Leg 2A-2 (South Kona landslide)

K210: Lower bench of south Kona area (M. Nakagawa)
Setp. 6, 2001

DIVE SUMMARY AND RESULTS

Abstract
The South Kona landslide from Mauna Loa volcano can be divided into two, the outer and inner areas. The inner area has two broad terraces (benches) with calved smaller blocks, whereas about a dozen much larger blocks are distributed in the outer area. In order to reveal the structure of the bench, the dive K210 investigated the steep slope of the bench about 1000 m height. The main results are as follows:
(1) The slope is topographically divided into upper and lower parts. There exists the gentle slope between the both parts, where shows a wrinkle structure.
(2) The slope is mainly composed of debris flow deposits often with massive sandstone and hyaloclastite layers, striking NE and dipping E. These deposits show fractured structure, and jigsaw like crack is commonly observed.
(3) Blocks in the debris flow deposits are mainly lavas, showing subrounded shapes with various vesicularity. Oxidized blocks are also found. Glass rims of these lava blocks are rarely found, and no pillow lavas are recognized. These suggest that these lavas are derived from subaerial eruption. However, lavas in hyaloclastite are often glassy. MnO coating and paragonite layer are thin (<0.5 mm).

概要
　South Kona landslideは複雑な構造を示し、遠方の巨大ブロックが点在する地域と、より近傍にあるベンチと小規模ブロックからなる地域に分けられる。本潜航ではその中でベンチの構造を知るために、ベンチの西麓より約1000の高度差を上昇して調査・サンプリングを行った。この潜航により以下の成果を得た。
１） 地形的には1000ｍの崖は下部と上部に分けられ、両者の間には緩傾斜面が存在する。そこには崖の伸びと平行に振幅10−20ｍのしわ状の起伏が存在する。
２） 観察された急崖の断面はほとんどがdebris flowであり稀にマッシブ砂岩あるいは狭義のハイアロクラスタイトと互層する。層理面も認められ北東走行で東落である。これらは破砕を受けており、jig-sawクラックが普通に認められる。
３） 採取された溶岩の発泡度は様々で、角がとれた円磨されたものも多く、稀に酸化していることもある。枕状溶岩は認められなかった。これらは陸上噴出の溶岩と考えられる。ハイアロクラスタイト中にはガラス質の溶岩も含まれる。MnOコーテイングやパラゴナイト層は薄く0.5mm以下である。

Video Highlights
(1) 9:37 - 9: 48
Massive sandstone layer in the debris flows
(2) 11:01 - 11: 44
Continuous outcrop of debris flows occasionally with jigsaw crack
(3) 12:53 - 12: 55
Jigsaw cracks in debris flows
(4) 14:20 - 14: 28
Massive outcrop of hyaloclastite

Purpose of Dive K210
There are several landslides in the area of western Hawaii Island. These are topographically divided into 6, North Kona, Alika 1 and 2, South Kona, West Kalae and East Kalae. The South Kona landslide is largest and show complex topographically features. The landslide is divided into the inner and outer parts. The inner part consists of two broad terraces (each about 10 km wide and 20 km long) associated with a cluster of smaller blocks at west of the terraces.　On the other hand, the outer part is composed of about a dozen of much larger blocks than those in the inner part. Although Moore et al. (1995) showed results of two dredges and one dive for two blocks in the outer part, other data especially for the inner part has not been reported.
In the Leg 2-A, we could carry out two dives (K210 and -211) at the area of the Kona South landslide, in order to reveal the structure and genesis of the landslide and to get samples of early Mauna Loa volcano. On the basis of these data, we will be able to provide important information on evolution of a giant Mauna Loa volcano. The dive K210 focused on the structure of the terraces (mid slope bench) in the inner part of the landslide. The terraces are composed of deeper southern and shallower northern ones. We decided to dive at the western steep cliff of the northern terrace (see attached figure). The purposes of the dive are as follows:
(1) Observe and describe the constituent of the steep slope and reveal the geology of the northern terrace.
(2) Collect various types of samples from the terrace to determine the source of the landslide, and to investigate early volcanic history of the Mauna Loa volcano.

Dive Results
Kaiko landed at the depth of 3918 m, at the foot of the steep slope. The landed area is covered by sandy mud with scattering rock fragments. The first outcrop of massive rocks (sand stones) was found at the depth of 3894 m. Good exposures had been observed almost continuously from there to the top of the slope. The slope is divided into lower and upper parts, and gentle slope zone between the both parts is recognized at the depth of 3400~ 3380 m. The lower steep slope consists mainly of debris flow deposits with much less amount of massive sandstone and hyaloclastite. Although the upper slope is also composed of debris flow deposits, sandstone layers were rare. The debris flow deposits contain coarse breccias, and mega block (>several m). Bedding planes are observed and show NE strike dipping inward towards Mauna Loa volcano. These debris flow deposits are well fractured. Jigsaw like cracks are often recognized in both matrix and breccia, suggesting the deposits had been moved to emplace there. At the nearly top of the slope, hyaloclastite layer was developed covered by thick sediments. The dip of the layer is nearly flat. It is monolithologic and contains angular aphyric, glassy lavas in fine matrix. The Kaiko left the bottom at the depth of 2784 m.
Most of the breccia in the debris flows is subangular to subrounded lavas. Although some lavas have glassy rims, no pillow lava is observed. Porous lavas are often collected. Strongly oxidized lava is also found. These suggest that most of the lavas in the debris flow are subaerial. Some of these lavas are often rounded, suggesting that these had been eroded at around shoreline. These breccia had been occupied from the subaerial edifice of early Mauna Loa volcano, and debris flow had occurred to enter into the sea. In addition, it is also suggested that lava flows had often entered into the sea to produce hyaloclastite. Considering the common occurrence of jigsaw like cracks in the debris flow deposits, debris flow deposits, hyaloclastite and sandstone of the bench has been deformed and/or slided before their emplacement. After that, the bench has been covered by upper most hyaloclastite and sediments.
Lavas in the debris flow deposits are aphyric and porphyritic olivine basalts, whereas picrite is often recognized. These lavas are slightly altered. Thin MnO coating and paragonite layer are recognized on these lavas. However, the thickness of these is very thin (<0.5 mm). We collected 26 samples from 16 sites, and two of them were lost during ascending from the seafloor. The sampling sites are shown in attached map. Descriptions on board are listed in the appendix.

Reference
Moore, J.G., Bryan, W.B., Beeson, M.H. and Normark, W.R., 1995, Giant blocks in the South Kona landslide, Hawaii. Geology, 23, 125-128.

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