Leg1B (West to southwest Oahu)

K203: Southwest Oahu (T. Kani)
Aug. 27, 2001

Abstract
KAIKO first dive was conducted at SW Oahu submarine volcano which is discovered by seabeam bathymetry during the 2001 JAMSTEC Cruise around Hawaii islands. KAIKO landed on the flat muddy bottom at the depth of 4180m. From this point, we took a course at fast due west ca. 1km approximately flat floor, and then northward, climbing up the steep slope to the summit of cone, 3831m depths. This dive achieved the following objectives:
1) In order to obtain geological information for newly recognized SW Oahu submarine volcano, we observed and videotaped volcanic construction and sediment distributions on one of the steep cone and the high-reflective lava flow. The sheet lava flow on the flat floor is younger than the pillow lavas based on the thickness of sediment and Mn-coating.
2) We collected rock 23 speciments for further petrological and geochemical studies. Also, we collected a sediment sample with the push core sampler.

概要
2001年JAMSTEC研究船かいれいのSeabeam調査により，オアフ島西南部の海底で火山地域が発見された．2001年8月27日，ROVかいこうの第203潜航を実施し火山円錐丘の一つの近くに着底し比較的平たんな面を東へ約１km進み，そこから北へ火山円錐丘の斜面を登りその頂上まで調査をおこなった．この潜航調査において以下の成果を得た．
水深4226〜3831mの範囲にわたって，火山地形や溶岩流の形態，堆積物の分布，などについて目視観察をおこなうとともにビデオ映像，スチル写真として記録した．平たんな場所では泥質堆積物は薄く，シート状溶岩が露出していた．火山円錐丘の斜面はより厚い堆積物に被覆されており，枕状溶岩の露頭が斜面のいたるところに認められた。火山円錐丘は枕状溶岩およびハイアロクラスタイトからなり，堆積物，MnO-コーティングの厚さから平たん面で観察されたシート状溶岩とは年代が異なると考えられる．．られッルっるがトビデオ映像，スチル写真として記録した。急斜面 lava flow,
化学組成分析をおこなうため２３個の岩石試料を採取した．

Video Highlights (Dive #203, 27 August, 2001)
Start: 11:09 , End: 11:15; Camera #2
This portion of video-recording shows sheet flow lava exposed on a thin muddy floor. This sheet flow outcrop are only found in this site during this dive. This is located at the water depth of ca. 4210m.
Start: 13:30, End: 13:32; Camera #2
This portion of video-recording shows pillow lava slope at the steep slope. This is located at the water depth of ca. 3938m.
Start: 14:17, End: 14:27; Camera #2
This portion of video-recording shows the pillow lava outcrop near the summit. This is located at the water depth of ca. 3865m.

Dive Results
Purpose
SW Oahu submarine volcano which occupies 100*100 km in area and 0.5 km in peak height is discovered by seabeam bathymetry during the 2001 JAMSTEC Cruise around Hawaii islands. The high reflectivity area in the GROLIA image covering the newly recognized submarine volcano is presumed the youngest lava flow field in this volcanism. The youngest lava flow is expected to similarity in age and chemical composition to North Arch Volcanic Field and/or rejuvenated stage alkali volcanism. Concerning the large volume of volcano, geochemical and petrological study of this volcano is important to speculate about cause of rejuvenated stage volcanism and volcanism on the Hawaiian Arch. Isotope geochemistry of this submarine volcanism is important to reveal temporal and spacial distribution of mantle components of Hawaii hot spot region. The first ROV KAIKO dive (#203) was conducted on the newly recognized submarine volcano and efforts mostly went into the one of the steep cone and the high reflectivity area in the GROLIA image. The prime objective of Dive #203 was obtain geologic information of the newly recognized SW Oahu submarine volcanism and to collect well-located rock samples there. In order to examine the volcano evolution, it is obvious that we have to sample volcanic products formed at the early stage and late stage of its growth process.

Dive Plan
Dive 203 was our first chance in Leg1B devoted to explore the SW Oahu submarine volcano. Thus, to utilize this opportunity fully, we planned to land on the seafloor at 20° 40.0’ N, 158° 51.5’ W and then westward ca. 1km and then heading north and climb up the slope to the summit of the steep cone. This particular landing site was chosen, so as to collect rock samples from early stage to late stage of the volcanism and to reveal stratigraphic relations of basal lava flows with different lineage. The actual dive was carried out approximately as planned above.

The main purpose of this dive is twofold:
Observe and videotape volcanic constructions, tectonic features, and sediment distributions along the survey line, in order to obtain information on the geology of the SW Oahu submarine volcano.
Collect hard rock samples to study geochemical characteristics of the sources involved in the SW Oahu submarine volcano.

Dive results
The seafloor at the landing site is covered completely with a thin layer of light yellowish beige sediment (silty to clayey ooze). The KAIKO which attempted collecting sediment with the push corer at this site suggests that the sediment thickness is probably less than 10cm. Sediments are ubiquitously present throughout the area surveyed during this dive, and it appears that the sediment of the steep slope of the cone is thicker than that of flat floor. In terms of sediment thickness, therefore, we could divide the Dive #203 area into the cone and sheet-like flow with different ages.

The first two rock samples (i.e., #1 and #2) collected around landing site at the depths 4186m and 4181m are siltstone with very thin (<1mm) Mn-coating, the sample #3 collected from sheet flow lava outcrop on flat floor is dense olivine basalt with max 5 mm glass rim and 1mm Mn-coating. Sample # 4, 5 are also dense olivine basalts with max 5mm glass rim and 1mm Mn-coating, which are collected from pillow lava outcrop at the bottom of the cone. The lava flows we encountered during climbing the steep slope occur exclusively in the form of rounded or elongated pillow lavas. Climbing up the slope of the steep cone, we collected hyaloclastite and vesiculated pillow lavas. Sample # 4, 5, 6, 8, 11, 12, 13 are aphyric basalts or olivine basalts with max 10mm glass rind and 2-3mm Mn-coating. Some samples (#7, 9, 10, 12, 14) from the summit and step slope of the cone are siltstones with max. 6mm Mn-coating. The pillow lavas from the cone look older compared to the sheet lava flow from the high reflectivity area based on Mn-coating thickness.

We could collect from the different aged lava flow during this first dive on newly recognized volcano. As was expected, the lavas from the high reflectivity area is likely similar in age to the North Arch lavas.

Dive log

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