Formation of submarine flat-topped volcanic cones in Hawai'i

High-resolution bathymetric mapping has shown that submarine flat-topped vo lcanic cones, morphologically similar to ones on the deep sea floor and nea r mid-ocean ridges, are common on or near submarine rift zones of Kilauea, Kohala (or Mauna Kea), Mahukona, and Haleakala volcanoes. Four flat-topped cones on Kohala were explored and sampled with the Pisces V submersible in October 1998. Samples show that flat-topped cones on rift zones are constru cted of tholeiitic basalt erupted during the shield stage. Similarly shaped flat-topped cones on the northwest submarine flank of Ni'ihau are apparent ly formed of alkalic basalt erupted during the rejuvenated stage. Submarine postshield-stage eruptions on Hilo Ridge, Mahukona, Hana Ridge, and offsho re Ni'ihau form pointed cones of alkalic basalt and hawaiite. The shield st age flat-topped cones have steep (similar to 25 degrees) sides, remarkably flat horizontal tops, basal diameters of 1-3 km, and heights <300 m. The fl at tops commonly have either a low mound or a deep crater in the center. Th e rejuvenated-stage flat-topped cones have the same shape with steep sides and flat horizontal tops, but are much larger with basal diameters up to 5. 5 km and heights commonly greater than 200 m. The flat tops have a central low mound, shallow crater, or levees that surrounded lava ponds as large as 1 km across. Most of the rejuvenated-stage flat-topped cones formed on slo pes <10 degrees and formed adjacent semicircular steps down the flank of Ni 'ihau, rather than circular structures. All the flat-topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years t o decades. These, and other submarine volcanic cones of similar size and sh ape, apparently form as continuously overflowing submarine lava Tends. A la va pond surrounded by a levee forms above a sea-floor vent. As lava continu es to flow into the pond, the lava flow surface rises and overflows the low est point on the levee, forming elongate pillow lava flows that simultaneou sly build the rim outward and upward, but also dam and fill in the low poin t on the rim. The process repeats at the new lowest point, forming a circul ar structure with a flat horizontal top and steer pillowed margins. There i s a delicate balance between lava (heat) supply to the pond and cooling and thickening of the floating crust. Factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents , moderate to high confining pressure at moderate to great ocean depth, lon g-lived steady eruption (years to decades), moderate effusion rates (probab ly ca. 0.1 km(3)/year), and low, but not necessarily flat, slopes. With hig her effusion rates, sheet flows flood the slope. With lower effusion rates, pillow mounds form. Hawaiian shield-stage eruptions begin as fissure erupt ions. If the eruption is too brief, it will not consolidate activity at a p oint, and fissure-fed flows will form a pond with irregular levees. The pon d will solidify between eruptive pulses if the eruption is not steady. Lava that is too volatile rich or that is erupted in too shallow water will pro duce fragmental and highly vesicular lava that will accumulate to form stee p pointed cones, as occurs during the post-shield stage. The steady effusio n of lava on land constructs lava shields, which are probably the subaerial analogs to submarine flat-topped cones but formed under different cooling conditions.