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

K212: East of Hana ridge (K. Johnson)
 Sept. 9, 2001

Results of Dive K212

Date: September 9, 2001
Place: Inner wall of arcuate shaped eastern end of Hana Ridge
Observer: Kevin Johnson
Back-seat drivers: Eiichi Takahashi, Peter Lipman

Purpose of Dive K212
The Hana Ridge is the 140 km-long submarine extension of Haleakalafs east rift zone and is the longest submarine rift zone in the Hawaiian Islands. The eastern tip of the Hana Ridge is characterized by a unique, 25 km-wide arcuate-shaped amphitheater-like termination which bifurcates the ridge into northern and southern extensions. Seabeam contours of the inner wall of this structure are remarkably regular and define a slope of ~25‹ over a distance of nearly 3 km and its morphology has led to the idea that the amphitheater was formed by a large landslide. The purposes of this dive were twofold: 1) to characterize the deposits and the surface of the inner wall of this arcuate amphitheater to test the hypothesis that this feature was formed by a landslide, and 2) to collect a stratigraphic section of lavas comprising the entire thickness of the lower Hana Ridge.

Dive Results
The dive began on soft sediments on the lower slope of the amphitheaterfs inner wall at a depth of 4855 m. A push core was collected at the start of the dive in the soft sediments. No rock outcrops were visible, but scattered angular rock fragments were observed over the first 120 m of the dive track. Attempts at sampling the rock fragments, which appeared to be lying loosely on the sediment, proved to be difficult throughout the dive due to strong MnO cementation of the surfaces. However, talus samples 1a and 1b were successfully collected at 4773 m water depth.
The first rock outcrop was encountered 120 m into the dive. Samples 2a, b, and c were collected at 4747 m depth from the base of this outcrop. The outcrop was a wall of broken pillows exposing cross-sectional surfaces. There were no intact pillows observed here, but at 4730 m water depth, an outcrop consisting of elongated pillow lavas oriented downslope was observed. Proceeding along the dive track, the outcrop graded into talus with a thin sediment dusting and then into mainly soft muddy sediment with angular, blocky talus consisting of pillow fragments and semi-indurated turbidite sediments coated with 1 mm rinds of MnO. After traversing talus and sediment over several hundred meters of relief, a second, and final, outcrop of oriented, intact elongated pillows was observed at 4489 m water depth. With the exception of these two outcrops of intact elongated, oriented pillow flows, the entire inner wall of the amphitheater consisted of repeated sequences of relatively low slope-angle areas of soft sediment containing angular fragmented pillow and turbidite talus, grading into dense talus piles at the base of outcrops of broken pillow lava flows exhibiting beautiful cross-sections. At several places along the traverse (e.g. 12:25 pm, 4306 m depth), the lava flows were observed to dip into the slope, the slope face truncating the lava flows at a high angle. The steep broken pillow outcrops were often capped by semi-indurated, MnO-encrusted turbidites 25-30 cm thick, either intact or broken into large plates that slid downslope. These turbidite deposits often contained blocks of pillow fragments embedded within them, indicating coeval deposition.
Overall, the morphology of the inner wall deposits is consistent with the hypothesis that the amphitheater was formed by a large landslide. The sequence of events that is recorded in the deposits is that the original surface consisted of pillow lava flows produced at the submarine rift zone. Slope failure occurred and the arcuate amphitheater was formed by a landslide that produced large turbidity currents, which then blanketed the excavated surface. This surface remained relatively undisturbed for a long enough period of time to allow a 1 mm-thick MnO coating to precipitate over the turbidite and rock surface, perhaps up to 1 my. Soft pelagic sediments accumulated on top of these slide deposits, and occasional slope failures ensued, breaking the blanketed surfaces further at localized sites, exposing new broken rock outcrops and fragmenting the semi-indurated turbidite blanket. Later stage small-scale volcanic eruptions produced slope-conforming intact, elongated pillow lava flows.
A total of 27 samples were recovered from 13 sampling sites. A push core sample was obtained from the flat at the landing point. This sample may represent sand cover material observed along the dive, and also represent the recent sediment cover of the inner wall of the amphitheater. Lava samples were primarily picritic basalt with >20 vol% olivine and plagioclase-olivine basalt with </= 10 vol% phenocrysts. Vesiculation is generally less than several percent. Olivines in picrites reached 5 mm in size. The lava samples are either sections from pillow lava lobes or fragments of pillow lava talus. Interiors of pillows were massive and compact with small vesicles that become larger toward the pillow surface, sometimes being elongated perpendicular to the cooling surface. Normally, the outer one to several centimeters of pillows are very fine grained and some samples contain glass rinds 1-2 millimeters thick. Surfaces of the pillows were invariably coated with MnO, sometimes up to 1 mm in thickness. However, interiors of the pillow fragments were generally only mildly weathered and showed only minor alteration. Several samples consisted of angular fragments of semi-indurated ungraded turbidite sands. These fragments were collected together with angular basalt fragments in talus piles in the lower part of the section. Sand grains appear to consist of mixtures of palagonite and fairly fresh glass shards cemented together with palagonitic clays and muds. The turbidite talus blocks are derived from the 25-30 cm blanket of turbidites mantling the brecciated pillow lavas and broken pillow outcrops.

Video highlights of Dive K212
time depth (m) altitude (m) heading (degree) Narrative
10:07 4773 2.3 337 Sample #1a,b - MnO-cemented talus in soft mud
10:17 4747 2.2 345 Sample 2a, 2b, 2c - fractured outcrop and talus in soft mud
10:19 4736 4.1 330 outcrop of elongated, oriented pillow basalts draping the slope
11:00 4568 4.1 319 Sample 3a, 3b - blocky, MnO-cemented talus
11:20 4496 4.3 275 Sample #4; scattered blocky talus and pillow fragments on crest of NNW-trending ridge
11:23 4489 4.5 331 broken surface of pillow outcrop w/elongated, oriented pillows
11:28 4474 324 Mn-encrusted semi-indurated slabs of sediment mantling surface have slid down slope, sample #5
11:47 4402 4.6 329 Sample #6a, 6b - broken pillow lava outcrop, pillow cross-sections exposed in cliff face, capped by semi-indurated slabs of mantling sediment
12:17 4319 2.4 308 Sample #7 - broken pillow outcrop
12:25 4306 4.3 330 Truncated flows appear to dip into the slope; slope face truncates the flow layers at a high angle.
12:30 4281 3.4 330 pillow outcrop on the sediment, the NE slope truncates the flows
12:36 top of ridge; large flattened lobate pillows
12:42 4219 2.3 326 sample #8 from broken pillow outcrop
13:31 4038 2 302 sample #9 - angular pillow talus
13:44 3945 3.3 304 sample #10 - pillow lava outcrop; broken pillow x-sections
14:21 3739 316 sample #11a,b,c - angular pillow talus
14:32 3666 2 306 sample #12a,b,c - angular pillow talus
14:38 3619 3.8 333 sample #13 - broken pillow outcrop
14:41 3614 5.8 334 end of dive - mixture of broken pillow outcrop and angular pillow talus; leave bottom.
time depth (m) altitude (m) heading (degree) Narrative Sample
9:40 4716 300 197 Kaiko detaches from launcher
9:47 4837 30 339 muddy bottom with scattered rocks, push red core in muddy bottom PC 1
9:52 4828 3.2 338 muddy bottom with scattered rocks
9:55 4786 2.9 331 muddy bottom with scatted rocks
9:58 4775 4.7 327 muddy bottom with scatted rocks, attempting to sample
10:00 4776 2.3 337 heavily cemented talus blocks - no sample
10:07 4773 2.3 337 sample#1a,b - MnO-cemented talus in soft mud 1a, 1b
10:14 4747 332 outcrop of pillow
10:17 4747 2.2 345 sample2a, 2b, 2c 2a, 2b, 2c
10:20 4736 4.1 330 outcrop of elongated, oriented pillow basalts draping the slope
10:22 4707 4.4 330 outcrop of pillow covered with thin muddy sediment
10:25 4686 2.8 330 outcrop of pillow, with steep slope
10:33 4652 3.2 295 outcrop of pillow, attempt to sample
10:43 blocky talus, MnO cemented
10:50 4592 2.9 330 outcrop of pillow in thin muddy sediment
10:56 4581 1.9 313 scattered blocky talus on muddy sediment
11:00 4568 4.1 319 sample3a, 3b 3a, 3b
11:04 4527 2.3 330 scattered blocky talus and pillow fragments on muddy sediment, steep NE slope
11:20 4496 4.3 275 sample#4; scattered blocky talus and pillow fragments on crest of NNW-trending ridge 4
11:23 4489 4.5 331 broken surface of pillow outcrop w/elongated, oriented pillows
11:25 4482 1.7 330 slope covered with mud
11:28 4474 324 Mn-encrusted semi-indurated slabs of sediment mantling surface have slid down slope, sample #5 from sediment block with Mn coating , 10 cm abyssal clay 5
11:41 4436 3.6 329 many angular blocks on muddy floor; large pillow boulders
11:47 4402 4.6 329 sample#6a, 6b(large), from pillow lava outcrop 6a, 6b
11:50 pillow lava outcrop, broken pillow cross-sections
11:54 4376 330 mantling sediment-covered pillow lava
11:56 4370 2.6 332 muddy floor
12:00 4355 2.5 329 muddy flat floor
12:03 4347 1.7 329 muddy floor
12:08 4343 1 332 muddy floor
12:12 4337 1.7 330 muddy floor with some scattered rocks
12:15 4327 2.4 329 outcrop of pillow
12:17 4319 2.4 308 sample#7 7
12:25 4306 4.3 330 cemented talus
12:30 4281 3.4 330 pillow outcrop on the sediment, the NE slope truncates the flows
12:42 4219 4.1 331 top of ridge; large flattened lobate pillows
12:44 4219 2.3 326 sample #8 from pillow outcrop 8
12:46 4211 2.7 327 many rock fragments on muddy floor
12:50 4197 1.8 327 brecciated pillow talus on W-facing slope
12:57 4160 3.6 330 soft sediment at base of slope
13:02 4150 3.3 331 a shrimp
13:06 4128 3.2 330 pillow talus embedded in partly indurated mantling sediment
13:10 4118 2.9 329 pillow talus on muddy bottom
13:12 4108 3.2 329 muddy bottom with gravels
13:14 4103 2.6 330 pillow talus on muddy bottom
13:16 4094 2.2 329 KAIREI at a speed of 0.5 knot
13:19 4085 2.7 327 some blocks scattered on muddy bottom
13:21 4081 2.1 329 muddy floor
13:22 4077 2.7 328 partly indurated mantling sediment
13:23 4070 2 328 some angular rock fragments on muddy bottom
13:29 4043 4.8 330 many rock fragments on muddy floor
13:31 4038 2 302 sample #9 put in box 9 (15cm*15cm) 9
13:34 4024 2.4 330 many angular blocks on muddy floor
13:35 4017 3.8 329 many angular blocks on muddy floor, the slope down rightward
13:40 3976 3.5 313 pillow lava outcrop; broken pillow x-sections in face of cliff
13:43 3948 5.6 311 pillow lava outcrop at top of slope
13:45 3945 3.3 304 sample #10 (20cm*8cm) from pillow lava outcrop 10
13:47 3942 2.4 316 sediment covered pillow lava
13:48 3927 2.1 308 muddy floor
13:51 3901 3.5 307 linear exposure of talus embedded in mantling sediment
13:52 3896 3.6 304 layered sediment with scattered rock fragments
13:53 3887 2.8 310 rock fragments in a line
13:56 3872 2.1 310 muddy flat floor
13:58 3862 3.6 287 angular blocks on muddy bottom
14:07 3814 3 319 muddy floor with some scattered angular fragments
14:15 3767 2.5 319 muddy bottom with many angular rock fragments
14:19 3740 313 outcrop of pillow
14:21 3739 316 sample #11a,b,c - blocky pillow talus 11a, 11b, 11c
14:25 3721 2.6 318 rock fragments with sediment
14:26 3705 3.2 319 scattered angular rock fragments on muddy bottom
14:29 3675 2.3 319 abundant angular pillow talus
14:32 3666 2 306 sample #12a,b,c - angular pillow talus 12a, 12b, 12c
14:35 3656 4.2 321 angular talus
14:36 3645 3.6 320 many blocks and fragments
14:38 3619 3.8 333 sample #13 - broken pillow outcrop 13
14:40 3614 5.8 334 end of dive, leave bottom

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