Submarine slope micromorphology and volcanic substructure of the Island of Hawaii inferred from visual observations made from U.S. Navy deep-submergence vehicle (DSV) “Sea Cliff”

Abstract

Based on visual observations made during forty-four submersible dives around the Island of Hawaii we are able to identify distinct morphological provinces which comprise the submarine slopes of the island. The micromorphology of each submarine slope is primarily a function of the distance from active volcanism, and age and structure of the adjacent subaerial volcanic shield. The ages of the submarine slopes around Hawaii progress from young submarine extrusive terrane off the east coast on Puna Ridge, clockwise to older carbonate-mantled submarine slopes and terraces off the northwest and northern coastlines. The transition between each submarine slope morphology is gradational and depends on the mechanical and chemical weathering of the pillow lavas and the structural setting of the adjacent shield. An exception to the clockwise age-progression of the submarine slopes was discovered west of Kealakekua Bay, midway along the west coast of Hawaii. Very fresh submarine lava flows and possible submarine vents overlie older submarine extrusive terrane and normal, volcanic rubble and sediment-covered submarine slope terrane. Extrapolation of lateral and vertical contact relationships, and submarine slope micromorphologies into the center of Hawaii, coupled with previously published geological and geochemical data allows us to construct a schematic model of the volcanic substructure of Hawaii. This model calls for the three-dimensional interfingering of volcaniclastic wedges with subaqueously extruded pillow lavas below and subaerial shield lavas above. Visual observations and our substructure model indicate that the major portion of the volcanic pedestal which underlies Hawaii is composed of pillow lava sequences.