Leg 2A-3 (Hana ridge and Kohara area)

K216: Hana ridge (Y. Orihashi )
Sept. 14, 2001

Results of Dive K216

Date : September 14th, 2001
Place : Middle - upper slope of the northern Hana Ridge
Observer : Yuji Orihashi
Advisors : Kevin Johnson, Eiichi Takahashi

Abstract
Our purpose on this dive is to observe geology for middle to upper slope and two cones with E-W alignment of northern Hana Ridge. On the middle to upper slope, basaltic pillow lavas which formed “step structurre” were exposed, and variably mantled by talus deposit by small scale landslide and/or debri flow. Two cones with W-E lineation on the top of the Ridge consisted of highly vesicular subrounded volcanic fragments, which has suggested that two cones might be likely “scoria cone” formed in relatively shallower submarine or subaerial condition. In addition, an outcrop of the pillow lava with Mn coating of 3 mm thickness (about 3 Ma as roughly estimated age) was found on the top of the Ridge at 2519 m depth from sea level. The formation age of Hana Ridge might be older than that we expected around 1 Ma.

要旨
今回のダイブはハナ・リッジ北壁の中腹から頂上部の構成岩と頂上部に分布し, E-W方向のリニエーションを持つ2つの円頂丘の地質調査を行なうことを目的とした．北壁の中腹から頂上部はカンラン石玄武岩質の枕状溶岩がステップ構造を呈して露出し，部分的に小規模な地滑りや土石流による崖錐性堆積物で覆われている． 頂上部を形成する円頂丘群は著しく発砲した亜円礫の火山岩片から構成され，同円頂丘群は浅海性の，いわゆる，”スコリア丘”のようなものかもしれない．また，同リッジ頂上部（水深2519m）において最大3mmのMnコーティングを持つ枕状溶岩の露頭 (推定 3Ma) が発見された．ハナ・リッジの形成年代は予想（約1 Ma）以上に古いのかもしれない．

Purpose of Dive K216
Hana Ridge, extending eastward 200 km from Haleakala volcano, is one of the largest submarine rift zones in Hawaiian shield volcanoes. The detailed bathymetric map of Hana Ridge, perfomed on Shinkai/Yokosuka cruises (Y99-07, -08) in 1999, revealed a 25 km-wide arcuate-shaped horseshoe-shaped concavity on the eastern margin and many cones with E-W lineation, of which each size was 0.5-2.0 km wide and 150-300 m high, on top of Hana Ridge (Smith et al., in press). The previous two dives (K212 and K214) have observed geology from the bottom (about 4,500 m water depth) to middle slope (about 3,200 m water depth) of the horseshoe-shaped concavity on the eastern margin and southern slope of Hana Ridge, respectively, and successfully collected a stratigrahic section of lava around 1,000 m high, although the section in K214 dive is more complicated. Therefore, this dive has attempted to observe geology and collect samples from the middle slope to top of the northen shoulder (from 3,200 m to 2,500 m water depth) and two cones which showed parallel aligment on the top in order to understand the overall geological structure of Hana Ridge.

Dive Results
The dive landed on soft sediment on the middle northern slope of the Ridge at 3182 m depth from sea level. The slope consisted of muddy soft sediment with scattered angular volcanic fragmments. At 3162 m water depth, a push core was collected in the soft sediment. At 3141 m water depth, an outcrop of partly broken pillow lava with flow direction of N40-50?W was exposed and observed over 60 m of the dive track. Two samples were collected from the outcrop of pillow lava (Sample #1a, b). A small bench covered with Mn-coated muddy to sandy sediment with was observed to be formed above the outcrop of the pillow lava, and small outcrops, maybe in-situ, of partly broken pillow lava were exposed in patches. Two sample were collected at 2976 m and 2913 m water depth, respectively (Sample #2 and #3). A talus deposit which mainly consisted of pillow lava faragments covered the slope from 2,880 m to 2,760 m water depth, and subsequently outcrop of pillow lava was continuously observed over about 100 m of the dive track. The size of pillow lava was relatively large and strongly broken, and its flow direction was observed to change at 2727 m water depth from N-S to N20-30?W. One sample was collected from volcanic fragments in the talus deposit at 2807 m water depth (Sample #4), and two samples were collected from the outcrops of pillow lava at 2761 m and 2700 m water depth, respectively (Sample #5 and #6). Proceeding along the dive track, talus deposit with partly channel mark was observed to cover the slope until 2600 m water depth. The matrix in the talus deposit, consisting in muddy sediment, was hardened due to Mn coating of the surface. Two samples were collected from the volcanic fragments in the talus deposit (Sample #7a, b). At 2660 m water depth, subrounded volcanic fragments predominated in the talus deposit, and the first cone of which summit was at 2420 m water depth was observed to be mainly covered with the subrounded volcanic fragment. Three volcanic fragments were collected as Sample #8a, b, c. Climbing up the slope of first cone along the dive track, a few thin lava flows which showed aa -like lava were scattered the flank, and then a depression of maximum 3 m depth were observed on the summit, although the depression was filled with soft sediment. Proceeding along the dive track, moreover, there was a small graben about 300 m wide between first and second cones. At the bottom (2520 m water depth), muddy to sandy sediment with Mn coating was distributed, and E-W directional current ripples were observed on the surface. A outcrop of pillow lava with about 2-3 m thick was exposed on the southern slope of the small graben (2519 m water depth). The pillow lava with Mn coating was partly broken and thought to flow with N-S direction on almost flat floor. One sample was collected from the pillow lava (Sample #10). The slope of second cone began at 2490 m water depth. The second cone was also covered with subrounded volcanic fragments, like the first cone, and some small outcrops of partly broken pillow lava were observed to be rarely exposed on the slope of second cone. Seven samples were collected from the subrounded volcanic fragments at 2481 m and 2401 m water depth (Sample #11a, b, c and Sample #12a, b, c, d). One sample was collected from the in-situ pillow lava at 2349 m water depth on the slope (Sample #13), and the dive was ended at the same point of sampling site 13th.
Total of 20 samples were collected from 13 sampling sites. A push core sample was obtained from the slope near the landing point. This sample may show muddy to sandy material observed along this dive, that seems to have recently covered the whole field of Hana Ridge. All lava samples were olivine basalt with 2-10 vol% olivine phenocryst. Vesicularity was, however, different in some sampling locations, i.e., pillow lava and its fragment which formed the slope of Hana Ridge have 2-10 vol% vesicularity while subrounded volcanic fragment and aa-like lava obtained from the slope and/or summit of two cones have 10-30 vol% vesicularity. It is obvious that volcanic fragments of the two cones are unusually vesicular even if we take difference of water depth into account. The thickness of Mn coating for lava samples shows the range from 0.2 mm to 3 mm, which is not correlated to water depth of each outcrop. The lava sample which represented the most thick Mn coating (maximum 3mm) was exposed at 2519 m water depth near the tip of Hana Ridge. Considering that growth of Mn coating may be approximately 1 mm/year, eruption age of the pillow lava is thought to be, at least, 3 Ma.
Observation and sampling in this dive have revealed the following matters, 1) The middle to upper slope of Hana Ridge is formed of pillow lavas which mainly consist of olivine basalt with 2-10 vol% olivine phenocrysts, and variably mantled by talus deposit from small scale landslides and/or debris flows, 2) Two cones with W-E alignment on the top of the Ridge consist of highly vesicular subrounded volcanic fragments, which has suggested that two cones might be “scoria cone” and/or “cinder cone” formed in relatively shallower submarine condition, and 3) Outcrop of the pillow lava with Mn coating of 3 mm thickness (about 3 Ma as estimated age) was found on the top of the Ridge at 2519 m depth from sea level (Sample #10). It suggests that the formation age of Hana Ridge might be older than that we expected (around 1 Ma).

Video hlights of Dive K216

Time Depth (m) Altitude (m) heading Narrative
(degree)
09:33 3141 2.1 145 Sample #1a, b - broken pillow lava
10:02 3004 2.8 198 firmer mud sediment with Mn coating
10:17 2976 1.2 139 Sample #2 - broken pillow lava
10:49 2913 3.0 181 Sample #3 - well-preserved pillow lava
11:06 2877 4.7 188 talus made up of volcanic fragments covered with mud sediment
11:16 2807 2.7 197 Sample #4 - relatively large pillow lava outcrop
11:44 2668 4.0 205 strongly broken pillow lava outcrop
11:50 2654 5.9 191 talus with channel mark (debris flow?)
12:01 2602 3.4 188 Sample #7a, b - volcanic fragments into Mn coated talus
12:07 2602 3.4 188 talus, made up of subrounded volcanic fragment
12:18 2505 1.7 218 Sample #8a, b, c - subrounded volcanic fragements into the slope of first cone
12:29 2468 5.9 190 huge lava lobe with flow direction of N60?W
12:32 2444 4.0 189 southern flank of first cone covered with mud sediment
12:37 2435 2.4 210 sample #9 - aa-like lava covered with mud sediment
12:58 2415 1.3 181 summit of first cone, mud floor
13:47 2501 1.3 180 current ripple on muddy seafloor with Mn coating in small graben between first and second cones
14:03 2519 1.0 190 Sample #10 - broken pillow lava
14:09 2488 2.5 178 talus, made up of subrunded volcanic fragments on the slope of second cone
14:12 2481 1.1 173 Sample #11a, b, c - subrounded volcanic fragments into talus
14:30 2370 2.8 178 Sample #12a, b, c, d - subrounded volcanic fragments into talus
14:31 2353 3.4 188 Sample #13 - broken pillow lava
14:34 2350 4.4 178 broken pillow lava outcrop covered with subrounded volcanic fragments

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