Leg 2B (South arch and North Kona landslide)

K217: South arch lava(E. Takahashi)
Sept. 16, 2001

Purposeof Dive K217
The South Arch lava was first recognized during the GLORIA sonar survey in 1986. Lipman et al (1989) reported its first geologic observation using deep tow camera and analyzed dredged rock specimen. Two dredge hauls were conducted both at the western and eastern lava field and showed that there are small variation in petrology and chemistry of the lavas (West flow; alkali basalt, East flow; basanite). The primary purpose of this dive is to observe the geologic nature of the South Arch lava using high-resolution video camera on ROV and collect fresh rock specimens. Prior to the ROV dive, Seabeam bathymetry mapping was carried out together with ship based side scan sonar image. Observation route was chosen so that to traverse northern lava field towards a cone (with 100m high) locating at the center of the north, west and east lava fields.

Results of Dive K217
The ROV landed on so-called oldest lava flow field (Lipman et al., 1989) 200m north of the north lava field. The base composed of unconsolidated clay. A push core was taken. The dive proceeded to south heading ROV towards the northern lava flow. First small outcrops of bedded lava showing pull apart structure (1-5 m wide graven with vertical wall of 0.5-1 m high). They are thick Mn-coated and appeared as layered sediment. Sampling was attempted several times for these bedded rock but failed because they are soft. Second push core was taken to sample the specimen from the bedded sediment. Rock sample-1 was taken from one outcrop of rock fragments covered with thick mud probably is a lava tumulus. The recovered rock was vesiculated lava with severe alteration (probably of Cretaceous age).

As dive continues to south, series of outcrops appeared showing a sidewall of the graben. They are bounded by vertical normal faults with east-west strikes, each with several meters step down to the south. Total height of the wall is 30m judging from the depth monitor on the ROV. At the base of the graben, there were scattered piece of young lava fragments on the muddy surface. These scattered fragments represent marginal facies of the South Arch north lava flow. The scattered lava is irregularly shaped inflated pillow 1 to 3 m long (many of them are irregularly shaped such as cabbage or even like sea urchin with coils, spines, etc.). Sample-2 and -3 were taken from those isolated pillow fragments.

As dive continues to south, the density of the scattered pillow lava decreases and replaced by wrinkled surface of sheet flow. Finally smooth low ridge (2 to 4 m height) indicating natural levier of the South Arch north lava (sheet flow) appeared. Different from the earlier observation on western lava flow using deep two camera (Lipman et al., 1989) which showed little sediments on the lava flow, the surface of the north flow is mantled with some sediments and the surface of the central flow unit could not be seen. Average thickness of the lava is no more than 3 m judging from the depth profile of the ROV.

Near the center of the traverse of the north lava flow, the ROV passed a depression (1 km in NS and 0.5 km in EW and more than 20m in depth). The edge of the depression is sharp and the lava is flowing down slope into the depression. Winkled surface of the sheet lava was tried to sample inside the depression, but it was very fragile and could not be collected. Shape and texture of these materials are very similar to those on subaerial lava (skin of shelly pahoehoe lava). It is possible that the depression could be (one of) the crater (eruption site) of the north lava flow and the lava was drained back into the crater that was filled with magma (lava lake) during the eruption.
Near the end of traversing the lava field, the sheet lava changed to the marginal lava facies dominated by many discrete pillow blocks. Size and the shape of the inflated pillows are even more spectacular than those in the earlier. Rocks (No.5,6,7 8) were collected from the pillow blocks.

Finally the ROV came to the cone (130 m in height and 1 km across). As soon as we saw the video from the ROV, we understood that it is a Cretaceous seamount but not the volcanic cone related to the eruption of North Arch lava. Thickness of Mn-coating on pillow lava is more than 10 cm and they were very difficult to sample. In several outcrops, fractured surface of stacked pillow lava was observed. Judging from the relatively thin Mn-coating, these outcrops may have been formed relatively

Significance
The South Arch lava has been sampled for the first time with in-situ ROV observation. As first discovered by Lipman et al (1989), the lava was a thin sheet flow with less than a few meter in thickness. The edge of the thin sheet flow consists of numerous discrete lava spatters with various shape (inflated pillow to cabbage or even sea urchin shape). These lava spatter must have been formed as splash of very fluidal magma when it was emplaced. Absence of natural levee made of pillow lava (which is common in Arch type lava and was reported in the west flow of South Arch, Lipman et al, 1989) may indicate very fluidal condition of the magma. This is consistent with our interpretation that K217 site is in the vicinity of the eruption center of North Arch lava. Although the cone finally we visited was not the eruption center of the North Arch flow, it is likely that the calderas-like depression visited in the middle of K217 was one of the vents.

Numerous glassy fresh rock specimens were collected by ROV in K217. These specimen may have unique significance to elucidate the origin of South Arch at the front edge of the Hawaiian plume.

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