Dive Objectives:

Hana Ridge, submarine east rift of Haleakala volcano

E.Takahashi, Kevin Johnson and Susumu Umino

Introduction

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. During the 1999 JAMSTEC survey around Hawaiian Islands, first seabeam mapping was attempted at the distal end of Hana ridge (Smith et al., 2002). A noteworthy morphologic feature of the Hana Ridge is it’s arctuate, 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 of 2001 JAMSTEC Kaiko (K-212) was carried out in order to collect samples at the inner wall of the arcuate cliff to test this hypothesis. Another important seabeam based observation is that the Hana Ridge is made up of more than two diverging volcanic lineaments (see MBARI and JAMSTEC bathymetry maps). Two Hana Ridge dives in 2001 (K-214 on south flank and K-216 on north flank) sampled these two branches of the rift zone.　　Petrologic and geochemical study on the recovered rocks from these ROV-dives revealed that there is a chemical variation of Haleakala magma through time from Kilauea-type to Mauna Loa type (Ren, Takahashi, K.Johnson, 2002). Further study to verify the geochemical variation of Haleakala volcano is needed.

Year 2002 Dive targets
Five submersible dives were scheduled in Hana ridge and its adjacent area. Due to the bad weather, however, in total three submersible dives were made possible on Hana ridge.

S-686: Deep Southern end (K.Johnson)

The purpose of the dive was to observe the structure and morphology of the lava lobe on the basal floor of Hana ridge and those of the southernmost limb of Hana Ridge and assess whether the amphitheater at the ridge tip was formed by a landslide or by constructional volcanism. Both the basal lobe and the walls on the Hana ridge is found to be intact volcanic constructions and not formed by a landslide.

S-687: Cones on Hana rift axis (S.Umino)

Hana ridge has a number of volcanic cones that are exclusively flat topped. The only exception is a steep-sided cone at depth of 2300 m in the middle of the rift. Although a Kaiko dive in 2001 was conducted at the base of a flat-topped cone and sampled tholeiitic basalt, the steep-sided cone was not sampled. Dive 687 was carried out on the steep-sided cone on the middle rift zone and a flat-topped cone adjacent to the south of it to address these questions. The aim of this dive is to study origin of the morphological variation of the submarine cones and also to collect rock specimens that represent the final growth stage of Hana ridge.

S-691: Lower slope on north flank (E.Takahashi)

This dive was designed to visit the north flank of Hana ridge, submarine Haleakala volcano. Research purpose of this dive was two-fold: 1) to study nature of the geologic construction of the steep slope of the northern flank; 2) collect rock specimens representing the northern flank. Rock sampling was attempted to cover entire slope when combined with the ROV-Kaiko dive K216 in 2001 which covers from 3100 to 2300m water depth.

Scientific significance
The three Shinkai dives this year, together with the three ROV-Kaiko 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; 4) Investigate the geologic evolution of Hana ridge. 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.

Work plans
A research team was established for the Hana Ridge. A Seabeam bathymetric map will be made from data collected on our cruise. Gravity profiles across Hana ridge and along its ridge axis were carried out to determine its internal structure. 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.).