術センター　PD特別研究員）

　　　　　　荒牧重雄 （日本大学 文理学部 地学科 教授）

　　　　　　高橋栄一 　（東京工業大学 理学部 地球惑星科学科 教授）

　　　　　　海野進 （静岡大学 理学部 地球惑星科学科 助教授）

　　　　　　木村純一　　（島根大学　理学部　地球科学科　助教授）

横瀬久芳 （熊本大学理学部 助手）

　　　　　　中川光弘　　（北海道大学　理学部　地球惑星科学科　助手）

　　　　　　折橋裕二　　（東京大学　地震研究所　助手）

　　　　　　松本拓也　　（大阪大学　理学部　地球惑星科学科　助手）

　　　　　　原田　靖　　（ハワイ大学　地球物理学科　PD特別研究員）

　　　　　　下司信夫　　（東京大学　理学部　地球惑星科学科　大学院生D3）

任鐘元 　 （東京工業大学 理学部 地球惑星科学科 大学院生D２）

　　　　　　丸山麻里　　（東京工業大学 理学部 地球惑星科学科 大学院生M1）

　　　　　　石井英一　　（北海道大学大学院　地球惑星科学専攻　大学院生M1)

　　　　　　國清智之　　（島根大学　大学院　地球科学専攻　大学院生Ｍ１）

米国側乗船研究者

　 Dr. Peter Lipman, (USGS): volcanology, landslide processes, evolution of Hawaii Island

Dr. James Moore (USGS Emeritus Scientist): Marine geology, volcanology, Hawaiian geology

Dr. Thomas Sisson (USGS): basalt petrology and geochemistry

Dr. Carl Thornber (USGS, HVO geologist): basalt petrology and eruptive processes

　 Dr. David Clague, (MBARI): Marine geology, petrology, ocean-island geology

Dr. Kevin Johnson (Bishop Museum): Petrology, marine geology

Prof. Greg Moore (Univ. of Hawaii): marine geophyisics

　　 Prof. Alex Malahoff (Univ. of Hawaii): Loihi geology

Dr. Brian Midson (Univ. of Hawaii): Loihi geology

Dr. John Smith (Univ. Hawaii): bathymetric and side-scan surveying and data processing

Prof. Julia Morgan (Rice University ): structural analysis, landslide mechanics

（２）目的・背景

　地球上最大のホットスポットであるハワイは，深部から上昇するマントルプルームのダイナミクスを理解するもっとも重要な鍵としての位置を占める。ハワイ火山は米国地質調査所ハワイ火山観測所（HVO）、ハワイ大学等の観測・調査により世界でもっとも研究の進んだホットスポット火山である。しかしながら陸上からの火山調査は現在活動中の火山に限られ、ハワイ大学の潜水調査も2000m以浅に限定されている。即ち水深5000mを越す深海底から隆起するハワイホットスポット火山の全貌は深海底部分に関しては全く知られていない。

　本研究の先駆けを成す第1次ハワイ航海（98−99年）で、我々はハワイ諸島周辺の精密な海底地形をシービームにより調査した（図−１）。更に潜水調査船による海底地質調査、採取した岩石の地球化学分析などにより、１）ロイヒ火山の水深4000mを越す基底部がアルカリ系列およびソレイアイト系列の2種ピクライト質溶岩から構成されている事、２）キラウエア火山南山腹のヒリナスランプ深海底部分に初期のアルカリ岩が露出している事、３）地球上最大の火山体崩壊であるオアフ島北方のヌーアヌ地滑りとワイラウ地滑りの時期を推定し、オアフ島のコーラウ火山成長史を復元した事、４）プルームの中心部から数100km離れたノースアーチ玄武岩の噴出口に潜り海底洪水溶岩の厚さが10mに満たない事を発見するなど、ハワイホットスポット火山の誕生から崩壊に至るまでの火山成長史に関する数多くの重要な発見をした。

本研究はこれまでの研究成果を更に発展させホットスポット火山の誕生から山体崩壊による終焉までの活動史を解明する目的で組まれた。第1次の研究で日米の研究者の間に生まれた緊密な研究協力体制はこれまでハワイなどホットスポット火山における研究史の浅かった我が国の地球科学にとっては貴重な財産である。我々は第2次調査航海を実施する事によりこれを継続し発展させたい。

（３）中期計画での位置づけ

　　ホットスポット火山活動とその原動力であるマントルプルームのダイナミクスはJAMSTECの中期計画の中でも特に重要視されている。我が国はプレートのサブダクション帯に位置するためにこれまでサブダクションプロセスに関する研究が地球科学の主流を占め、ホットスポットやマントルプルームに関する研究は少なかった。　我々の研究グループは1998年および1999年の2年に渡りJAMSTECの深海調査船を用いてハワイ諸島の深海を調査し、主として陸上調査に基づいて構築された従来のハワイ火山成長史を大きく変えるいくつかの発見を行った。JAMSTECの中期計画ではハワイホットスポット火山のこれまでの研究成果が高く評価され、今後積極的に推進されるべき分野であると明記されている。ホットスポット火山活動史に焦点を当てた第2次ハワイ航海の後には地震学的にハワイプルームの3次元構造を探り、ハワイ天皇海山列のホットスポットトラックからプルームの長期活動史を解明する第3次ハワイ航海をJAMSTECに設立する固体地球科学統合フロンティアと協力して実施したい。

(4) 研究内容：主要な研究課題のみを以下に掲げる。（その他の課題は　補足資料−１　参照）

4-1ハワイ火山成長史　

（高橋, 木村, 中川,　折橋 K.Johnson, D.Clague, T. Sisson, Thornber, E.Hauri）

ハワイホットスポットの起源およびホットスポット火山の成長に伴うマグマの化学組成や温度の変遷を明らかにするためにはキラウエア火山などの表層部の火山活動のみに頼るこれまでのハワイ火山研究は十分とは言えない。巨大山体崩壊などによって露出したハワイ火山の山体内部構造をドレッジ、潜水調査船による調査によって解明する。我々は既に98−99年航海によって得た地形、地質、岩石の情報からオアフ島コーラウ火山の成長史復元に成功しており、一つのハワイ楯状火山がケアトレンドとロアトレンドをまたぐ成長史を記録する従来のモデルでは説明不可能な事実を発見している（Takahashi　他、AGUモノグラフ、補足資料−４参照）。第2次ハワイ航海ではこの研究手法を用いて他の火山（ロイヒ、マウナロア、コハラ、ワイアナエなどが候補に上がっている）の成長史研究を行いたい。

4-2. 山体崩壊の時期とメカニズム

(金松、仲、木村、横瀬、荒牧、P. Lipman, T.Sisson, J.G.Moore, J. Smith, G.Moore, J. Morgan)

オアフ島の北方数１００ｋｍ四方に広がるヌーアヌ地滑りは５０００ｋｍ^３以上の崩壊体積を持つ地球上最大級の山体崩壊である。またキラウエア火山の南に広がるヒリナ地滑り地帯は1970年代のカラパナ地震に代表されるごとく現在も活動中であり、近い将来津波災害を伴う山体崩壊を起こす危険性を含んでいる。ハワイ火山が成長のどの段階で、いかなるメカニズムにより山体崩壊を起こすかは地球科学のみでなく災害科学からも重要な研究課題である。我々は98−99年航海で明らかにしたキラウエア火山のヒリナ地滑り帯の地質学的研究（米国側申請書　Fig.2）およびヌーアヌ地滑りの地質学的研究（米国側申請書　Fig.4）を継続し山体崩壊の時期とメカニズムを解明する。更に金松を中心とするJAMSTECチームはハワイ諸島周辺の10個所でピストンコアーを実施し、タービダイトの古地磁気編年、ガラスの化学分析等から各火山の山体崩壊時期を解明する。

4-3. 海底火山リフトの内部構造とマグマ活動

（高橋、中川、海野、下司、K.Johnson, J．Smith，D.Clague,　Thornber）

　ハレアカラ火山から南東に伸びる海底リフトは延長200kmにも及びハワイ楯状火山の内最大の規模である。99年のハワイ航海で我々はハレアカラリフトのシービームによる海底地形調査を始めて行い、その先端部に山体崩壊により生じたと思われる馬蹄形の窪地を発見した（付図−３）。もしこれが最近生じたリフトの崩壊地形であるとすれば2000mに達するその崩壊崖はリフトの内部構造を露出している可能性が高い。リフトは山頂火口と並んでハワイ型火山の成長線に当たる。プルームから供給されたマグマの大部分は山頂火口の下にあるマグマ溜りから数１０ｋｍ（ハレアカラ火山の場合は最大200km）の距離を岩脈として水平移動した後で海底に噴出する。リフトの形成、内部構造、マグマ移動の仕組みはハワイ型火山を理解する上で第1級の研究課題である。

４−４．プルームの3次元構造：　North　Arch玄武岩

（高橋、木村、宇都、趙大鵬、末広潔、D.Clague, C.Wolfe）

地震学トモグラフィー、海底地形、重力異常から推定されるハワイプルームの広がりは直径1000kmにも及ぶが、その火山活動はハワイ島南部の直径100km以下の狭い範囲に限定されている。中軸から水平に広がったプルームの傘を構成する物質の化学組成、温度などの情報はプルームダイナミクスを考察する上で中軸部のソレイアイト楯状火山と同様に重要である。米国地質調査所の行った海底音響探査（GLORIA）により、オアフ島はるか北東350kmの深海底にNorth　Arch　玄武岩が発見されハワイプルームの中軸から数100km離れた縁辺部で起こる火成作用として注目を集めている。JAMSTEC−99航海ではNorth　Arch　玄武岩の噴出点2個所に潜水調査を行いその噴出様式に関する貴重な資料を得たが、第2次航海では更に調査地域を広げて、North　Arch　玄武岩の全体像を明らかにする（海底地形調査地域に関しては　付図−１参照）。　選られた岩石資料を用いて高温高圧実験を行いプルームの傘を構成する物質の温度と化学組成を推定する。得られた物性情報を地震トモグラフィーの結果にフィードバックし、ハワイプルームの3次元構造を解明する。

４−５．ハワイ諸島海底測量の完成

（J.Smith, 原田、高橋、金松、および全乗船研究者）

シービーム測量　

　精密な海底地形図はすべての海洋調査の基本資料となる。ハワイ諸島は米国のもっとも重要なホットスポット火山でありながらこれまでシービームなどの精密な海底地形図はハワイ島南海域など極限られた範囲をカバーしていたに過ぎない（付図−１）。JAMSTECによる第1次ハワイ航海の結果ハワイ諸島周辺海底地形図領域は一挙に倍以上に拡大した。JAMSTEC地形図はAGUモノグラフにCD-ROMとして添えられる予定でありその利用価値はハワイ火山関係者にとどまらず広範な地球科学研究者に及ぶものと考えられる。我々は第2次航海においてハワイ諸島周辺の精密海底地形図を完成する事を目指す（付図―1参照）。

重磁力調査

「かいれい／よこすか」の航走調査でこれまで知られていなかったこと明らかとなった。例えば、ヒロ海嶺は逆帯磁しており（約８０万年より古い）、ハワイ島のいかなる火山のこれまでの年代よりも古い(Kanamatsu et al. 2000)。このよな結果はハワイの火山の活動期間等に大きな制約を与えるもので、極めて重要な情報である。そこで、今後の調査で回航等を効果的に重磁力調査を行い、これまでの（ピストンコア等も含め）年代結果等の再検討を行う。

（５）期待される成果

ハワイ火山成長史　

我々は９８−９９年航海の結果オアフ島のコーラウ火山がこれまでのハワイ火山の常識を覆すマルチステージのマグマ活動（マウナケア型からマウナロア型へ変化し更に活動の最末期にコーラウ型という特別なステージに以降した）を経験した事を見出した。これは火山の表層部のみを研究するこれまでのハワイ学に重大な変更を強いる発見である。第２次航海では巨大山体崩壊により露出した他のハワイ火山の深部構造を研究する事により、マルチステージマグマ活動の普遍性を検証する。火山の成長史によるマグマ活動の変遷からハワイプルームの溶融域でのマグマ形成機構が解明されると期待する。

山体崩壊の時期とメカニズム

山体崩壊の時期とそのメカニズムを解明する事により、津波災害などの予測が可能となる。従来の予想ではハワイ火山の巨大山体崩壊はソレイアイト火山の成長時に起きると考えられてきた（Moore et al., 1989）。しかしながら第１次航海の研究結果、特にピストンコアのタービダイト層の古地磁気層序に基づく我々の解釈では、オアフ島の崩壊は1.8Ma、モロカイ島の崩壊は1.0Maと推定され（Kanamatsu et al, AGU Monograph, 資料−４参照）これらはいずれもソレイアイト火山の活動期より新しい山体の侵食期に相当する。したがってハレアカラ火山など現在は主活動期を過ぎたハワイ火山も今後巨大崩壊を起こす危険性がある事になり、そのメカニズムを理解し、時期を予想する事により将来の自然災害を防ぎ得る。

海底火山リフトの内部構造とマグマ活動

海底火山リフトはハワイなど海洋火山島に限られるものではない。2000年の三宅島噴火で明らかになったごとくわれわらの身近な日本列島でもリフト帯に沿った玄武岩マグマの長距離水平貫入移動が起きている。海底火山リフトの内部構造はこれまで全く研究例がなくその結果は火山学、マグマ学の上で貴重である。研究結果は将来のハワイに限らず地球上多くの火山活動予測、噴火災害予知などに役立つと期待される。

プルームの3次元構造：　North　Arch玄武岩

ハワイプルームは固体地球のダイナミクスを検討する最重要フィールドの一つである。現在カーネギー研究所のS.Solomon博士を中心としてハワイプルームの地震学観測計画が練らている。我々がNorth　Arch　玄武岩のネフェリナイトマグマ生成モデルとキラウエアなどプルーム中心部のソレイアイトマグマ生成モデルを結んでプルームの3次元的熱構造、物質分布を解明する事により将来の3次元地震学研究に検証するべきチェックポイントを設ける事が出来る。我々の第2次ハワイ航海はプルームの大構造を研究する第3次航海の布石としての役割を担う。

ハワイ諸島海底測量の完成

言うまでもなく広域的な海底地形図はハワイプルームの活動を解く出発点となる重要な情報源である。今回の航海でハワイ島からオアフ島までの精密な海底地形図が完成するとその利用は地球科学にとどまらず海洋学、海洋生物学、その他に広がると期待される。シービーム地形図が得られた事により新たな海底地滑りが発見されたり、未知のリフト地形が見出される（付図―３参照）など、ハワイ火山活動史を探る上で貴重な発見が多数あるものと期待される。

（６）実施計画案

第2次ハワイ調査航海として我々は2001年および2002年の2回を申請する。2年間継続の研究実施スタイルは98−99年ハワイ航海で既にテスト済みである。2001年は「かいこう・かいれい」を用いた調査海域の予備的な調査、2002年は「よこすか・しんかい６５００」による本調査を実施したい。日本側の調査予定個所は付図−２に示した。ハワイ諸島を取り巻く海底地形図を作成しタービダイト層の年代を決めるためのピストンコアーを実施するためには最低2週間程度のかいれい単独行動が必要である。米国側は多数の潜水調査を未調査の地滑りに行いたいとしている（米国側申請書Table-2,3）。日本側の潜航希望調査点と米国側のそれと完全には一致していないが、これは12月にサンフランシスコで開催されるAGU会議で小集会を関係者で開いて更に擦りあわせる予定である。双方15−20回の潜航希望を持っているので「かいこう･かいれい」の3週間程度の行動が必要である。

2001年夏は「かいれい」単独行動2週間、「ROV−かいこう」による潜水調査3週間を申請する。横須賀とホノルルとの往復、乗員の交代および休養を兼ねたホノルルでの滞在期間を考慮すると、本航海は日本出発ベースで合計約2ヶ月を要する。「かいれい」単独行動は主としてシービームによる海底地形調査、ピストンコアーを用いた柱状採取泥（10個所程度）、ドレッジによる海底岩石採取（1５個所程度）に当てる。「ROV-かいこう」を用いた潜水調査では、多数の研究者が同時に操縦ピットに入れるROVの特長を生かして、日米双方の出来るだけ多数の研究者による意見交換を行いながらの地質調査を目指す。ROVの採取した岩石についても日米双方の研究者で分担協力しながら研究を進める。日本側の参加者には今回から参加した潜水調査未経験者も多いので2001年の「ROV−かいこう」による調査は2002年の「しんかい−６５００」を用いた潜水調査の予行演習の意味を兼ねている。

2002年夏には前年に予備調査した海域の中から研究上もっとも重要と判断された部分について、「よこすか・しんかい６５００」による潜水調査６週間を申請する。

（７）事前調査データ（航海の日取りについて）

　98年および99年行われた調査はいずれも8月−9月に航海が行われた。99年の「よこすか・しんかい6500」では、予備日を使って全潜水調査計画の９８％を実施できた（98年は１００％）。これは調査の下準備が入念になされていたことにもよるが、天候の好条件に恵まれた要素が大きい。ハワイ大学・USGSなどの経験者によれば、ハワイ海域での海上気象条件は8−9月の盛夏において貿易風が弱まるために潜水に最適となるが、それ以外の時期は波が高く潜水調査には適さない。従って我々は第2次ハワイ調査航海も8−9月に行うことを強く希望する。

　なおハワイ海域の気象状況は貿易風に支配される所が大きいが、巨大な火山の風下に当たる西側は常に風が弱く潜水調査が可能である。したがって我々はマウナロア火山の西側地域（米国側申請書　Fig.5）およびオアフ島西側海域（米国側申請書　Fig．6）を荒天時調査地域と考えている。

（８）研究業績

98年99年JAMSTECハワイ航海の研究論文集は2000年6月WPGM国際会議で2日間に渡る特別セッションを開催して報告された。これらの論文は米国地球物理学会からモノグラフ（単行本）「Evolution of Hawaiian Volcanoes: recent progress in deep underwater research」として2001年出版する予定で高橋・仲・Lipman, Garciaらが編集作業を進めている。モノグラフには我々研究グループから30編の論文が出版される予定である　（補足資料−４　参照）。

申請代表者の最近5年間の研究論文リストを（補足資料−５）に掲げる。

（９）国内・国際協力体制　（＋科学研究費の裏付け）

　本研究を実施する上で第1次ハワイ航海において培われた日米研究者の協力は第2次航海を計画する上でもっとも重要な遺産として継承される。下記の協力研究者は第2次航海に直接乗船はしないが第1次に引き続き協力研究者として、我々の航海を成功するための便宜供与したり、乗船研究者のみでは実現できない岩石試料の精密な同位体分析などに協力を申し出ている。このように本研究は真の意味での国際共同研究であり、またJAMSTEC、大学など国内の多数研究機関の密接な協力の基に行われる。

　代表者の高橋は2000−2004年度の5年間文部省特別推進研究「ホットスポットの起源」を受領している。特別推進研究から海外旅費を支出し、可能な限り多数の大学院生を第2次ハワイ航海に参加させ、米国側研究者から直に学ぶチャンスを与えたい。

主な国内協力研究者

（JAMSTEC:　末広潔）（愛媛大学：趙大鵬）ハワイプルームの地震学的構造

（地質調査所：宇都浩三、佐竹健治）　Ar-Ar年代測定、シービームデータの解析　

（岡山大学固体地球研究センター：中村栄三、田中亮史）鉛、Nd,　Sr,　B、の同位体分析

（東京大学地震研究所：兼岡一郎）希ガスの同位体分析

（岡山理科大学自然研究センター：板谷徹丸）K-Ar年代測定

（京都大学地殻熱学研究施設：巽好幸）Re-Os同位体分析

主な国外協力研究者

（カーネギー研究所：Eric　Hauri）水中急冷ガラス中の揮発性分の定量分析

（ハワイ火山観測所：Don　Swanson、Dave　Sherrod）火山活動モニターによる安全確認

（ハワイ大学SOEST元副学長： Lorentz Megaard）ハワイ大学関連研究者の取り纏め

（ハワイ大学地球物理学科: Cecily Wolfe）ハワイプルームの地震学的構造

（10）添付資料一覧

補足資料−１：　日本側研究計画の補足

補足資料−２：　米国側研究計画申請書

補足資料−３：　米国側申請書付図−２，４，５，６，７，８，９，１０

補足資料−４：　AGUモノグラフ「海底から探るハワイ火山成長史」論文リスト

補足資料−５：　研究代表者業績リスト（1996−2000年）
補足試料−１　　日本側研究計画の補足

（１）ハワイ諸島海底地形図（シービーム測量計画）

ハワイ諸島は米国のもっとも重要なホットスポット火山でありながらこれまでシービームなどの精密な海底地形図はハワイ島南海域など極限られた範囲をカバーしていたに過ぎない（付図−１）。我々は第2次航海においてハワイ諸島周辺の精密海底地形図を完成する事を目指す。

付図−１　ハワイ諸島周辺の海底地形図。シービームなどの精密海底測量が行われた地域を黄色枠（ハワイ大学、米国地質調査所）、オレンジ枠（モントレー湾海洋科学研究所）、赤枠（JAMSTEC−９８，９９航海）など。2001年航海では新たに黒破線枠内のシービーム測量を行い、ハワイ島からオアフ島までの海底地形図を完成する。2002年にはオアフ島周辺海域での潜水調査の夜間およびメンテナンス日を用いて更に西のカウアイ島周辺のシービーム測量を行いたい（黒鎖線枠内）。

（２）ピストンコアーによる火山活動・巨大地滑りの編年（金松・仲・久保）

２−１．ハワイ島南方海域の火山層序　(仲，金松、USGS Lipman)

９８年の「かいれい」によるピストンコア約６mのP6（ハワイ島南東方約８０km）にはキラウエアおよびマウナロアの火山性の堆積物が含まれていた。このうちキラウエアの砕屑物をい含む部分の下部には、９８、９９年の「しんかい６５００／かいこう」で確認されたアルカリ岩（Lipman et al., in edition）に由来すると考えられる、砕屑物が含まれていた。しかし、このコアは全て正帯磁期（８０万年より若い）で時代の特定が困難であった。一方約７mのP5（約１７０km）では2.3m付近に反転があり、それより上が約８０万年より若いことが判明していたが、P6に存在する、火山性のタービダイトに相当する層が明瞭ではなかった(Naka et al., in prep.)。そこで、P5.P6間で１本以上、さらに可能ならP5からさらに南東側で１から2本のピストンコアを実施し、現在からできるかぎりふるくまでの連続した火山層序を研究する。

２−２．他地滑り帯の年代検討（金松，仲，久保）

98年に採取したオアフ島北方のコアに含まれるタービダイトの年代から，巨大海底地滑りはシールド形成が終わった頃に起こったという結果になった．この年代的制約は巨大海底地滑りのメカニズムを考える上で重要である．このタイミングはハワイ諸島の他の巨大海底地滑りにも当てはまることなのか，その一般性を検討すべく他の地滑り地帯（例えば Waianaeでの2-3本のコアを採取し）で検討を行い，その結果から巨大崩壊地滑りのメカニズムについて言及したい．また今までハワイ周辺で採取されている堆積物の年代から堆積速度が異様に速い時代があることが分かっている．これはハワイ諸島周辺で斉一的に起こった何らかのイベントの記録である可能性がある．この普遍性がハワイ諸島周辺の他海域でも当てはまるか上に述べた年代決定とともに同海域にて実証したい．

２−３．タービダイトの分布・形態（久保，金松，仲）

タービダイトの分布から過去の地形，火山島の形成史・成長史を再現する．複数本のコアサンプルと高解像度のsub-bottom profilerからタービダイトの面的な分布を決定する．また流れが地形から受ける影響（blocking, reflection,deflection etc）の痕跡を堆積物から探す．その結果を水路実験・数値計算と比較し過去の流れと地形を推定する．ハワイ周辺でこの研究をする理由は乱泥流発生原因を海底地滑りに特定でき，地形が比較的単純で流れに及ぼす影響を想定しやすい．また火山砕屑物の化学組成から発生源を特定できる可能性が高い．このような研究の結果，遠方への輸送を含めた堆積物の総量の評価　(タービダイトMass wasting の量の見積もり）が可能になりまた古地形の再現が可能になる．実施海域は比較的情報が多くタービダイト層の分布が確実にあることが分かっているオアフ島北方やハワイ島南方の前縁が適当である．

付図−２　2001年航海でピストンコア（黒丸）、ドレッジ（三角）、を行う候補地を地図上に示す。更にROV−かいこうによる潜水調査候補地点（白丸）を示す。これらの候補地域は米国側の希望する調査地域と必ずしも一致していない。調査地域についてはAGU2000年秋季大会においてハワイ航海関係者で小集会を開催し更に議論を深める予定である。

ハレアカラ火山海底リフト

（高橋・木村・下司・K.Johnson・J.Smith）

ハレアカラ火山は（マウナロア、マウナケアに続いて）地球上で3番目に大きな火山である。ハレアカラ火山の活動は今から100万年以上前に開始したと考えられ、その活動はハレアカラ火口内部のアルカリ玄武岩の噴出が有史時代にも起きている。他のハワイ楯状火山の成長史から考えて、ハレアカラ火山もその体積の大部分はソレイアイト質玄武岩が占めるものと考えられるが、陸上に露出するハレアカラは厚さ1000m近いアルカリ玄武岩、ハワイアイト、トラカイトなどの溶岩に覆われ、ソレイアイト玄武岩は海岸付近に極僅か露出するに過ぎない。

ハレアカラ火山から南東に伸びる海底リフトは延長200kmにも及びハワイ楯状火山の内最大の規模である。99年のハワイ航海で我々はハレアカラリフトのシービームによる海底地形調査を始めて行い、その先端部に山体崩壊により生じたと思われる馬蹄形の窪地を発見した（付図−３）。もしこれが最近生じたリフトの崩壊地形であるとすれば2000mに達するその崩壊崖はリフトの内部構造を露出している可能性が高い。リフトは山頂火口と並んでハワイ型火山の成長線に当たる。プルームから供給されたマグマの大部分は山頂火口の下にあるマグマ溜りから数１０ｋｍ（ハレアカラ火山の場合は最大200km）の距離を岩脈として水平移動した後で海底に噴出する。リフトの形成、内部構造、マグマ移動の仕組みはハワイ型火山を理解する上で第1級の研究課題である。

　　リフトゾーンに沿った噴出物の岩石学的特長および化学組成の水平方向変化から、リフトゾーンにおけるマグマの分化・混合過程を推定する。マグマ貫入と定置・冷却が頻繁に繰り返すリフトゾーンにみられる火山岩組成の多様化過程は、複成火山にみられる一般的な現象であり、大陸地殻の影響をうけないハワイ火山のリフトゾーンでの解析は、側火山マグマ供給系における火成岩の組成バリエーションの形成過程をより明瞭に解析できる可能性をもつ。山体崩壊によってリフトゾーンの断面が露出している地点では、リフトゾーン内に定置・固結したマグマポケットで形成された沈積岩相に相当する斑れい岩やかんらん岩試料が得られる可能性があり、噴出岩とあわせた岩石学的解析を行うことによって、リフトゾーン内での分化作用の詳細を噴出岩・深成岩の両面から解析する。

付図−３　ハレアカラリフトとハワイ島北東部の海底地形図。JAMSTEC−９９航海の結果長大なハレアカラ海底リフトの精密な地形が始めて明らかになった。（米国側申請書　Fig．３と共通）

海底溶岩の形態学研究　（海野・荒牧・J.G.Moore）

　ハワイ型盾状火山の特徴は長大なリフトゾーンの発達である。このリフトゾーンの拡大はもっぱら海面下で行われており，その実態の解明はハワイ型盾状火山の成長過程を理解する上で重要である。そのメカニズムとして有力なものは，（１）断層にそった山体の構造運動（スランプ），（２）リフトゾーン先端の伸長と溶岩原の拡大である。キラウエア，マウナロアのように火山島にまで成長した段階では，構造運動が盛んに行われ，リフトゾーン側方は崖錐堆積物によって構成されている。一方，ロイヒ海底火山は成長の初期段階にあり，崖錐堆積物が少なく，山体の構造運動の様子（断層）を観察するのに好都合である。また，ロイヒリフトゾーンおよびプナリッジ先端では若い噴出物が確認され，下方には大量の溶岩原が広がっている。この溶岩原の産状とそれを生じた火山活動の様子，山体の構成物としての役割を明らかにすることは，

リフトの拡大メカニズムの理解につながる。

（１）ハワイ型盾状火山の成長過程

潜航ターゲットA：ロイヒ火山南リフトゾーン，プナリッジ先端およびヒロリッジ北東方

これらのリフトゾーン先端に広がる溶岩原の実態を明らかにする。火山噴出物の産状の観察と試料採集を行い，溶岩形態の解析によって，噴出率や噴火継続時間についての情報を得る。また水底シート溶岩の形成過程を明らかにする。

潜航ターゲットB： ロイヒ火山南リフトゾーン，プナリッジ上の新鮮な円錐火山98-99年度の調査でロイヒ火山南リフトゾーンの段丘状地形のいくつかは，積み重なった円錐火山の一部らしいことがわかってきた。このような中心火山体の構造と，それらがリフトゾーンの全域にしめる体積（面積比）を明らかにする。また，形態，大きさ，体積，個体密度などについて陸上のリフトゾーンに見られる盾状側火山との比較研究を行うためのデータベースとする。

潜航ターゲットＣ： 南リフトゾーン崩壊壁

南リフトゾーン中央部には南に大きく崩壊した部分があり，リフトゾーンが東に屈曲している。この崩壊壁には古いリフトゾーンの名残りである岩脈群など，火山体の内部構造が露出している可能性がある。この崩壊壁上部から尾根上にかけて溶岩と火砕物の産状，岩脈の走向，傾斜，厚さなどの観察，試料採集を行う。

（２）水底溶岩ローブの形態解析

水底溶岩ローブの形態を解析することによって，溶岩ローブへの溶岩の供給率，火口での噴出率を推定する。とくに，ローブの形態が供給源からの距離，溶岩の供給率，基盤の微地形とどのような関係があるかを明らかにし，陸上の溶岩ローブとの比較研究を行う。

潜航ターゲットＡおよびＣ：（１）のＡおよびＣ

低噴出率の枕状溶岩，パホエホエ溶岩と高噴出率のシート溶岩についてのデータが得られる。

潜航ターゲットＤ：ノースアーチ

ここでは広大なシート溶岩が分布している。このような大規模溶岩の形成メカニズムの解析は，陸上の洪水玄武岩や海洋底の巨大火成岩区の火山活動がどのようにして起こったかを解明する手がかりとなろう。

事前調査実績

ロイヒ火山南リフトゾーンについては98年のかいこう，プナリッジとノースアーチについては99年のしんかい6500による潜航調査が行われている。火口列にそって噴出したと思われる新鮮な溶岩原と陥没孔が確認された。また，リフトゾーン基底部に広がる溶岩原はHolcomb et al. (1988)によるGLORIA音響映像が撮られている。(Holcomb,　et al., Geology, 16, 400-404)

Volcaniclastic apron の堆積様式と海洋島の内部構造に関する研究　　

（横瀬, J.G.Moore, J.Morgan, ）

　海洋島における大規模な斜面崩壊に関する研究は，防災上の観点から重要であるばかりでなく，海洋島の生長過程，鉱床の形成過程および炭化水素のリザバーに関する理解においても重要な情報を与えてくれる．ハワイ諸島は，海洋島の代表であり，最も研究の進んでいる地域の一つに数えられる．しかしながら，ハワイを含め海洋島の研究は，陸上および浅海の地質情報にもとづいているため，海洋島の内部構造に関する情報は乏しい．ハワイ島の成長過程ですら，1970年代のモデルを踏襲しているにすぎない．また，主にリフト帯にそった研究が多く，溶岩類の研究にその大部分が費やされている．しかしながら，実際多くの山体崩壊は，リフト帯そのものではなく，リフト帯と平行に存在する火山麓斜面で発生している．これらは，火山麓斜面における堆積様式や噴火様式が極めて重要な要因である事を示唆する．実際，海面下における大規模な崩壊は，火山活動とは無関係な大陸棚斜面においても頻発している．つまり，海面下の斜面がもつ特異性も，大規模崩壊に対して重要な役割をになっていたことが伺われる．

　本研究では，火山麓斜面の形成機構を堆積学的に考察することを目的としている．そのためには，火山麓斜面の構成物に関して，垂直および水平方向における岩相変化をできるかぎり追跡する．

堆積構造の理解を深めるためには，方位のしっかりしたサンプリングは必要不可欠である．また，粗粒堆積物の場合は，堆積構造が確認できる程度のサンプルサイズが必要である．この様にしてえられた露頭情報や岩石情報に基づいて層相分析を行い，火山麓斜面の形成機構をより具体的にモデル化する．また，この様にしてえられた火山麓の堆積様式と大陸縁辺部の堆積様式の比較から，大規模海底地すべりに関する新知見が得られるであろう．また，リフト系で構築された海洋島の形成機構と火山麓斜面の発達機構を融合することで，より具体的な海洋島発展モデルが導き出せることが期待される．

ロイヒ海底火山岩中の窒素同位体地球化学　　（松本拓也）

本研究の目的は「マントル窒素の起源およびその進化をハワイ海底火山岩中の窒素の存在量・同位体組成を基に明らかにする。」ことにある。周知の通りハワイのホットスポットは非常に始源的なヘリウム・ネオン同位体成分を持っており、数十年前から現在に至まで希ガス研究の中心の一つである。一方、窒素の同位体地球化学は希ガス程理解が進んでいるとはいえない。しかし比較的データの揃っているMORBの窒素同位体組成は、大気よりも数‰軽い窒素を持つことがわかっており、ある種の始源隕石物質中の窒素との関連等が議論されるなど、トレーサーとしての潜在性は高いと考える。しかしながら現在までに、全マントル的な窒素の分布を知る上で欠かすことのできないプルーム関連の試料中の窒素データは非常に乏しい状況である。従って現在の地球大気の主要成分である窒素がマントル内部でどのように分布し、地球史を通じてどのような進化を経てきたのかは未だ明らかでは無い。マントルの窒素に関する我々の理解を進めるためにはハワイホットスポットの海底火山岩中の窒素研究は是非執り行うべき課題だと考える。今までの希ガス研究の成果からも明らかなようにハワイ海底火山岩は大陸の存在による汚染なしにマントル深部の情報を与える理想的な試料である。そのためハワイに関する希ガス研究は近年の深海調査研究課題としても執り行われているが、同じく揮発成分である窒素の同位体組成に関する系統的な研究は未だ行われていない。中でもロイヒの急冷ガラスは、マントル中に太陽組成のネオンの存在が確認された最初の試料であり、非常に興味深い。自然界で様々な価数をとり様々な化学種が存在する窒素が、果たして希ガスと同じように地球の始源成分の情報を与えるかどうかは未知数ではある。現在までに報告されているプルーム関連試料の窒素同位体組成もかなりの不均一性を示しており、一概にプルーム=始源成分といった解釈が許されるのかも不明である。しかしその同位体組成は地球の物質循環にマントルプルームが果たす役割に関する情報を持っているはずであり、非常に興味深い課題だと考える。

補足試料−２　　米国側研究計画

　　Hawaiian Magmatism, Volcanism and Landslides

2001-2003 (2nd Stage ) JAMSTEC Hawaii Program -- USA Proposal

By scientists from the U.S. Geological Survey, Monterey Bay Aquarium Research Institute, and the University of Hawaii (Oct.25,2000)

OVERVIEW

A three-year program of ROV and submersible dives in Hawaii is proposed to continue study hotspot magmatism and the relationships between volcanism and large-scale landsliding. Oceanic islands generate enormous landslides that represent a major tsunami hazard for the Pacific Basin. The proposed dives will study deep parts of the constructional volcanic rift zones that are the primary structural features of the volcanoes, the giant slide blocks and other debris from the submarine landslides, and the internal structure of the volcanoes that are dissected by them. This project is a continuation of the highly successful 1998-99 joint Japanese- U.S. Hawaii program, which utilized the ROV KAIKO and SHINKAI 6500 on landslide and volcanologic research of importance to both countries. We also propose additional dredging, piston coring, and making a detailed bathymetric and sidescan sonar surveys of the seafloor offshore of the Hawaiian Islands. Much of this region has never been surveyed with multibeam system, and we expect to discover many new features about ocean-island volcanoes from such surveys. Results of the 1998-99 cruises were reported in a special session on “Magmatic and Tectonic Evolution of Hawaiian and Other Hot-Spot Volcanoes” (E. Takahashi, convener), at the AGU Western Pacific Geophysics Meeting in Tokyo, June 2000, and an AGU Monograph based on the special session at WPGM is currently in preparation.

Introduction

Hawaiian volcanoes are classic examples of mantle plume volcanism. These volcanoes host frequent large eruptions, generate major earthquakes, and are prone to flank failure causing enormous landslides and gigantic tsunami. Geologic hazards associated with these volcanoes are not just restricted to those living in the Hawaiian Islands but also extend to the populated coastal regions of the Pacific. Hawaiian volcanoes are the best known volcanoes in the world, but most previous studies have focused on the easily accessible subaerial parts of these volcanoes and largely ignored their submarine flanks. Thus, knowledge of these immense volcanoes is based largely on the tips of these volcanic 'icebergs'. More is known about the backside of the Moon than for many areas surrounding this classic suite of volcanoes. The problem is that the vast bulk of Hawaiian volcanoes are under the ocean, and exploration of these areas has been difficult, costly, and very limited until recently, leaving many societally critical issues unresolved (Table 1).

Many previous studies have focused on subaerial parts of Hawaiian volcanoes, but the deep-water flanks of the edifices (maximum depths, 5,700 m) remain poorly known because of difficult access. In 1998, a collaborative Japan-USA program was initiated to explore the evolution of Hawaiian volcanoes including their growth and degradation, making use of the deep-sea research capabilities of the Japan Marine Science and Technology Center (JAMSTEC). During a four week cruise in 1998, the ROV Kaiko (Remotely Operated Vehicle), supported by its mother ship RV Kairei, made 10 dives at depths to 5,200 m for direct sampling and video observations, supplemented by dredging, piston cores, and SeaBeam bathymetric surveys. During a seven week cruise in August-September 1999, the Shinkai 6500 submersible made 29 dives from the RV Yokosuka for sampling and direct observations as deep as 5,560 m, and further SeaBeam surveys were obtained. Participating scientists are largely from JAMSTEC, the University of Hawaii, several universities in Japan, the Monterey Bay Aquarium Research Institute, the U.S. Geological Survey, and the Geological Survey of Japan.

Our knowledge of the submarine region around Hawaiian volcanoes has changed dramatically in the last few years as a result of these JAMSTEC expeditions to Hawaiian waters. These expeditions have generated improved images of sea-floor bathymetry, utilized ROV and submersible vehicles to collect a large suite of precisely located samples for petrologic study, and acquired detailed photo and video images for many critical areas. Such materials, supplemented by petrologic, geochronological, and other laboratory studies, provide the basis for investigating a wide variety of geophysical phenomena. These including the sources for and extent of plume magmatism, processes associated with active and ancient landslides on oceanic island volcanoes, seismic structure and tectonic processes on active volcanoes, nature of rift zone and other submarine volcanism, growth of oceanic island volcanoes, and hydrothermal processes. In concert with the field and laboratory studies, recent theoretical investigations have explored the causes and consequences of Hawaiian plume magmatism and the landslides associated with these unstable volcanoes.

The Hawaiian Islands are products of a mantle plume, which is thought to have originated at a boundary layer deep within the mantle, perhaps at the 660 km discontinuity or at the core- mantle boundary. Thus, the volcanism associated with plumes provides a window into the deeper mantle and the possibility of access to the geochemical record of plate recycling and mantle). The Hawaiian plume is the Earth's hottest, most productive and most thoroughly studied mantle plume. Nonetheless, a lively debate continues on such fundamental questions as the nature and scale of heterogeneities within the plume, magma generation processes and the composition and temperature of the magmas that supply Hawaiian. A controversy continues over whether the magmas that supply Hawaiian volcanoes are basaltic, similar to those commonly erupted (8-12 wt% MgO), or whether they are picritic (MgO >15 wt %). Resolution of these questions is critical if we are to understand the nature and dynamics of the Hawaiian plume and the processes of magma generation within it. We must determine the long-term compositional variations of lavas from the tholeiitic shield-building stage of Hawaiian volcanoes to resolve these questions. The subaerial portions of oceanic volcanoes provide us with a very small fraction of a volcano's overall history (5-10%), biased in favor of its most recent development, and largely ignores the vast bulk of the volcano. Submarine landslides provide special opportunities opportunity to sample the deep interior of Hawaiian volcanoes, and much new data, especially on extremely alkalic early volcanism from ancestral Kilauea volcano, has already been obtained from the initial JAMSTEC program. Especially important for further petrologic studies, these initial results for the presently most active volcano (Kilauea) provide confirmation of appropriate sampling strategies that can now be applied elsewhere along the Hawaii-Emperor Ridge.

One objective of this JAMSTEC cooperative program is to explore the evolution of oceanic islands including their growth and degradation. Giant landslides are now widely recognized along the flanks of many oceanic volcanoes, such as Hawaii, Marquesas, La Reunion, Galapagos, and Canary Islands. The landslides form during the growth of the volcano as it is centered over the hot spot and after it drifts off. The abundance of landslides demonstrates that mass-wasting processes play an important role in the construction and evolution of oceanic-island volcanoes. Not only do such processes modify the surfaces and slopes of the islands, they also are closely linked with major geologic hazards, including earthquakes associated with slope failure, large-scale submergence or emergence of coastlines, and massive tsunamis which can destroy life and property. Due to the unpredictable and sporadic nature of such massive landslides, the processes and timing associated with these events remain poorly understood. Yet the significance of landslide features in the evolution of volcanic islands, and their extraordinary destructive potential, make it imperative that we understand their history and behavior, and assess their impact on human development of volcanic islands and adjacent coastal areas.

A focus on landslide deposits and the scars they produce provides a window into Hawaiian volcanoes. This will provide an opportunity to reconstruct the deformational sequence of Hawaiian slides and to better constrain static and kinematic models for landslide initiation and movement, and to provide data for the development of models for destructive landslide-generated tsunamis. High-resolution seafloor mapping of the U.S. Exclusive Economic Zone (EEZ) using the GLORIA side-scan sonar system has revealed the presence of more than 68 giant landslides along the flanks of the Hawaiian volcanoes. Using the slides from Kilauea and Koolau as models, we can gain greater insight into the landslide processes and better assess the potential hazards they present to human life and property in Hawaii and around the Pacific Rim from the associated earthquakes and seismic sea waves. Through collaborative Japanese-US geophysical surveys and deep submersible dives, the structure, morphology, and lithology of the submarine flanks of Hawaiian volcanoes will be revealed. These studies will offer critical insights into the evolution of the oceanic island volcanoes.

Study methods and equipment needs

Methods will be similar to those used successfully during the 1998 and 1999 Hawaiian cruises and follow-up laboratory analysis and interpretation. Shipboard activities will include: continued high-resolution SeaBeam bathymetry and sidescan sonar surveys of additional unmapped seafloor adjacent to the Hawaiian Ridge, imaging of seafloor features by ROV-mounted video cameras and by direct observation during manned submersible dives, collection of seafloor rock samples directly from outcrops during ROV and submersible dives, additional rock sampling by dredging, and sampling of turbidite-sand stratigraphic sequences by gravity piston coring on the deep seafloor out beyond the basal slopes of the Hawaiian volcanoes.

This proposal has been prepared in anticipation that the next phase of work will involve: (1) 4-5 weeks of RV Kairei time in Hawaiian waters in 2001), including shipboard surveys, dredge and piston-core sampling, and ROV Kaiko dives; (2) a similar-length cruise in 2002 or 2003 utilizing the Shinkai 6500, and (3) a possible third cruise with the the Kairei and Kaiko. Most of following discussion focuses on priority research opportunities for the first of these proposed cruises (in 2001).

SEABEAM MULTIBEAM MAPPING PROJECT

New SeaBeam surveys during the 1998-99 cruises (Fig. 1) provided the needed framework for planning and interpreting dive results and identifying possible sites for future cooperative studies. The SeaBeam 2112 sonar system was operated most nights and submersible-maintenance days at survey speeds of 10-15 knots, depending on sea conditions. The Nuuanu and Wailau landslides now have nearly complete bathymetric and sidescan sonar coverage, along with part of the Koolau platform. Part of the North Arch field was also surveyed for the first time with a multibeam system. New bathymetric coverage southeast of the Hilina slump, identified numerous rounded mounds 1.5-5 km in diameter and with 150-300 m relief. None were examined by submersible or ROV sampling, but the round shapes of these features resemble small volcanic shields, such as on the North Arch Field, more closely than angular landslide blocks of the Nuuanu and Wailau slides.

During the 2001 cruise, additional extensive areas of the Hawaiian Ridge will be surveyed during ROV and submersible service days. Though the entire Hawaiian EEZ has been mapped with the low resolution GLORIA sidescan sonar system, only limited areas have been ensonified with medium resolution systems such as SeaBeam. Existing areas of multibeam coverage include the areas northeast and north of Oahu and Molokai, respectively, and the southern flanks of Hawaii Island and the Cretaceous-age seamounts west of the Big Island (154 to 158 degrees W, and 18.5 to 20 degrees N). Coverage was added to existing surveys in these areas during 1998-99 Hawaii program.

Numerous offshore features, including more landslides, submarine rift zones, seamounts, and drowned reefs, would be imaged in much greater detail that the existing single-beam random trackline data can provide. We have already found that the R/V KAIREI and YOKOSUKA SeaBeam 2112 systems also provide better sidescan data, with more solid navigational control, than the GLORIA data. Additional dive targets will certainly be found that can be studied in following years if collaborations such as these continue.

Proposed participants for SeaBeam Multibeam program:

J.R. Smith, K.Satake, E. Takahashi

OCEAN-FLOOR TURBIDITE STRATIGRAPHY, SAMPLED BY PISTON CORES

One of the most important outcomes of the first-stage JAMSTEC cruise has been the identification of turbidite basaltic glass sands, and their magnetic stratigraphy, obtained from gravity piston cores.

Four piston cores ~7 m long, containing many turbidite layers were recovered from the vicinity of the Nuuanu and Wailau landslides during the 1998 Kairei cruise (Fig. 4). The cores contain reversals and excursions of the geomagnetic field, as revealed by marked changes in the inclination, declination, and intensity profiles, based on sampling at 5-cm depth intervals. Preliminary ages assigned to the polarity events yield estimates for the bottoms of cores PC1 (>1.0 Ma), PC2 (~1.8 Ma), and PC3 (>1.0 Ma), in agreement with paleontological age estimates from microfossils. Sedimentation rates, based on these ages, change from relatively fast (7-46 mm/ky) prior to the Brunhes-Matuyama reversal (~0.8 Ma) to slower (1-2 mm/ky) afterward. The distribution and compositions of the turbidite sands also provide constraints on ages of the landslides.. Major sliding probably occurred near the peak of shield building: 1.75-0.75 Ma at East Molokai volcano (Wailau slide), 2.6-1.8 Ma at Koolau volcano (Nuuanu slide).

Two sediment piston cores (~7 m long) were recovered 100-200 km seaward of the Hilina slump (Fig. 1). Core PC6, located about 100 km offshore, is entirely of normal polarity, suggesting that its 10 glass-sand turbidite layers (Fig. 4) are all <0.8 Ma. In contrast, microfossils from 2.5-3.7 m depth in core P5, located about 200 km offshore, suggest a Middle to Early Pleistocene age, and a polarity boundary at 2 m depth may represent the top of the Olduvai subchron (1.8-1.9 Ma).

Scientific participants: T. Kanamatsu will continue this project, with widely spaced additional piston cores in 2001, in order to study timing of the landslides and their relation to the volcano growth history more broadly in time and space. Most American participants will collaborate in most aspects of this study, which has important bearing on several of the other research targets. Determination of glass compositions for the turbidite sands by electron microprobe will provide especially important information on source volcanoes for the turbidites, and in turn on growth histories of the source volcanoes

PROPOSED TARGETS FOR BOTTOM SAMPLING AND IMAGING

Many highly important research opportunities, including some newly defined by the 1998-1999 JAMSTEC cruises, remain offshore of Hawaii, Oahu, and the other young islands (Table 1). Results from the work to date have defined additional study areas and dive targets, that would further our understanding of slope-failure and landslide processes, and also could sample the early magmatic history of ocean-island hotspots. Comparable observations on older parts of the Hawaii-Emperor chain, which will be complicated by thicker sedimentation and weathering alteration, would be most productive if designed within a strong framework of studies on the young islands.

We endorse the concept of a three-year program, involving two cruises Table 2). The first cruise would build on successful dives from prior years to complete studies on the Nu’uanu and Hilina slide areas, and do exploratory dives for several new research targets. In later years, emphasis would shift from landslide and slump studies to deep parts of rift zones (assuming initial dives are productive), both offshore of the Hawaiian Islands and along the Hawaii-Emperor Ridge.

DIVE TARGETS IN THE HAWAIIAN ISLANDS

For the younger islands, particularly attractive targets are (A) Slump structures on island flanks, (B) Distal portions of the large debris avalanches, (C) Basal areas of submarine rift zones, and (D) Deep-water alkalic lava fields (Table 2).

(A) Slump structures on island flanks

Flanks of some volcanoes have undergone slumping under influence of gravitational spreading and dike injection along rift zones. Such sites provide evidence of the processes that may lead to more catastrophic landslide failure and also have the potential to provide samples of the early petrologic evolution early in the growth of the volcanoes.

(1) The Hilina slump area, south flank of Hawaii Island, is currently the most actively deforming flank of any oceanic island worldwide. A key part of the first JAMSTEC Hawaii project has been the submersible investigation of the Hilina slump, an active landslide at least 40 km wide on the southeast flank of the currently active Kilauea volcano that is presently moving seaward at rates up to 10 cm/yr, which has been mapped by recent detailed multibeam bathymetric. In 1868 and 1975 this region moved abruptly several to tens of meters during major earthquakes (M7.9 and M7.2, respectively) with attendant destructive tsunamis. The tsunami generated in both 1868 and 1975 resulted in extensive damage and fatalities on Hawaii, and the 1975 tsunami produced minor damage in California. The possibility exists that future detachments of this type, or far more extensive and catastrophic debris avalanches, will occur in the future. The entire south flank of the island shows evidence for slumping and collapse. This proto-slump has now broken into two slumps that are buttressed in the middle by Loihi Seamount. These slumps are the Punaluu slump west of Loihi and the Hilina slump east of Loihi. The presence of debris avalanche deposits along adjacent island flanks indicates the potential for catastrophic failure of such. The continuous creep and incremental movement associated with the large earthquakes are apparently driven by both magmatic processes within the active volcanoes and gravity. However, the mechanisms by which these slowly creeping slumps fail catastrophically are unknown, as are the precursors to such activity.

Submersible observations and samples from the 1998-1999 cruises show that the lower south flank of Hawaii, offshore from Kilauea volcano and the active Hilina slump system, consists entirely of compositionally diverse volcaniclastic rocks; pillow lavas are confined to shallow slopes. Submarine-erupted basalt clasts have strongly variable alkalic and transitional basalt compositions (to 41% SiO₂, 10.8% alkalis), contrasting with present-day Kilauea tholeiites. The volcaniclastic rocks provide a unique record of ancestral alkalic growth of an archetypal hotspot volcano, including transition to its tholeiitic shield stage, and associated slope-failure events. The geometry and diverse constituents of the slump terrain require initial slumping during submarine alkalic volcanism. A conspicuous NW-trending scarp, aligned with Papa’u Seamount, bounds the slump terrain on its SW side but has no subaerial expression. In contrast, a parallel scarp, which bounds the Punalu’u slump from Mauna Loa, merges on land with a conspicuous fault zone that moved 2-3 m laterally during the 1868 earthquake. This contrasting subaerial expression, along with the dominance of alkalic clasts in the lower scarp, suggests that the distal slump and its western bounding scarp initially developed prior to shield growth at Kilauea and inception of the active Hilina faults; size and volume of the tholeiitic shield are smaller than previously inferred. Such early structures may have localized younger Hilina slumping. Many questions remain, but perhaps the active Hilina slump structures on Kilauea’s south flank are in an early growth stage, thus posing greater future potential for larger-scale landsliding and devastating tsunamis.

The results of the 1998-99 dives onto the deformed flanks and adjacent seafloor of Hawaii provide critical motivation to return to this area with the ROV KAIKO in 2001. Many of the original questions relating to the mechanics and history of deformation along the flanks are still unanswered, and in addition, new ones have been raised. For example, what is the volcanic flank really composed of? The presence of indurated volcanic sandstones throughout the deep portions of the south flank of Kilauea suggest that the distal slopes of Hawaiian volcanoes are largely composed of sediment. This possibility has implications for the mechanical strength and long-term stability of the deforming flanks. However, the occurrence of primary volcanic rocks upslope of the sedimentary strata on Kilauea suggests that the transition from volcanic to clastic environment may be relatively complicated. In order to interpret the evolution of the islands, and the kinematics of the deforming flanks, it is important to know the location and nature of this transition.

Several additional dives (and probably some dredging) in the Hilina area are needed to test the interpretations of early hotspot magmatic evolution and concurrent slope failures developed so far. Priority dive targets include the lower flank of Papa’u Seamount (dive lost in 1999), the large elongate ridges outboard of the lower Hilina scarp, and dives to refine boundaries of the compositional transition from submarine-erupted alkalic to tholeiitic basalt (Table 3; Fig. 2). Some complementary problems, on upper slopes of the Hilina slump, may be examined during the upcoming MBARI ROV in the spring of 2001. Additional piston cores south of Hawaii Island could test the important stratigraphic, paleomagnetic, and geochronologic correlations among basalt-glass turbidites and fill gaps between cores P5 and P6 collected in 1998.

These marine studies on the south flank slump terrain will closely interface with the abundant existing data and several in-progress studies on Hawaiian volcanoes and with ongoing slope-stability studies in Hawaii and elsewhere. The Hawaiian Volcano Observatory of the USGS monitors earthquake activity in Hawaii, including the active Hilina slump area. A joint Japanese-US research program is underway to study the seismicity of Kilauea Volcano. The University of Hawaii is completing interpretation of a seismic reflection survey of the Hilina slump that should allow first-order predictions about the types of materials that may outcrop at the seafloor along the flanks and in the slide blocks; these data can be used to guide future, particularly across the oversteepened toe seaward of the midslope bench and the incised flank above the bench. Results of the new KAIKO dives will provide important constraints on the seismic interpretations and models.

Successful interpretation of these stratigraphic, structural, and petrologic features, for which questions still outnumber answers, has critical implications for understanding the primary depositional growth of the submarine flanks of oceanic volcanic islands, and also for structural evolution of the Hilina slump system and development of large slumps elsewhere in the Hawaiian chain and on other oceanic islands.

Suggested dive sites: The attached map (Fig. 2) and Table 3 show six recommended dive sites, based on discussion by T. Ui, T. Sisson, and P. Lipman at the WPGM meeting in June 2000. Many of these sites also lie along the Univ. Hawaii seismic profiles obtained in 1998 by J. Morgan and G. Moore. The dive tracks with "K9x"and” S50x” numbers are locations of the completed 1998 KAIKO and 1999 SHINKAI 6500 traverses.

Participants: Lipman, Sisson, Smith, Morgan.

(2) Laupahoehoe slide. Dives on the deep ridges and elongate basins of the Laupahoehoe slide area, newly identified by the JAMSTEC SeaBeam surveys, would provide an especially interesting counterpart to Hilina, as well as potential samples of ancestral Kohala volcano.

The Pololu debris avalanche, previously interpreted to have its headwall in the large subaerial valleys on NE Kohala, appears to override an older slide located farther NE, newly named the Laupahoehoe slump. Characterized by an ENE failure direction and hummocky terrain containing blocks and/or cones 2-5 km in diameter of 50-200 m relief, the Pololu slide impinges upon structures at ~3000 m water depth (wd) that are proposed to constitute the Laupahoehoe slump or "accreted bench". These structures are NE oriented scarp-and-bench topographic features analogous to the Hilina slump on the mobile SE flank of Kilauea, and may have formed similarly by mechanisms of volcano spreading. Six enclosed basins (100-400 m deep, 4-10 km long, 1-5 km wide) lie at 3000-5000 m wd, fronted by 50-200 m high ridges on their seaward sides. The basins may result from local rotational slumps or from uplift above discontinuous thrust faults in the bench. Prominent slope breaks at ~1100 and ~400 m wd mark the end of tholeiitic shield building at Kohala and Mauna Kea respectively; the Laupahoehoe slump is overlain by both shield margins and must have been derived from an elongate Kohala edifice. The smoothly dipping slope of Mauna Kea at 1200-3000 m wd appears to have grown over the older slide terrane; it resembles the upper Kilauea flank of the Hilina slump, which is known to be well-sedimented and composed of primary volcanics. The two-slide complex is 90 km wide and abuts the distal Haleakala east rift zone. An outer debris apron continues 95 km from the base of the island.

Suggested dive sites: The attached map (Fig. 3) show three recommended dive sites targeting steep slopes along the lower benches, based mainly on the available JAMSTEC bathymetry and analogies with successful sampling strategies along the lower south flank of Hawaii Island.

Participants: Smith, Lipman, Sisson, USGS postdoc.

(3) Loihi Seamount

Loihi Seamount, rising to within 955 m of sea level from sea floor depths of 4800 m (Fig. 2), is the youngest Hawaiian volcano. It is seismically active, consists of alkalic basalts overlain by tholeiites that provide insights into the early growth of Hawaiian volcanoes, and was the site of volcanic activity as recently as 1996. Before 1998 only a few deep dives using the Russian Mir submersible had been made on Loihi, although numerous dives to depths of 2000 m with the Alvin and Pisces V had mapped and sampled the summit area. Four Kaiko dives and 11 Shinkai 6500 dives in 1998-99 sampled and mapped the young lava morphology along the south rift zone from 2000 to 4800 m.

These dives documented largely glassy pillow and tube lavas extruded from rift-axis fissures and axial cones. The deep lavas at Loihi all appear to be young, although a dusting of sediment covered the axial flows and thicker sediments mantle the basal flows. Observed vent and flow features include pillow ramparts, subsidence pits, complex lava channels, and marginal levees. Olivine-rich basalt (picrite) is the dominant rock type sampled along deep segments of the south rift zone.

Upper parts of the steep eastern and western flanks of the volcano, resulting from landslide failures, contain a record of the progression from alkalic to tholeiitic eruptions. By analogy with the recent JAMSTEC results for the deep south flank of Kilauea, lower parts of the Loihi slopes, which have not previously been studied, may contain alkalic rocks more diverse than those sampled along the rift zones or higher on the flanks. Several dives on the deep flanks are proposed for 2001.

Sampling of in-place flows on the steepest part of the west flank for comparison with the fractionation model based on the geochemistry of the SE-rift zone being developed by Malahoff. This area is also interesting because of the contact between the apparent slide-scar and the unusually broad rift-like feature of the west flank.

In addition there is continued interest in sampling any hot-water systems discovered on the deep west flank, although this must be viewed as exploratory as no sites are identified.

Participants: Takahashi, Thornber, Malahoff, Midson

(4) The Kaena slide, northwest of Oahu (Moore et al., 1989), also contains tilted fault blocks whose long axes are normal to the direction of movement. As an older and more heavily sedimented feature than the block slump areas offshore of Hawaii Island, dives on this feature would test our ability to use sampling and analytical techniques on small basaltic glass samples that have worked well in the Hilina area. This would become a priority dive site for a second-phase cruise, if successful results were obtained during the Laupahoehoe dives in 2001.

(B) Distal portions of the large debris avalanches

Giant landslides are now widely recognized along the flanks of many oceanic volcanoes, such as Hawaii, Marquesas, La Reunion, Galapagos, and Canary Islands. The abundance of landslides demonstrates that mass-wasting processes play important roles in the morphologic evolution of oceanic volcanoes. Such processes drastically modify the surfaces and slopes of the islands, and they are closely linked with major geologic hazards, including earthquakes associated with slope failure, large-scale submergence or emergence of coastlines, and massive tsunamis which can destroy life and property. Processes and timing associated with these massive slides remain poorly understood due to their unpredictable and sporadic nature.

The north flanks of Oahu and Molokai are the source areas of two of the largest landslides on Earth, the Nuuanu and Wailau debris avalanches. The offshore expression of these slides is an extensive, rubbly field of debris extending across the Hawaiian Deep and Arch. Numerous large, irregular hills protrude up to 1.8 km above the abyssal sediments, and are thought to be fragments of the volcanic edifice carried downslope during flank collapse. Little bathymetric, side-scan sonar, or seismic data were available for this area prior to the JAMSTEC 1998-99 cruises, and little was known about the structure, morphology, or source areas of the slide debris. The magnitude of the slides suggests that the Koolau and East Molokai Volcanoes has been deeply incised, exposing an extensive stratigraphic sections through these volcanoes. These landslides provide two outstanding research opportunities for Japanese and US scientists: 1) to study the mechanics of giant landslide formation and 2) to determine the early magmatic history of the source volcanoes. Although some of the world's largest landslides have formed on the flanks of Hawaiian volcanoes, the mechanics of their formation are poorly understood

Nu’uanu-Wailau slide complex. Despite a substantial dive program in 1998-99 on the enormous Nu’uanu-Wailau slide complex, distal blocks remain unstudied, an important remaining problem. Structures and compositions of the distal blocks, some as much as 200 km from their sources on Oahu and Molokai, would provide evidence of landslide mechanisms and might contain petrologic records of early growth and magmatic evolution in the source volcanoes. In addition, collections from distal and mid-course blocks in these landslides can address the question of which parts of the landslides originated from which host volcano on Oahu and Molokai. A combination of manganese-oxide thickness and geochemistry may be utilized to make this separation, which is essential for development of models of tsunami generation.

The offshore expression of the slides is a rubbly field of debris (Fig. 4), extending ~230 km from the island across the Hawaiian Deep and onto the Hawaiian Arch (Fig. 1). Limited bathymetric or side-scan sonar data were available for this area prior to the JAMSTEC 1998 Kairei cruise, and little was known about the structure, morphology, or source areas of the slide debris. About 50 large blocks (5-40 km across, standing as much as 1.5 km above the sea floor) are thought to be fragments of the volcanic edifice carried downslope during collapse of the flank of the island. The size of blocks decreases with distance from the islands. The magnitude of the slide suggests that the Koolau volcano has been deeply incised, exposing an extensive stratigraphic section of the volcano. Adjacent to the Nuuanu slide is another large debris avalanche deposit, the Wailau slide from the younger East Molokai volcano. The Nuuanu and Wailau landslides provide outstanding opportunities: 1) to study the mechanics of giant landslide formation and 2) to determine the early magmatic and structural history of Koolau and Molokai volcanoes. Koolau Volcano is of special interest as Hawaii's most geochemically distinct volcano (e.g., high SiO₂).

Dredging and piston coring from the R/V Kairei supplemented two ROV dives in 1998. The ROV dives provided the first images of the structure of the landslide blocks and collected in-situ rock samples. The slide blocks are composed of gently-dipping (<15^o) fragmental volcanic debris, unconformably overlain by a few meters of slope mantling debris (dipping >30^o). A dive on the flanks of Koolau volcano sampled pillow lavas with the same distinct mineralogy and composition as subaerial Koolau lavas. Nine dives in 1999 with the Shinkai 6500 focused on the structure and chemistry of rocks from the Nuuanu and Wailau landslides to evaluate the mechanics and timing of sliding, the distinctive chemistry of Koolau lavas, and implications for deep-mantle recycling of oceanic crust. Landslide blocks are dominated by coarse breccia that is heterolithologic, crudely stratified, and contains both subaerial- and submarine-derived basalt clasts. Minor interlayered pillow lavas occur near the crests of several Wailau blocks, indicating derivation from near-shore sources. Thicker manganese crusts on Nuuanu than on Wailau rocks confirm a younger age for the Wailau slide.

One of the general themes to be addressed in the study of the giant Oahu-Molokai landslides is the processes involved in their failure, movement, and emplacement. To gain insight on this problem, we first need to know the extent of each landslide and the nature of the boundary between them, as well as the sequence and age of their movement. Next, an understanding of the place of origin of the landslide blocks is needed, including which segments were originally part of the subaerial versus submarine portions of the volcano and whether portions of the rift zone or summit reservoir systems are exposed on the blocks. Finally, a knowledge of the structures developed within the blocks (such as whether fractures common in subaerial landslide blocks are present) is needed. All of these features need be contrasted between landslide blocks that have only moved a short distance versus those that were transported more than 100 km at the distal end of the landslides.

Geometric relations and internal fracturing of Nuuanu and Wailau slide deposits support a debris-avalanche origin, even though some features are different from subaerial debris avalanches (Ui et al., 2000). For example, the topographic contrast between seamounts and surrounding seafloor is greater, and sizes of individual seamount blocks and scale of the landslide are an order of magnitude larger, although the degree of fracturing is less than in subaerial debris avalanches. Volcanic-sand/silt layers sampled by piston coring are turbidites produced by fragmentation during sliding. Ocean-floor sediments and interstitial water may contribute to decreased friction in the sliding mass.

Proposed 2001 dive sites: Potential dive sites are indicated on the attached map (Fig. 4). To address the landslide problems outlined above, the dives need be distributed along the length of both landslides so that both proximal and distal lithologies can be compared for each of the landslides. To date, all dives have been on proximal or mid-distance block. We urge dives on relatively large distal blocks as a high priority for future work. The dives should be conducted on the steepest slopes of the blocks so that sediment cover does not obscure bedrock.

Proposed USA participants for Nu’uanu-Wailau program (in collaboration with Takahashi, Yokose, &?):

James Moore (USGS): marine geology,

John Smith (Univ. of Hawaii): landslide processes, bathymetric and side-scan surveying

Julia Morgan (Rice Univ.): structural analysis, landslide mechanics, seismic interpretation and correlation

(2) South Kona slide complex. Parallel studies on the west (Mauna Loa side) of Hawaii Island, where multiple young landslide features (especially South Kona and Alika) provide counterparts to compare and contrast with Hilina and Nu’uanu results. The large distal blocks of the South Kona landslide have never been systematically sampled. The deeper parts of several of them provide exceptional places to search for lavas that may extend our view of Mauna Loa magmatism back in time. This would provide new views on the evolution of the Hawaiian plume. Evidence may also be obtained on the relative proportions of pillow lava vs hyaloclastite involved in construction of the submarine flanks of large oceanic-sisland volcanoes, a current topic of controversy.

Participants: J.G. Moore, Lipman, and USGS postdoc.

(3) Wai`anae slide complex (Figure 6). G. Moore of Univ. of Hawaii proposed this study as a comparison to the work that has been done on the Nu`uanu, Wailau and Hilina slides. As with the other areas, the scientific rationale is to understand the mechanics/timing of the large landslides. Nu`uanu and Wailau are large debris avalanches on an old volcano; Hilina is a slump on a young volcano. Wai`anae seems to be a slump on an older volcano, so it will provide a useful comparison to the other 3 study areas.

Participants: G. Moore, Smith.

(C) Basal areas of submarine rift zones

Dives near the base of the deep toes of rift zones: Puna Ridge (Kilauea), Maui E Rift (Haleakala--HERZ), SW Rift of Mauna Loa, Hilo Ridge (Kohala?), would provide new information on poorly known structures and volcanic processes, as well as samples that could bear on early volcanic evolution. Lower parts of such rift zones may record relatively early eruptive stages in the evolution of Hawaiian volcanoes, as suggested particularly by the SW rift of Mauna Loa, for which the lower subaerial segment and shallow submarine segments have been inactive for at least the last several tens of thousand years. The dominant rocks of lower portions of the Hilo Ridge are characterized by reversed magnetic polarity, as newly recognized by JAMSTEC results in 1999, and are accordingly older than the Bruhnes-Matayama boundary.

Bifurcated rift structures similar to the HERZ occur on many of the older volcanoes in the Hawaiian and Emperor Seamount chains. Dives on the deep distal parts of the young rift zones around Hawaii and Maui should provide an interpretive framework for proposed future dives on the much older deep rift zones of the Emperor Seamounts, where sedimentation and time-dependent alteration are likely to complicate interpretations.

Haleakala East Rift Zone. The distal east rift zone (HERZ) is especially attractive for study because no direct observations exist and this area is characterized by a poorly understood bifurcated geometry (Fig. 3), which may either be a primary constructional feature, or due to landslide failure of the central portion. Twice as long (140 km) as Puna Ridge of Kilauea, the tip of the HERZ displays a curious steep-sided arcuate rift tip, at 3000-5000 m wd with a diameter of 22 km. It displays the classic amphitheater scarp of a landslide, although the volume of the debris lobe embraced by the branching arms appears rather small. Numerous flat-topped pancake cones (1-2.5 km dia; 50-150 m relief) are distributed along the broad 8-13 km wide crest of the lower rift zone. Similar cones, though fewer in number, are also present on the Hilo Ridge, which is thought to be the continuation of either a Kohala or Mauna Kea rift zone.

Variations in gross surface morphology is observed on the Puna Ridge at upper, middle, and lower sections. Do similar variations occur on the Haleakala Ridge and if so what do they tell us about flow emplacement mechanisms as a function of other parameters such as volatile content, degassing, viscosity, magma temperature?

Suggested dive sites in 2001: 3-4, along bifurcation of deep distal toe and steep intervening headwall (Fig. 3). Participants: Takahashi, Johnson, Sherrod, Clague, Smith.

Puna Ridge studies: The Puna Ridge, the 75-km-long submarine continuation of Kilauea’s east rift zone, has been the site of several recent studies, in part because it may be analogous to mid-ocean ridges. One Shinkai 6500 dive examined and collected rocks from two terraces with contrasting surface morphology based on sidescan sonar images on the northeastern tip of the Puna Ridge at depths of 4200-3950 m (Fig. 1). A second dive sampled a deep alkalic lava flow, which had been previously identified from high backscatter on sonar surveys, near the south base of the rift zone at 5565 m depth. 2-3 dives along the distal toe of the ridge are desirable to search for evidence of early eruptive history from Kilauea and to evaluate the proportion of young alkalic lava that has reached distal parts of Kilauea volcano. More detailed sampling of the deep portions of the Ridge will also improve our understanding of deviations from the typical liquid line of descent of Puna Ridge magmas that has been observed in lavas from previous deeper sites. Participants: Takahashi, Johnson, Thornber, USGS postdoc.

Hilo Ridge: The Hilo Ridge, long considered to be the major exposed rift zone of Mauna Kea, is of particular current interest, because recent petrologic study has suggested that it may represent the distal portion of a long eastern rift zone from Kohala volcano, and results of the JAMSTEC surveys show that its rocks are at least largely reversely magnetized, indicating an older age (>760ka) than known on-land exposures of any Hawaiian volcano. In general, the Hilo Ridge size and morphology appear similar to that of Puna Ridge, although pancake cones are less obvious on Puna ridge. Suggested dive sites: 1-2, along deep distal toe. Participants: Johnson, Thornber, USGS postdoc.

(D) Deep-water alkalic lava fields

North Arch - The North Arch volcanic field is one area of off-axis volcanism located on the flexural Arch north of Oahu. A SeaBeam survey of about 1/3 of this huge flow field during the Shinkai dives in 1999 revealed about 100 vent structures and defined the gentle slopes down which many of these flows were emplaced. The two Shinkai dives (S502 and S503) recovered sheet flows, pillow lavas, and hyaloclastite from a composite cone in the southern part of the flow field and demonstrate that the eruptions began with eruption of gas-rich lavas that formed steep structures made of vesicular pillow basalt and hyaloclastite. As the eruption progressed, the central part of the cone collapsed and a low shield formed by steady eruption of fluid, dense lavas.

South flank of Puna Ridge - Six samples collected on Shinkai dive S495 from a highly sonar reflective sheet flow in the moat on the southern margin of the Puna Ridge are alkalic with 0.95 wt\% K₂O. These lavas are sparsely olivine-clinopyroxene-phyric to aphyric, dense, glassy, and are very fresh in appearance. The flow covers an area of some 70 km² with bathymetric relief of only 2-3 meters over much of this area suggesting eruption of a highly fluid lava. These lavas have distinctly higher K/Ti ratios than Puna Ridge lavas at similar Mg/(Mg+Fe) precluding a common mantle source for the two populations. They plot in the broad field defined by rejuvenation stage lavas from the islands and the North Arch lavas, but are on the less enriched end of the spectrum of these lavas in incompatible minor elements such as Na, K, Ti, and P; in Mg and Fe they resemble North Arch lavas. The distal portion of this flow was sampled on s495, but the source vent has not been found and would be the target for further work.

The geochemistry of the lavas erupted off axis are similar to those of the rejuvenated stage in the Islands (such as the Honolulu and Koloa Volcanics). The similarity of the compositions of the North Arch, south flank of Puna Ridge, and rejuvenated stage lavas are difficult to understand given that the mantle sources beneath the islands and the flexural Arch have had very different histories. However, our ability to compare the data sets are impaired by the subaerial eruption of the rejuvenated stage lavas which leads to degassing of volatile components and to subsequent modification during subaerial weathering. Collection of submarine erupted rejuvenated stage lavas from near the islands would lead to better understanding of the distribution of the different mantle components that partially melt to form Hawaiian lavas. Clague will be sampling some of these flows and cones in the spring of 2001 with MBARI’s ROV Tiburon, but the vast majority are too deep for this ROV to access. The flow on the south flank of Puna Ridge is also too deep (5500 m) for "conventional" ROVs and submersibles to reach. Either Shinkai or Kaiko could, however, reach these deeper targets in the North Arch and South Flank volcanic fields, a region of previously unsampled flows south of Oahu, and the deeper cones on the flanks of Kauai and Niihau.

Participants: Clague (North Arch), Johnson (south flank Puna Ridge).

Older volcanoes of the Hawaii-Emperor Ridge:

Study of these volcanoes is complicated by the sediment cover, Mn-oxide encrustations, and diagenetic alteration associated with their greater ages increasing with distance from the Hawaiian Islands. Summits of most of the older volcanoes, which are commonly mantled by coral reefs, consist of deeply weathered subaerial basalt. ODP leg 55 drilled the summits of 4 of them in 1978, and another ODP cruise will drill into several of these during the summer of 2001. Most of the previous sampling was specifically aimed at recovering samples suitable for K-Ar dating, and most dredging was focussed on the break-in-slope where beach cobbles of resistant rocks were recovered. However, most of these samples are from the postshield alkalic stage and consist of differentiated alkalic lavas. A few dredges on the rift zones have recovered fairly unaltered tholeiitic lavas, commonly with some glass still preserved. Evaluation of the changes in the mantle sources for the Hawaiian-Emperor chain should focus on the tholeiitic shield lavas. Few fresh samples have been recovered due to the sampling bias imposed by locating ODP drill holes on the summits and dredging almost exclusively at the break-in-slope. The existing SASS bathymetry shows that many of the Emperor Seamounts have extremely long rift zones characterized by complex bifurcation of the rifts near their bases. The least altered petrologic samples and best records of the early growth of these seamounts may come from dives along the deep portions of their flanking rift zones. Recovery of such samples from the Emperor Seamounts or the Western Hawaiian Ridge would provide the means to evaluate how the Hawaiian hotspot has changed over long periods of time. Development of robust dynamic models of the mantle plume beneath Hawaii requires such knowledge of mantle source variations through time.

Ancillary Programs

In addition to the specific research projects outlined above, a general study of volatiles on any recovered glasses (H2O, CO2, F, S, Cl, D/H ratios) using the ion probe has been proposed by Erik Hauri at DTM/Carnegie Institute of Washington.

Tentative work plans and timetable

Dec. 2000: Meeting at the American Geophysical Union National Meeting in San Francisco of representatives from Japan and US to evaluate progress on interpretation of results from the 1998-99 cruises, preparation of the AGU Monograph, and discussions of plans for the 2001 Hawaii KAIREI/KAIKO program.

Early 2001: Continue editing revising reports for the AGU Monograph. These results and new models should provide critical information on the growth history of Hawaii Island, interfacing with the NSF-supported Hawaii Scientific Drilling Project that is also interpreting and writing up results of the first phase of deep drilling.

May-June, 2001: If the 2001 Hawaii program has been approved, collaborate with Japanese colleagues in developingplans for the field program.

September 2001: Participate in the KAIREI/KAIKO cruise.

Post- KAIREI/KAIKO cruise: Data collection and interpretation, based on materials obtained during the cruise dives. Prepare summary and other cooperative publications based on the JAMSTEC project to date. Begin planning for next phase, with SHINKAI 6500.

Proposed USA participants, and fields of expertise:

　

David Clague, (MBARI): Marine geology, petrology, ocean-island geology

Kevin Johnson (UH Manoa, Bishop Museum): Petrology, marine geology

Peter Lipman, (USGS): volcanology, landslide processes, evolution of Hawaii Island

Alex Malahoff (Univ. of Hawaii): Loihi geology

Brian Midson (Univ. of Hawaii): Loihi geology

Greg Moore (Univ. of Hawaii): marine geophyisics

James Moore (USGS Emeritus Scientist): Marine geology, Hawaiian geology

Julia Morgan (Rice Univ.): structural analysis, landslide mechanics, seismic

Thomas Sisson (USGS): basalt petrology and geochemistry

John Smith (Univ. Hawaii): bathymetric and side-scan surveying and data processing

Carl Thornber (USGS, HVO ): basalt petrology and eruptive processes

USGS Postdoc: to focus on the Japan-USA Hawaii work during the time frame of this proposal and provide a research interface between mainland scientists and those at HVO

Products of Collaborative JAMSTEC Hawaii Research

1. Bathymetric and reflectivity maps of the deep ocean floor around the Hawaiian Islands, based on SeaBeam and sidescan-sonar data.

2. Scientific papers addressing many aspects of Hawaiian magmatism, marine geology and landslide processes including: the emplacement processes, recurrence rates, and attendant hazards associated with large oceanic-island landslides; new interpretations of internal structure and growth of oceanic volcanoes; nature and petrologic evolution of the Hawaiian plume.

3. Participation in open-house and other public-outreach activities when the Japanese ships are in Honolulu; preparation of non-technical press releases and other scientific communications for the public; assistance to Japanese colleagues in communicating their written results to non- Japanese audiences.

TABLE 1. Utility of continued marine studies: Kilauea, Mauna Loa, Loihi

H As the youngest parts of Hawaiian chain; offshore features are theleast complicated by later structures & sedimentation

H Geodetically and seismically active volcanoes, providing geophysical data on subsurface structure

H On-land volcanic history & structural features (since ~100 ka) are well documented by geologic & petrologic study

H Some under-water features are older than any on land, uniquely documenting early evolution of hot-spot volcanoes

H Frequent eruptions and earthquakes generate continuing geologic hazards, some of Pacific-wide concern

H Kaiko-Shinkai results to date for the south flank have defined previously unanticipated problems that we now know how to study

H The offshore deep west flank of Mauna Loa,and its landslide complex (analogous to Nu’uanu), remains “terra incognita.”

H In the short term, the Japan-USA JAMSTEC group has special opportunities for deep-sea studies of early alkalic rocks from Kilauea and other Hawaiian volcanoes, but these may soon be taken on by other research groups if we fail to follow up on initial successes.

Table 2. Possible 5-Year Cruise/Dive Plan (most detailed for 2001)

[preliminary suggestions only]

Cruise #1 #2 #3

Year 2001 2002 2003-5

Ship Kairei/Kaiko Shinkai Kairei/Kaiko?

No. of dives(?) (20-25) (15-20) (15-20)

Research Targets

A. Slumps

1. Hilina 3-4 -- --

2. Laupahoehoe 2-3 X --

3. Loihi flanks 2-3 -- --

4. Kaena -- X --

B. Distal landslides

1. Nuuanu-Wailau 2-3 -- --

2. S Kona-Alika 2-3 X --

C. Deep rift zones

1. Puna Ridge 2-3 X X

2. Haleakala 3-4 X X

2. Hilo Ridge 1-2 X X

4. Mauna Loa SW -- X X

D. Deep alkalic lava fields

1. N. Arch -- X --

2. S. of Puna Ridge 1-2 X --

3. S of Oahu -- X --

E. Hawaii-Emperor Ridge

Deep rift zones 2+/- X XX

Table 3. Proposed 2001 Dive Sites, Hawaii Island

Site Research targets Dive depths, m

SE Hawaii (4-5 dives

1. S base Papa’u SM Alkalic lavas?, transverse structure 3600-2500

2. Central Kilauea slope Early tholeiite lava of Kilauea,

vs transit. basalt, submarine erupted?

(compare dive K95) 2700-1800

3. NE mid-slope bench Pillow lava vs sediment; compare with

main mid-slope bench 4700-3300

4. NE ridge, S of Loihi Rock types, source (ML or Loihi), and

origin; compare with dive K93 5200-4500

5. Outlying block Rock types, source (ML or Kilauea), and

origin; compare with dive K93 5000-4100

6. SE Papa’u SM Transition between sites # 1 & 2, if needed 2900-2000

7. Conical seamounts Small young volcanoes? ~5500

Also piston core between P5 & P6

8. Alkalic flow Puna Ridge Source vent for 70 km² flow 5500

9. Deep toe of Puna Ridge Most distal flows along the rift zone ~5000-5500

possible source of High-Mg glasses

West Hawaii (2-3 sives)

Alika & S Kona Slide Distal large blocks (Mauana Loa); 4-5000

compare w/Nuuanu

North Hawaii (2-3 dives)

Laupahoehoe Slide Lower benches (compare w/Hilina); 3500-4500

possible early alkalic Kohala?

補足試料−3 米国申請書付図

日本側と共通のFig.1　Fig.3　以外は容量オーバーのため省略した。

Fig 1. Index map of Hawaiian Islands, showing existing JAMSTEC SeaBeam bathymetric coverage, and those by others. (Same as Fig.1 in Japanese Proposal).

Fig. 2. Locations of dives by the ROV Kaiko in 1998 (K) and Shinkai 6500 (S) in 1999, south flank of Hawaii Island, and areas proposed for dive sites in 2001. Contour interval, 200 m. Star, epicenter of 1975 M=7.2 earthquake.

Fig. 3. Locations of proposed dive sites on Laupahoehoe slump and distal lower part of Haleakala East Rift Zone. (same as Fig. 3 in Japanese proposal).

Fig. 4. Locations of dives by the ROV Kaiko in 1998 (K) and Shinkai 6500 in 1999 (S), north of Oahu and Molokai Islands, and areas proposed for dive sites in 2001. Dashed lines indicate approximate boundaries of Nuuanu and Wailau landslides. Contour interval, 200 m.

Fig. 5. Locations of proposed dive sites in West Hawaii slide complex.

Fig. 6. Locations of proposed dive sites in Wai`anae slide complex.

Fig. 7. Location map for Loihi west flank dives.

Fig. 8. Alkalic flow along southern margin of Puna Ridge.

補足試料−４　AGUモノグラフ論文リスト

Evolution of Hawaiian Volcanoes:

recent progress in deep underwater research

Editors: Eiichi Takahashi, Jiro Naka, Mike Garcia, Pete Lipman, and Shigeo Aramaki

Chapter 1 Progress in Deep Underwater Geology Around Hawaii

Collapse features revealed by recent multibeam sonar surveys on the Hawaiian Ridge

J. Smith* & K. Satake (text 20-25 p) (fig 10 ) (tab 1) CD-ROM

(Hard copy maps possible as well if JAMSTE wants to pay for them to be published)

Deep-Sea Volcaniclastic Sedimentation around the Southern Flank of Hawaii Island

Jiro Naka*, Toshiya Kanamatsu, Pete Lipman, Tom Sisson, Nohoro Tsuboyama, Julia

Morgan, John Smith and Tadahide Ui

(text: 30 pages) (figures: 15) (number of tables: 4) (20photos in CR-ROM (3 table in CD-ROM)

Magnetostratigraphy on marine sediments in an adjacent area of Large Hawaiian submarine Landslides.

Toshiya Kanamatsu* and Emilio Herrero-Bervera (text 20p) (fig 10) (tab 3)

Eruption style and flow emplacement in the submarine North Arch Volcanic Field, Hawaii

David A. Clague*, Kozo Uto, Alice S. Davis

(text 18 pages) (figures: 12 some in colour)

Hydrogeology as a Possible Explanation for the Hawaiian Heat Flow Anomalies

Marcia K. McNutt* (text 10p) (fig 4) (tab0)

Chapter 2 Initial Stages of Plume Volcanism in Loihi

Emplacement and Inflation Structures of Subaerial and　Submarine Pahoehoe Lavas from Hawaii

Susumu UMINO*, Sumie OBATA, Peter LIPMAN, John R. SMITH, JR., Tsugio

SHIBATA, Jiro NAKA and Frank TRUSDELL

(text 17 pages) (figures: 7 (2color)) (tables: 1 )

Petrology of picritic basalts from the basal parts of Loihi submarine volcano, Hawaii

T. Shibata*, H. Kagami, J. Naka, S. Umino, K. Nogami (text 20 p) ( fig 10 ) (tab 5)

U-series isotopes geochemistry of lavas from Loihi Seamount and implications for the

dynamics of mantle melting on the margin of the Hawaiian plume

A.J. Pietruszka*, E.H. Hauri, M.O Garcia (text 15-20 p) (fig5-6) (tab 2)

The origin and geochemistry of basal Loihi hydrothermal systems

B Midson*, J Ishibashi, A Malahoff, I Kolotyrkina, T Oomori (text 6 p) (fig 2) (table 1)

Phylogenetic analysis of 16S rRNA and RuBisCO genes form the 4772 m-deep low-temperature vent

in the south rift zone of Loihi Seamount

Hosam E. El-saied, Makoto Sato, Takeshi Naganuma* (text 10p) (fig 2) (tab 1)

Chapter 3 Climax Stage Magmatism: growth history of Kilauea volcano and its instability

Correlation of seismic reflection data and submersible surveys of the offshore bench

along Kilauea's south flank

Julia Morgan*, Greg Moore, Denise Hills, John Smith, others?

(text 20-25) (figures 8-10) (tables - 1)

Structural variability along the submarine south flank of Kilauea volcano,

Hawaii, from a multichannel seismic reflection survey

Denise Hills*, Julia Morgan, Greg Moore (text ~25-30) (figures -~12-15) (tables - 1)

( Some figures may go on CD)

Volcanic structure of the Puna Ridge, Kilauea Volcano

Deborah K. Smith*, Laura S. Kong, Kevin T. M. Johnson, Jennifer Reynolds

(text: 30p) (Fig. 8-10) (Table 1-2) (foldout geologic map)

Ancestral submarine growth of Kilauea Volcano and instability of its south flank

Peter W. Lipman*, Thomas.W. Sisson, Tadahide Ui, Jiro Naka, Julia Morgan, John Smith

(text: 30 pages) (figures: 10, 2-3 in colour) (tables: 3)

Submarine alkalic to tholeiitic shield-stage evolution of Kilauea magmas

Tom Sisson*, Lipman,Ui, Naka (Kanamatsu?) (text 30p) (fig 10) (tab 3) {color Figs}

Geochemical and petrological systematics of submarine lavas from the Puna Ridge, Hawaii

Kevin T. M. Johnson*, Jennifer R. Reynolds, and Denys Vonderhaar

(text 25) (fig. 7-9) (fig. 1-3)

Petrology and geochemistry of lava erupted before, during and after the January 1997

magma-mixing event on Kilauea's east rift zone: Evidence for discrete shallow magmatic

processes associated with prolonged rift-zone eruptions

Carl R. Thornber*, James Budahn, Christina Heliker, David R. Sherrod,

W. Ian Ridley and Gregory P. Meeker

(~30p text) (9 figures :including 5 full-page composite figures) (4 data tables on 6 pages)

Chapter 4 Giant landslides in the Northeast of Oahu: when, why and how?

Mapping of the Nuuanu landslide

James G. Moore* and David A. Clague (text:23 pages) (figures:17)

Submarine geology of giant landslides in the Northeast Oahu:

SHINKAI 6500 submersible investigation

Yokose*, H., Takahashi, E., Shinozaki, K., Garcia, M. O., Moore, J.G., Takarada, S., Ui,

T., Kanamatsu, T. and Naka, J.

(text : 16 pages) (figures: 12) (tables: 1 ) (8 route maps in CD)

Geochemistry of volcanic glasses from piston cores taken north of Oahu and Molokai Islands

Sarah Sherman, Michael Garcia* and Eiichi Takahashi

(text 12 p) (fig 10 ) (tab 2) {Excel tab in CD}

Formation of Volcanic Breccia and Hyaloclastite in Landslide Blocks from the Wailau and Nuuanu Giant Landslides, Hawaii. David A. Clague*, James G. Moore, Alice S. Davis

(text in double space:15 pages)(number of figures: 8-10) (number of tables:2)

(microprobe glass analyses on CD-ROM)

Petrology of submarine rock samples from giant Nuuanu and Wailau landslides

K.Shinozaki*, Z.Ren and E.Takahashi (text 20), (fig. 10) {Excel table in CD}

Geochemical characteristics of Koolau volcano and the Nuuanu and Wailau landslide deposits, Hawaii

Ryoji Tanaka*, Eizo Nakamura, and Eiichi Takahashi (text 20p) (fig 10) (tab 3 )

Reconstruction of the Koolau volcano with multi-stage growth history

E.Takahashi*, H.Yokose, K.Shinozaki, Z.Ren, R.Tanaka, J.Moore and M.Garcia (text 20p) (fig.10)

Volume reconstruction and tsunami modeling for the Nuuanu and Wailau slides

K. Satake* and Smith (text 10p) (fig 10) ( tables: a few ) {color Figs} {Tsunami CD}

Comparison of giant landslides in the hotspot volcanoes of the Canary and Cape Verde Islands with those in Hawaii S. J. Day* (Text pages: 15) (Figures: 12) (Tables: 2)

Chapter 5. Hawaiian plume and magma genesis

Seismic structure of mantle plumes under Hawaii and other hotspot regions

Dapeng Zhao* and E.Takahashi (text 10 p) (fig 10)

Growth and Spacing of Hot Spot Volcanoes Akira Takada* (text 20 p) (fig 6) ( tab 1)

Tectonic control on the development of monogenetic cones on the flank of hot spot volcanoes:

comparative study from Hawaii and Cheju islands

Hasenaka*, T., Saiki, K., and Lee, M.W. (text 12 p) (fig 10) (tab 4)

Geochemistry of alkali basalts from North Arch volcanic field

Tomomi Kani*, Kozo Uto, David.A. Clague and Jiro Naka　（text: 12) (figures: 8) (tables: 2)

Pb isotopic compositions of olivine-hosted melt inclusions from Koolau and Kilauea volcanoes

Atsuhshi Utsunomiya*, Hisayoshi Yurimoto, Eiichi Takahashi, and Shigenori Maruyama

(text 10p) (fig 7) ( tables: 3)

Noble gas systematics related to the Hawaiian volcanism - based on　analyses for submarine rocks from Loihi, Kilauea and Koolau Volcano areas　collected by Submersibles "Shinkai 6500" and "Kaiko"

Ichiro Kaneoka*, Takeshi Hanyu, Junji Yamamoto and Yayoi N. Miura　(text 20-25p) (fig 10-12) (tab 3-4)

Submarine picritic basalts from Koolau Volcano: Implications for parental magma compositions

and mantle source Michael Garcia* (text 15 p) (fig 8) (tab 4)

Origin of the Garnet Pyroxenite Xenoliths from Salt Lake Crater, Oahu

Nanami Ichitsubo*, Eiichi Takahashi, David A. Clague (text 15p) (fig 10 ) ( tab 4)

Melting process in the Hawaiian mantle plume head: an experimental study

E.Takahashi* and K. Nakajima (text 15-20p) (fig 5-8) (tab 2-3)

補足試料−５ 代表者研究業績リスト（１９９５−２０００）

Primary magma composition in Koolau volcano, Oahu, Hawaii, R.Takeguchi and E.Takahashi, submitted to G-cubed, 2000.

Enrichment processes at the base of the Archean lithospheric mantle: observations from trace element characteristics of pyropic garnet inclusions in diamonds, W.Wang, S.Sueno, E.Takahashi, Y.Yurimoto and T.Gasparik, Contrib. Mineral. Petrol, in press, 2000

ハワイホットスポット火山の全貌が明らかになる　　高橋栄一・仲二郎　科学, 70, 694−696, 2000

Tectono-Magmatic Processes investigated at deep-water flanks of Hawaiian volcanoes, J.Naka, E.Takahashi and others, Eos Trans. Amer. Geophys. Union, 81, 221-227, 2000

Subsolidus and melting experiments of a K-rich basaltic composition to 27 GPa: implication for the behaviour of potasium in the mantle, W.Wang and E.Takahashi, American Minralogist, 84, 357-361, 1999

Subsolidus and melting experiments of K-doped peridotite KLB-1 to 27GPa: its geophysical and geochemical implications, W.Wang and E.Takahashi, J. Geophysical Research, 105, 2855-2868,2000

Origin of the Columbia River Basalts: Melting model of a heterogeneous plume head. E.Takahashi, K. Nakajima and T.L.Wright, Earth Planet. Sci. Lett., 162, 63-80, 1998.

Melting experiments on homogeneous mixtures of pridotite and basalt: Application to the genesis of oceanic island basalts. T.Kogiso, K. Hirose, and E. Takahashi, Earth Planet Sci. Lett., 162、45-61,1998.

Melting study of an alkali basalt JB-1 up to 10 GPa: behavior of potassium in the deep mantle. K. Tsuruta and E. Takahashi, Phys. Earth Planet. Inter., 107, 119-130,1998

Geochemical properties of lithospheric mantle beneath Sino-Korea craton. W.Wang, E.Takahashi and S.Sueno, Phys. Earth Planet. Inter., 107, 119-130, 1998

Hydrogen in molten iron at high pressure: the first measurement. T.Okuchi and E. Takahashi, in “Properties of Earth and Planetary Materials at High Pressures and High Temperatures” ed. by M. Manghnani and Y.Shono, Am. Geophys. Union, Monograph, pp249-260, 1998.

Solubility of Ne, Ar, Kr and Xe in binary and ternary silicate systems: a new view on noble gas solubility, T.Shibata, E.Takahashi, and J.Matsuda, Geochim Cosmochim Acta, 62, 1241-1253, 1998

地球内部ダイナミクス 岩波講座 地球惑星科学 第10巻 （分担執筆 ：谷本俊郎 他５名） マントルダイナミクスと地球内部進化 pp123−199、岩波書店、１９９７．

島弧火山の深部構造とマグマ変遷の仕組み 高橋栄一・東宮昭彦・宮城磯治、 火山、 ４２、特別号 マグマ学、 S209-218 、 １９９７．

洪水玄武岩の起源 高橋栄一・中嶋勝治、 科学 、６７巻、 ５１９−５２９、１９９７．

Water solubility in albite-orthoclase join and JR-1 rhyolite melts at 1000 C and 500 to 2000 bars, determined by micro-analysis with SIMS. I. Miyagi, H. Yurimoto and E. Takahashi, Geochemical J., 31, 57-61, 1997.

Field occurrence, geochemistry and petrogenesis of the Archean Mid-Oceanic ridge basalts (AMORBs) of the Cleaverville area, Pilbara craton, Western Australia, H.Ohta, S.Maruyama, E.Takahashi, Y.Watanabe, and Y.Kato, Lithos, 37, 199-221, 1996

Noble gas solubility in binary CaO-SiO2 system. T. Shibata, E. Takahashi and J. Matsuda, Geophys. Res. Lett., 23, 3139-3142, 1996.

何が島弧火山の深部構造を決めるか 高橋栄一・高橋正樹 科学、６５巻、６３８−６４９、１９９５．

Reconstruction of a magma chamber evolution beneath Usu-volcano. A. Tomiya and E.Takahashi, J. Petrol., 36, 617-636, 1995.

Melt generation by isentropic mantle upwelling. H. Iwamori, D.P. McKenzie, and E.Takahashi, Earth Planet. Sci. Lett., 134, 253-266. 1995