Leg 2B (South arch and North Kona landslide)

K219: Base of Mauna Loa (M. Coombs)
Sept. 18, 2001

Results of Dive K219

Date: September 19, 2001
Place: Deep underwater flank of Hualalai volcano, North Kona coast
Observer: Michelle Coombs
Co-observer: Pete Lipman
Advisors: Eiichi Takahashi, Toshiya Kanamatsu

Purpose of Dive K219
The deep underwater flanks of Hualalai volcano have not been previously studied. This dive was conducted, in tandem with K218, to investigate the nature of the mid-slope bench on Hualalai's submarine west flank. Dives on similar features on Kilauea, Kohala, Mauna Loa, and Waianae volcanoes have revealed sequences of volcaniclastic sandstones and breccias, though to be shed from the ancestral edifices of these volcanoes. K219 allows us to see whether Hualalai will follow this developing trend.
This dive site was chosen because of its location directly under the summit of Hualalai, thus increasing the likelihood of finding either volcaniclastics shed from the central edifice, or primary volcanic material associated with the central vent system. The dive track climbs a prominent rib on the mid-slope bench; several smaller benches are crossed as the track ascends the rib. The site for dive K218 is ~35 km north of this site, and the lavas observed there are likely a component of Hualalai's NW rift zone.

Summary of dive
K219's dive track ascends a series of small benches on the lower bench of North Kona. The Kaiko vehicle lands at a depth of -3566 m on a muddy slope. As we ascend the first bench, talus becomes visible, and at -3514, pillow lava outcrop appears. We observe broken, fragmented pillows until -3437, where a small section of sheet lava is crossed before gaining the flat, sedimented top of the bench. Two samples are collected on the face and a third at a small isolated outcrop on top of the bench (#s 1-3). All three are picritic pillow fragments.
The vehicle ascends the base of the second bench, reaching outcrop at -3293 m. Again, this bench is composed of fragmented pillows that appear very much like ones of the first bench. They have palagonitized exteriors and truncated ends. A few exhibit downslope flow direction, but they are scarce. Samples 4 and 5 are taken on the face of this bench; again, they are picritic pillow fragments. The sediment-covered top of the bench is reached at -3132 m. Sample six is taken from a flow outcrop on the flat top.
Again ascending, Kaiko soon passes into more pillow lavas. These are not as fragmented as the ones from the lower two benches, and have more downslope pillow tubes. The dive track traverses up the sides of two more subtle benches, during which time almost continuous outcrop of pillow lavas is observed. Samples 7, 8, and 9 are collected during this portion of the dive. These lavas, as well as sample 6, are olivine-rich basalts. At -2820 m, Kaiko ascends the side of a smaller well-defined bench; sample 10 is collected here. The pillows are particularly well-formed and tube-like. After crossing the left top of the small sediment-covered bench, pillow lavas are seen again at -2800 m, and sample 11 is collected. For the next 60 m of vertical rise, pillow outcrops are observed. Sample 12 is collected just before the slope decreases in angle and becomes sediment covered. At this point we reach the top of the main bench structure, and pass over sediment for 200 m , during which time a few scattered blocks of unknown origin are passed. At -2713 m, a breccia outcrop is observed and sampled (13a-c); Kaiko left the bottom just following their collection. These final samples are breccias (hyaloclastites) with clasts of vesicular crystal-poor lava in an oxidized porous matrix.

Geologic Interpretation
During this dive, we observed and sampled three units that are distinct in outcrop appearance and/or lithology: 1) fragmented pillow lavas, 2) less fragmented, downslope-elongate pillow lavas, and 3) breccia. The first two units are composed of olivine-rich to picritic, finely vesicular basalt lavas; further geochemical study will determine whether they are compositionally distinct. Clasts in the breccia vary in phenocryst content and vesicularity (some being extremely vesicular), though most have fewer olivine phenocrysts than the picritic pillow lavas. Sample12, collected below the breccia unit, presumably still in pillow outcrop, is in hand sample more like the clasts in sample 13 - fewer olivine and a rough, aa-like exterior.
The breccia itself is notable for its highly oxidized appearance. It is fines-poor and the matrix is primarily composed of clasts several mm in diameter. All matrix and larger clasts have oxidized rims ~2-3 mm in width. This fragmental material could have formed as aa lava entered the sea, or perhaps during a phreatomagmetic event. We identified very little, if any, glass in the breccia. Regardless of its exact origin, it is likely to be of subaerial origin given its oxidized appearance and highly vesicular clasts. In contrast, all of the pillow exposures were devoid of debris or breccia material, suggesting that the lavas have a completely submarine origin.
This dive illustrated that not all mid-slope benches of Hawaiian volcanoes are created equal. Unlike Hilina and other benches, the slope of the Hualalai bench is draped almost completely with pillow lavas. Perhaps these lavas drape the scarp left by the North Kona landslide, suggesting that deformation at Hualalai was concurrent with predominantly submarine eruption. Perhaps the lavas are analogous to the transitional lavas seen above the midslope bench of Hilina. Hualalai's bench is smaller, and perhaps more easily covered by later eruptions. Analysis of the volatile contents of the pillow glass will give an indication at the depths that the lavas were erupted, and chemical analysis will show whether the lavas were erupted early or late in the compositional evolution of the volcano.

Video Highlights of K219

Dive log

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