Results of Dive K215

Date: September 13, 2001
Place: Distal Hilo Ridge, 40 km east of Hawaii Island
Observer: Peter LIPMAN

Abstract
The dive traverse NW up a steep spur on the southern side of the distal Puna Ridge. All rocks exposed were basaltic pillow lavas, variably mantled by sand and talus. The lavas form two groups by composition and elevation: a lower sequence of olivine-bearing (5-10%) basalts to 3815 m depth (6 samples), and an upper group of coarsely porphyritic picrites (25-30% olivine).from 3777 m to the top of the dive at 3208 m (13 samples). Small-scale bottom morphology was far more complex than illustrated by the bathymetric map. Notably many pillow mounds rise 20-40 m above the general slope, probably marking sites of satellitic vents fed either by subsurface lava tubes or fractures generated by dike injection along the rift axis. The presence of such vents, along with the general coherence of lava compositions suggests that all the samples may have been generated by just two eruptive events (something to be tested by geochemical study. While much of the slope was characterized by virtually unbroken pillows and pillow lobes representing primary depositional surfaces, some steeper slopes expose mainly broken pillows and pillow rubble that likely represents the break-away scars from landslides and slumps. A particularly large slide scar is present along the southwest side of the traverse ridge for about 600 m laterally from about 3610 to 3575 m depth. Mn coatings on primary lava average about 1 mm; thicknesses are less in landslide-scar samples.

Purpose of Dive
The Hilo Ridge is one of the three major submarine rift zones that are well defined on Hawaii Island, along with Puna Ridge (Kilauea) and Ka Lae (Mauna Loa). The Hilo Ridge, previously interpreted as the submarine extension of an east rift zone of Mauna Kea, may alternatively be the east rift zone of a highly elongate large Kohala edifice (Holcomb et al, 2000; Kauahikaua et al., 2000); as such, it could also provide important samples of early Kohala growth. Basalt samples from the deep-water lower slope of the Hilo Ridge will have far better quality for petrologic study than subaerial tholeiite material from Kohala, and may provide the first reliable materials to characterize this large Kea-trend volcano. The distal site may provide abundant picrite samples for comparison with submarine rift zones of other volcanoes, and study of Mn coatings in comparison with Hana Ridge (Haleakala) and South Kona bench (Mauna Loa) may provide information on eruption ages. This dive will provide opportunities along the distal Hilo Ridge to make geological observations, collect rock samples, evaluate rift structure and morphology, and make petrologic correlations with source volcano.

Dive Results
The dive traversed commenced at 4051 on coarse sandy sediment (PC-1), but soon encountered coarse angular talus from basal outcrops of the ridge (sample R1). In contrast, sample R2 and most subsequent samples were from outcrop.
All rocks exposed were basaltic pillow lavas, variably mantled by sand and talus. The lavas form two groups by composition and elevation: a lower sequence of olivine-bearing (5-10%) basalts to 3815 m depth (6 samples), and an upper group of coarsely porphyritic picrites (25-30% olivine).from 3777 m to the top of the dive at 3208 m (13 samples). Some picrite samples (13A, 15) appear to contain slightly smaller-size olivine phenocrysts, but otherwise no hand-lens-detectable differences were noted. On this and other submarine rift ridges, it is becoming increasingly clear that picrites are far more common than for on-land portions of the same volcano.
Small-scale bottom morphology was far more complex than illustrated by the bathymetric map. Notably many pillow mounds rise 20-40 m above the general slope; these probably mark sites of satellitic vents fed either by subsurface lava tubes or fractures generated by dike injection along the rift axis. The presence of such vents, along with the general coherence of lava compositions suggests that all the samples may have been generated by just two eruptive events (something to be tested by geochemical study. All the pillowed hills, including the large flat-topped knob at the end of the dive, lie south of the main rift axis, which terminates or becomes diffuse at the dive site; perhaps this area marks the eastern limit of dike penetrations, thus contributing to magmatic inflation and fracturing in this area, and resulting complex diffusion of lava eruption sites. More such dive observations are needed on the deep flanks of young submarine rift ridges to test such hypotheses; thus, far most submarine rift-zone dives in Hawaii have been along the ridge crest (Puna--Kilauea) or along a fault-disrupted flank (Ka Lae--Mauna Loa).
In general, talus was relatively sparse along the dive track, in accord with the widespread exposure of little-broken primary depositional surfaces of pillow lavas. While much of the slope was characterized by virtually unbroken pillows and pillow lobes representing primary depositional surfaces, some steeper slopes expose mainly broken pillows and pillow rubble that likely represents the break-away scars from landslides and slumps. A particularly large slide scar is present along the southwest side of the traverse ridge for about 600 m laterally from about 3610 to 3575 m depth.
Shipboard magnetic data indicate that the overall central and distal Hilo Ridge is reversely magnetized (Naka et al., in press), and a special effort was made to collect pillow samples that could be reliably oriented to test whether deep surface lavas might show reverse magnetic polarity. Such mates.rial proved difficult to collect, because most pillows were massive, and little fractured, and cemented by Mn coating. One exceptionally good sample (R6) good samples was collected from the roof of a hollow pillow, containing dripstone stalagmites as an additional orientation indicator, may provide material for a valid paleopole direction.
Mn coatings on primary lava average about 1 mm; thicknesses are less in landslide-scar samples. In comparison with samples from dives on the Hana Ridge (Mn thickness up to 2 mm) and from the South Kona bench (thickness ~0.5 mm or less), some approximate age estimates should be possible for each of these submarine areas, calibrated by isotopic ages on subaerial lavas of comparable composition.

Video Highlights:
1025-1027: Sample hollow pillow of olivine basalt for paleomagnetic polarity determination
1035-1038: Top of pillow mound vent (sample R6), separated from main slope by a saddle
1040-1041- Top of another pillow mound vent
1132-1133: Top of a nice pillow mound
1135-1137: Pillow tubes on flank of another mound
1150-1151 Steep wall, with down-slope tubes; probable slumped flank of pillow mound
1212-1225: Steep wall, with near-horizontal broken tubes; probable large landslide scar; to NE are unbroken pillows on gentle primary surface (best seen by starboard camera)
1338-1348: Broadly flat upper slope of elongate ridge at 3270-3250 m that is less deep and more morphologically complex than shown by SeaBeam: probably marks an elongate fissure vent
1418-1421: Small sheet flows (lobate pillows) on a gentle slope above a steep scarp of broken pillows that is probably a landslide or slump scarp

Videolog of Dive K215