Leg 2B (South arch and North Kona landslide)

K218: North Kona landslide (H. Yokose)
Sept. 17, 2001

Abstract
In order to obtain geological information for mapping and for petrology of the lower bench in the north Kona landslide area, dive K218 was conducted submarine western flank of Hawaii Island on September 17, 2001. K218 traversed 2.4 km east from the base of the western slope of Hualalai ( -3970 m) and up to the highest place of the lower bench (-3019 m).

We observed various type of lava morphology, such as elongated pillow, bulbous pillow, aa lava, lobate flow and ropy textured lava during this dive. Within a single mound, lava morphology was observed to change according to the steepness of the slope. Steepest part of the mound are covered by elongated pillow lava and/or pahoehoe lobes. Gently dipping slop and flatted top of the mounds are mainly covered with bulbous pillows and pillow cones. This correlation between lava morphology and the slope angles of the basement may correspond to effusion rate of the lava flows. Morphological change of lava suggest that the pillow lavas may not be a single compound flow but has vents at the top of the each mound.

On board descriptions indicate that these rock specimens are mostly picrites with abundant olivine phenocryst and glassy rind. Glassy rind has slightly altered to paragonite but no Mn coatings. It seems that the abundance of the olivine phenocryst in the lavas increases with increasing of the depth of sampling sites.

概要
北部コナ地滑り地域の下部ベンチに関する地質図作りや岩石学のための地質学的情報を得目的で，Ｋ218潜航調査は，ハワイ島西部海底山麓において2001年9月１7日に実施された．Ｋ218潜航調査は，フアラライ火山の西部斜面基底部（-3970ｍ）からその下部ベンチの最高位（-3019ｍ）まで上昇する，東向き2.4ｋｍのコースを横断した．

我々は潜航調査中様々な溶岩形態を観察した：例えば，伸張した枕状溶岩，球根上枕状溶岩，アア溶岩，ローベイト溶岩や縄状溶岩など．一つの小山内において，溶岩形態は斜面の角度に対応して変化する事が観察された．小山の最もきつい斜面は，伸張した枕状溶岩やパーホエホエローブによって覆われている．ゆるく傾いた斜面や平らな頂上は，主に球根状枕状溶岩やピローコーンによって覆われている．溶岩の形態と基底部の斜面角度との間の相関は，その溶岩流の噴出率の対応しているのかもしれない．溶岩の形態的な変化は，この溶岩が単一の複合流ではなく各マウンドの頂上に火口を有している事を示しているのかもしれない．

船上における記載は，岩石試料のほとんどがガラスの殻をもち大量のカンラン石斑晶が含まれるピクライト質であることを示す．ガラスの縁は，若干パラゴナイト化しているがマンガンコーテングなない．溶岩中のカンラン石斑晶量は，採集地点の深度が増加にともなって増加しているようである．

Purpose of Dive K218
Many slump structures and small scale landslide topography have been recognized in northwestern flank of island of Hawaii based on the GROLIA image (e.g., Moore and Chadwick, 1995; Lipman et al 1988). This area, flank of Hualalai volcano, is called north Kona landslide region. There are bench system like a Hilina slump region（from -4000 to 3000m in depth）. Because submarine investigation on the deep submarine western flank of island of Hawaii did not carry out, the relationship between bench system and landslide mechanism is leaved as an unsolved problem.
Geological information of the north Kona area is very important to consider systematically the development and decline of volcanoes in Hawaii islands, especially Hualalai and Mauna Loa volcanoes. The purpose of the dive Ｋ218 is to characterize the volcaniclastic apron of Hawaii islands. Northwest flank of island of Hawaii is the best place to investigate on the geologic structure of bench system on submarine volcanic apron. Furthermore, it is expected that the rocks of early product of Hualalai will be collected during this dive. These rocks will serve us petrological and geological information about early stage of either Hualalai volcano or Mauna Loa volcano.

ＤＩＶＥ　ＲＥＳＵＬＴ
Topographic feature around dive K218 is bench structure which is trending NNW-SSE , on the submarine flank of Hualalai volcano (ca. 30 km long, 2 km wide and 1km height). The slope of the bench has rough topographic surface unlike the upper slope. Since the small hills (ca. 120m high) overlaps each other, the slope of the bench looks like a stairs case as a whole. We identified 5 steps on the curse of this submersible investigation.

K218 investigation reveals that these small hills are composed of four geological units: soft sediment, angular pillow rubble, elongate pillow, and pillow cone with bulbous pillow.
Soft sediments were observed at the base of steep slope and depressions between pillow cone and steep slope. It seems to deposit thickly on the place where the slope angle changes from flat to steep. At the toe of steep slopes are mantled with pillow joint-block rubble. The rubbles are well sorted on their appearance. Almost all escarpments are draped with elongate pillow preserved their original surface structure. We observed truncate pillow on the escarpment at the fourth and fifth hills. The escarpment is inferred to be fault scarp based on the occurrence of the truncated pillow. Surface of the pillow lavas on the steep slope have relatively smooth. We did not observe corrugation texture and the contraction crack of tortoise-shell shape on the surface of the draped pillow lavas.
On the other hand, a large majority of the pillow lava occurred on the relatively gentle slope and flat top area have some feature of pillow crust such as corrugation, contraction crack of tortoise-shell shape and trapdoor texture. Not only elongated pillow but also branching pillow and pillow bud were observed.
As already mentioned above, the lava morphology was observed to change according to the steepness of the slope within a hill. The steepest part of the each hills is covered by elongate pillow lava and pahoehoe lobes. Top of the each hill are mainly covered with bulbous pillows and pillow cones. This correlation between lava morphology and the slope angles of the basement may suggest effusion rate of the lava flows. This mean that this site is not covered with a single compound flow but has vents at the top of the each bench.
We did not observe both landslide deposit and slump structure during dive. It is doubt that huge landslide events happened in this area. It is reasonable that this bench system is thought to be a simple ridge system.

On board descriptions indicate that these rock specimens are mostly picrites with abundant olivine phenocryst and glassy rind. Glassy rind has slightly altered to paragonite but no Mn coatings. The phenocryst abundances seems to increase with water depth of the location .The thickness of volcanic glass from the pillow surface vary from 5 to 10ｍｍ. There is no pebbly mudstone.

Video highlights
10:09: aa lava
1036: pillow cone: bulbous pillow with tortoise shell texture
10:42-10:45: joint-block talus at foot of pillow, hollow pillow, loate flow
11:01: pillow cone
11:31: elongated pillow
14:02: a pillow lobe with a hollow interior and ropy textured lava

DIVE LOG

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