Abstract
In order to understand both geological and petrological growth history of the Mauna Loa that is the biggest volcano in the world, dive K211 was conducted one of the distal block in the south Kona landslide area southwest of Hawaii Island on September 7, 2001. K211 traversed 2.4 km west from the base of the eastern slope of the distal megablock and up to the highest place of it.

The succession of the landslide block can be divided into upper part (volcaniclastics deposits: -4200 to -4400 m) and lower part (alternation of hyaloclastie and sheet lava: -4400 to -4700 m) . The lower part of the block is composed of product of volcanic eruption, hyaloclastite and sheet flows. Both deposits show brownish orange in their appearance due to palagonitization. The size of clasts in each hyaloclastite layers indicate upward fining through the succession. On the other hand, the upper part is composed mainly of debris flow deposits. These deposits contained coarse subrounded boulder of basalt. This imply that the clasts were abraded by shoreline process.
On board descriptions indicate that these rock specimens are mostly olivine basalt. Glassy rind has slightly altered to paragonite but no Mn coatings.

概要

世界一巨大な火山であるマウナロアの地質学的および岩石学的成長史を理解するために，ハワイ島南西部の南コナ地滑り地域に存在する遠く離れた山体崩壊ブロックに関して潜航調査を2001年9月７日に実施した．Ｋ211潜航調査は，遠く離れた巨大ブロックの東斜面の麓から開始し，そのブロックの最高点に達する，西向きのコースを2.4km横断した．

その地滑りブロックの層序は，上部（火山砕屑性堆積物：-4200　から　-4400　ｍ）と下部（ハイアロクラスタイトとシート状溶岩の互層：-4400　から　-4700ｍ）．に二分できる．下部は，火山噴出物，ハイアロクラスタイトとシート状溶岩，によって構成されている．両堆積物は，パラゴナイト化のために外観状茶色を帯びたオレンジ色を示す．ハイアロクラスタイト層中の礫径は，層序を通して上方細粒化を示す．一方，上部は，主に火山砕屑性堆積物を主体としている．これらの堆積物には，粗粒の玄武岩質亜円礫が含まれる．これは，その礫が，海岸の過程で円摩されたことを示唆する．
船上の記載では，これらの岩石資料がカンラン石玄武岩であることをしめす．ガラス質の縁は，弱く変質しておりパラゴナイト化しているものの，マンガンコーテングは認められない．

Purpose of Dive K211
Topographic features of submarine landslide have been recognized in many volcanic island. It is important to evaluate the hazard of tsunami produced by submarine slope failures. There are four large submarine landslides on the western flank of Hawaii: the North Kona, Alika phase 1, Alika phase2 and South Kona landslides. These landslides have been discussed based on the GLORIA image and detailed bathymetric maps（e.g, Lipman et al., 1988; Moore et al., 1989）. It is expected that such megablocks produced by a gigantic landslide can serve us essential information about internal structure of a volcanic island.
In the case of South Kona landslide area, hummocky terrane extends to western offshore of Hawaii (ca. 80 km ). Two of the landslide blocks in this area were investigated using submersible and dredge（Moore et al., 1995）.Unfortunately, the submersible investigation could not collected in situ rock samples due to manipulator malfunction.
The purpose of K211 are (1) to describe geological succession of landslide block, (2) to collect in situ rock sample from outcrops，(3) to compare with geological succession of bench on southwest flank of Hawaii, which was investigated by K210. Rock samples collected in situ during this dive will be a good clue for early volcanic history of Mauna Loa volcano.

Result
Dive Ｋ211 investigated one of the megablock South Kona landslide area offshore of island of Hawaii. We landed on the base of east wall of the megablock. The strata outcropping on this megablock can be divided into two major groups: consolidated strata (product of volcanic eruption and volcaniclastic deposit) and unconsolidated landslide deposits. The megablock composed of the former. The later deposited as a buttress unconformity.

Geology of landslide block
The megablock is composed mainly of layered hyaloclastite accompanied with debris flow deposits, sheeted lava and pillow lava.
We observed hyaloclastite layer in the lower part of this megablock succession approximately -4400 m to -4650ｍ in depth. The hyaloclastite layers which have different in average clast size and thickness of the layer (50 cm to 100cm ) are deposited coherently. The hyaloclastite layers have basically clast-support texture and indicate NW-SE striking and incline to southward. Average clast size vary from sand to pebble . The shape of the clasts are completely angular without any abrasion. On board descriptions indicate that hyaloclastite layer which is composed of sand grain have sedimentary textures. The breccia in the hyaloclastite layers are monomictic and is the same as matrix of the layers．
Sheet lava shows well developed columnar joint. During the dive course, only one outcrop of pillow lava was observed at the depth of 4360ｍ. The lava occurs as a truncated pillow and are covered with pebbly mud layer．Outcrops of pillow joint-block rubble were observed on the southern gentle slope in the depth from -4400 to -4300ｍ.
The debris flow deposit intercalated with hyaloclastite are composed of subangular to subrounded cobble to boulder and are clast-support texture. Clasts in the debris flow deposits indicates imbrication.

Landslide deposite
Unconsolidated pebbly mud layers which composed of reddish-brown mud with clast (about 5% in volume) covered partly with manganese coating was observed everywhere during the diving. The bottom of the layers are rich in clast. The clast often indicates normal grading. The size of the clast seems to correlate positively with the thickness of the layer. In muddy matrix, we observed fine lamina in some layers. The unconsolidated pebbly mud layers cropped out on the east wall of the megablock shows a buttress unconformity. Almost every gentle slope are mantled with upper fine grained part of the layer (ca. 50-100ｃｍ in thickness)

Rock samples
Volcanic rocks collected from this area are composed mainly of picritic basalt and olivine rich basalts.The rocks of lava flows and hyaloclastite layers in the landslide block, are picritic basalts. On the other hand, breccia in landslide deposits and debris flow deposits are olivine basalt. The vesicularity of the rocks are high compared with picrites. A rock sample collected from southern slope have high vesicularity up to 20%. On board descriptions indicate that these rock specimens are mostly olivine basalt. Glassy rind has slightly altered to paragonite but no Mn coatings.