Abstract
On the basis of GLORIA image and Seabeam survey, it has been considered that several cones and lava flows are distributed in the area at west of the Oahu Island to form a volcanic field. Dive K204 focused on one of the lava cones in the area. The KAIKO landed on the southeastern flank of the cone at the depth of 4211 m. From this point, we took a course approximately toward NNW, climbing up the gentle slope to a flat top of the cone. This dive achieved the following objectives:
1) The cone has a flat top (500 m in a diameter) with a very gentle slope (2 km in a diameter and 150 m of elevation), and is composed mainly of pillows accompanied with small amount of sheet flows and broken pillows. The top of the cone is covered by mud, and did not show crater, suggesting that the cone had tapped lava at the top to form a lava lake.
2) Most of the collected samples are nearly non-altered, dense pillows, with relatively thick quench glass rinds (5 ~ 15 mm in thick). One porous sample is also collected at the flank of the cone. Mn-coating and paragonite layer are thinner than 1 mm in thick, indicating that these samples are possibly younger than 1~0.5 Ma.

概要
　この潜航ではグロリアイメージやシービームによって認識されていたオハフ島西方海域に分布する火山群のひとつの溶岩平頂丘を調査した。「かいこう」は平頂丘の南東麓に着底し、北北西方向に平頂丘斜面を上り頂上部まで調査した。この潜航により以下の成果を得た。
１） 平頂丘は約2kmの底形で約150mの比高であり、なだらかな斜面と山頂部の直径約500ｍの平坦な頂部を持つ。斜面は丸みをおびた枕状溶岩に覆われており、その他にシート状溶岩や破砕されたピローなども認められた。頂部はわずかに枕状溶岩が露出するものの、厚く泥に覆われた平坦面である。明瞭な火口地形は認められない。これらの特徴から、この溶岩平頂丘は頂部に溶岩湖を形成していたと考えられている”flat cone”であろう。
２） 採取されたサンプルは新鮮で緻密なカンラン石（斜長石）斑晶を含む玄武岩質の枕状溶岩であり、特徴的に厚い（5〜15 mm）急冷ガラス縁を有する。発泡したサンプルは１つだけ山麓部で採取された。MnOコーテイングやパラゴナイト層は薄く1mm以下であり、これらの試料が100~50万年より若いことを示している。

Video Highlights
(1) 13:06 - 13:12
Elephant shaped pillow lobe and its internal structure
(2) 13:24 - 13:45
Various shapes of pillow lavas

Purpose of Dive K204
On the basis of GLORIA image, Holcomb (1997) suggested that relatively younger volcanic field probably including lava flows and several cones at the west of Oahu Island. Similar volcanic fields are also recognized at southwestern of Oahu. The relationship between the volcanic fields near Oahu and North- and South Arc lavas, and Honolulu volcanics has not been discussed. During this Kairei cruise Leg 1-B, Seabeam and side-scan sonar has revealed topography of the area in detail. Kaiko dives of the Leg 1-B concentrate on Geological, petrological and geochemical study on these recently recognized volcanoes.
In the area of west of Oahu Island, we can recognize several volcanoes with cones and lava flows. The Dive K204 focuses on one of cones at the western part of the area, because we cannot dive anywhere due to dense cables. The purposes of the dive are as following:
(1) Observe and describe the constituent and its morphology of the cone, and reveal the geology of the cone.
(2) Observe and make sure whether flat top of the cone is normal crater and lava lake.
(3) Collect various types of samples from the cone.

Dive Results
Kaiko landed at the depth of 4200 m, about 800 m southeast off the foot of the cone. The landed area is covered by probably thick mud. Muddy bottom without any rock fragments continues during the first 300 m in distance. The first out crop of pillow lobes in mud was found at the depth of 4200 m, about 600 m off the foot. In addition, strong refraction probably of the northern extension of the pillow lobes was observed by sonar. This suggests that lava flows are distributed at the southeastern flank of the cone, being consistent of lobe-like topography in the Seabeam map. However, we could not approach to central part of the flow, and could contact at southern side of the flow. From the sampling point 1, thick muddy surface continue again about 700 m in distance. We could not find any outcrops and rock fragments in the thick mud.

At the depth of 4197 m, the Kaiko began to climb at base of the cone. The slope of the cone was very gentle, 150 m height in 1000 m distance. The cone is composed mainly of pillow lavas and fragmental pillows, with minor amount of sheet flows. The outcrop of lavas had been occasionally found from the depth of 4188 m. Clusters of pillow lavas several meters thick are distributed in a muddy gentle slop. These pillows often occur with sheet flows, which show wrinkle. These pillows have preserved their primary morphology, and show elongated or globular shapes. Elephant nose-like and stalactite-like lobes are often recognized. The slope with scattered out crops continue until ~ 4080 m depth, and becomes more gentle at the top. The top did not show crater, and was covered by thick mud. We tried to push the core at the top, and collect mud easily. The Kaiko left the top at the depth of 4034 m. After leaving the bottom, strong refraction was observed at the west of the vehicle by sonar image. This might be exposed lavas along the crater rim.

Most of the collected samples are very dense basalt, and their vesicularity must be less than several %. Only one sample from the foot of the cone is porous (vesicle > 20 %). The pillow samples usually have thin glassy rim (5 ~ 15 mm in thick). MnO coating is usually thin (< 1 mm), and paragonite layer has not been developed well, suggesting that these lavas are relatively young (0.5 ~ 1 Ma). Although collected samples might be petrologically similar, these are the first samples collected in this volcanic field of west of Oahu. Locations and description of collected samples are shown in attached appendix in this onboard report.