2-2. Newly recognized volcano southwest of Oahu �@[E. Takahashi]

During the 2001 Hawaiian cruise of the JAMSTEC research ship Kairei, Seabeam mapping revealed a previously unidentified volcanic edifice (submarine shield) located about 100 km southwest of Oahu (here called SW Oahu volcano). The SW Oahu volcano (centered at 21�‹35�fN, 158�‹45'W) is approximately 100 km in diameter and 0.5 km high with its summit at 4200 m depth. Near the top of the volcano, a lava flow field with high reflectivity in the GLORIA image had been previously reported (Moore et al., 1989) but the presence of the shield volcano was not known.

The low submarine shield is studded with numerous flat top cones typically less than 100m in height and several km across (similar to those described by Clague et al, 2000). In addition, more than 30 steep cones (circular to irregular in shape; typically 300 to 500 m in height) are distributed over the submarine shield volcano. Much of the east side of the volcano is mantled by thick sediment perhaps derived from landsliding of Waianae volcano. The maximum thickness of such material is more than a few hundred meters. Accordingly, the flat top cones are not visible (if present) and only some steep cones are exposed on the east side.

Year 2001 Dive targets.
ROV dive K203 (20�‹40.0'N, 158�‹51.5'W) collected samples from the high-reflectivity lava flow shown on the GLORIA image as well as from one of the steep cones. Judging from the thin sediment and the thickness of the Mn-coating (1-2 mm), the high-reflectivity lava flow may be similar in age to the North Arch alkalic lavas (0.5 to 1.5 Ma, Clague et al., 1990). The steep cone consists of vesiculated pillow lava and hyaloclastite and is apparently older than the flow, judging from the thick sediment cover and the Mn-coating (up to 6 mm) similar to that on the north slope of the ca. 3 Ma old Koolau volcano (Shinozaki et al., 2001). The high vesicularity of some of the lavas (collected at depths of 4000 m) indicates a high volatile content and almost certainly an alkalic basalt composition. Dive K206 (20�‹39.0'N, 158�‹47.5'W) on one of the flat top cones imaged and sampled dense aphyric pillow lava (also with thick Mn-coating) cascading down the steep flank. Preliminary data collected from the recently completed cruise indicates that this newly mapped submarine Hawaiian shield volcano (2000-4000 km3) is similar in age to those on Oahu Island (Waianae and Koolau: 2-4 Ma), and apparently grew during several stages of magmatic activity.

Scientific significance
From the geographical position, SW Oahu volcano could be contemporaneous with, and related to Waianae or Koolau volcanoes on Oahu island. Alternatively, it could be one of the eruption centers of the arch-type magmas similar to North Arch (Clague et al. 1990) and South Arch (Lipman et al., 1989). If this volcano is a member of the tholeiitic shield type, its location should indicate that the central magma feeding zone of the Hawaiian plume at 2 to 4 Ma was much larger than previously thought. On the other hand if the southwest Oahu volcano as a whole is an eruption center of the arch type magmas, its location centered somewhat inside the axis of the Hawaiian Arch requires a revision of our models for the Arch volcanoes; that is, that they form by eruption of magma ponded in the crust beneath the Arch axis. The extremely low shield nature of this submarine volcanic edifice indicates lavas of low viscosity in the submarine environment. This fact, and the apparent alkalic (high volatile) character of the lava support the Hawaiian Arch setting for the volcano. The low shield shape of the volcano compared with other arch type volcanic fields with low-relief lava on the flat ocean floor for both North and South arches suggests that the SW Oahu volcano was a more productive, long-lasting magmatic center with more spacially restricted vents.

In any case, the discovery of a volcano with a volume of more than several thousand cubic kilometers is an important episode in the study of Hawaiian geology. Further study to elucidate its geology, eruption age, petrology and geochemical nature will all be the object of concerted effort.

Work plans
A research team was established for the SW Oahu volcano. A Seabeam bathymetric map will be made during our cruise. A single channel seismic profile to test the sediment coverage on the northwest flank of the volcano has been acquired. 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 series of lava flows in the SW Oahu volcano will be examined by a number of methods (i.e., stratigraphy, sediment thickness, thickness of Mn-coating, K-Ar and Ar/Ar dating, etc.)

References
Clague, D.A., Holcomb, R.T., Sinton, J.M., Detrick, R.S., and Torresan, M.E., Pliocene and Pleistocene alkalic flood basalts on the sea floor north of the Hawaiian islands, Earth and Planetary Science Letters, 98, 175-191, 1990.
Clague, D.A. Moore, J.G. and Reynolds, J.R. Formation of submarine flat-topped volcanic cones in Hawaii. Bull. Volcanol., 62, 214-233, 2000.
Lipman, P.W., Clague, D.A., Holcomb,R.T., South Arch volcanic firld- Newly identified young lava flows on the sea floor south of the Hawaiian ridge. Geology, 17, 611-614, 1989.
Moore, J.G., Clague, D.A., Holcomb, R.T., Lipman, P.W., Normak, W.R. and Torresan, M.E., Prodigious submarine landslides on the Hawaiian ridge, Journal of Geophysical Research, 94, 17465-17484, 1989.
Shinozaki, K., Ren, Z.Y. and Takahashi, E. Petrology of landslide blocks in Northeast of Oahu, Hawaii. In E.Takahashi et al eds. AGU Monograph, Evolution of Hawaiian volcanoes: Deep Underwater Perspectives., 2001.