2-6. Hana ridge 　[K. Johnson]

During the 2001 Hawaiian cruise of the JAMSTEC research ship Kairei, we sampled and surveyed with Seabeam the distal portions of the Hana Ridge. Hana Ridge is the submarine extension of the east rift zone of Haleakala volcano on Maui. It extends 140 km from the eastern shoreline of Haleakala, making it the longest submarine rift zone in the major Hawaiian Islands
.
A noteworthy morphologic feature of the Hana Ridge is its arcuate, bifurcated eastern tip. Seabeam bathymetry of this feature reveals a 25 km-wide amphitheater-shaped inner wall. Bathymetric contours of the inner wall of this structure are remarkably evenly spaced and regular and define a slope of ~25° over a distance of nearly 3 km. This morphology has led to the idea that the amphitheater was formed by a large landslide, and the first dive site was planned in order to collect samples to test this hypothesis.

Another important observation is that the Hana Ridge is made up of two diverging volcanic lineaments (see MBARI and JAMSTEC bathymetry maps). The other two Hana Ridge dives sampled these two branches of the rift zone.

Year 2001 Dive targets.
Three ROV Kaiko dives, K212, K214, and K216, were carried out on the Hana Ridge during the KR01-12 expedition. The dive track for ROV dive K212 (20°32.5'N, 154°56.2'W) was designed to traverse the inner N wall of the arcuate tip of Hana Ridge over the depth range 4850 - 3600 m. The purposes of this dive were twofold: 1) to characterize the deposits and the surface of the inner wall of this arcuate amphitheater, and 2) to collect a stratigraphic section of lavas comprising the entire thickness of the lower Hana Ridge in order to characterize the age and composition of the stratigraphic sequence of lavas. The dive traversed alternating broad fields of featureless pelagic mud, grading into zones of scattered, angular pillow +/- turbidite talus, and then steep outcrops of broken pillow flows. This sequence repeated itself several times up the slope through the end of the dive. The pelagic sediment zones corresponded to areas of low slope angle and the outcrops corresponded to areas of high slope angle. In places, semi-indurated MnO encrusted turbidite sediments mantled the surface to depths of 25-30 cm and often incorporated angular blocks of pillow talus. MnO thickness was about 1 mm.

ROV dive K214 (20°28.1'N, 155°16.05'W) collected samples from south to north on the south-facing flank of the Hana Ridge at a depth range of 4450 - 3050 m. The purpose of the dive was to characterize the age and composition of volcanic rocks from the south branch of the rift zone and to describe the nature of surface flows. Compared to dive K212, the outcrops of K214 comprised more intact pillows and elongated pillow flows. Although some zones of brecciated lavas were observed and sampled, the majority of the outcrops were intact pillows.

ROV dive K216 (20°38.7'N, 155°08.0'W) was laid out in a north-to-south direction on the north-facing flank of the Hana Ridge between 3200 m and 2300 m water depth. The purpose of this dive was to characterize the age and composition of volcanic rocks from the north branch of the rift zone and to describe the nature of surface flows. Based on sonar backscatter data (Smith et al., 2001 AGU Monograph), we expected the lavas of this branch of the rift to be the youngest sampled. However, the lavas did not appear to be noticeably younger than the other dive sites, and in fact, fairly thick pelagic sediment covered large areas of the low-angle slopes. The rock outcrops of K216 comprised intact pillows and elongated pillow flows with talus piles at their base. We passed over the top of one flat-topped cone at 2450 m and found it to be entirely sediment covered.
In addition to the three ROV dives on the Hana Ridge, one single-channel seismic line was run from east to west into the amphitheater to observe the nature of the sediment and debris deposits.

Scientific significance
The three dives and one seismic line carried out on the Hana Ridge will allow us to: 1) Evaluate the hypothesis that a large landslide formed the arcuate tip of the Hana Ridge; 2) Characterize the stratigraphic progression of lava compositions in the three sections of the rift zone; 3) Determine eruption ages of the lavas in the three sections of the rift zone. Together, these objectives will provide a strong foundation for understanding the age and geologic history of the rift zone and will allow us to view the entire Haleakala volcanic system as a genetic unit.

Overall, the morphology of the inner wall scarp deposits is consistent with the hypothesis that the amphitheater was formed by a large landslide. The sequence of events that is recorded in the sampled deposits is as follows: 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 unsorted, polygenetic sediments which then mantled 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 turbidites and rock surfaces. Soft pelagic sediments accumulated on top of these slide deposits, and occasional slope failures ensued, breaking the mantling turbidity deposits and mantled rock surfaces further at localized sites, exposing new rock outcrops and fragmenting the semi-indurated turbidite blanket. Later stage small-scale volcanic eruptions produced slope-conforming intact, elongated pillow lava flows.

Work plans
A research team was established for the Hana Ridge. A Seabeam bathymetric map will be made from data collected on our cruise. A single channel seismic profile to test the sediment in the mouth of the arcuate tip of the ridge has been acquired and will be used to evaluate the landslide hypothesis for the formation of this unusual arcuate amphitheater. Major and trace elements of the lavas, mineralogy and petrology of the recovered rock samples, combined with isotopes will be carried out. Eruption ages of lava flows from the Hana Ridge will be measured by a number of methods (i.e., stratigraphy, sediment thickness, thickness of Mn-coating, K-Ar and Ar/Ar dating, etc.).