The morphology of submarine volcanic terrain on the crest of the Puna Ridge, Hawaii, was observed during two manned submersible dives in water depths of as much as 2,000 m. Steep-walled linear ridges 30 m high, trending 060°, and composed of lava pillows and narrow fissures with the same strike were the principal volcanic features observed on the ridge crest. Lava tunnels, tubes, and pillows frequently were observed to be partially broken through, thereby exposing water-filled voids, which are inferred to be interconnected to great depths within the extrusive submarine volcanic pile. Low compressional wave velocities reported by other authors for the crustal layer composed of submarine volcanic extrusives and the low effective density of this layer, as determined from published surface gravity observations, are attributed to the large intraflow and interflow porosity of submarine volcanic terrain. Areal variations in heat flow through the crust of submarine volcanic features are also attributed to the high porosity and consequent permeability of submarine volcanic terrain which is likely to persist to the bottom of the submarine extrusive pile.

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