The formation of deep-sea Limu o Pele
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Deep-sea sheet hyaloclastite consists mostly of sand-sized blocky and splinter-shaped shards, but also contains subordinate mm- to cm-sized thin, curved, and wrinkled plates and sheets of sideromelane. This latter type of shard, termed Limu o Pele, has been observed forming subaerially on Kilauea by entrapment of water in flowing lava followed by expansion of steam to form large bubbles which burst into thin fragments. Deep-marine limu has been inferred, from comparative morphological studies and assessment of physical bubble-forming conditions, to form in a similar way but with the increased ambient pressure and the higher viscosity of water reducing bubble expansion. Differing mechanisms of heat transfer and rates of magma chilling also modify the limu-forming process in the deep sea. This paper evaluates a variety of deep-sea limu-forming processes and develops a new and quantitatively supported model, based on observed limu-forming processes and criteria derived from dive samples and observations at Seamount Six, Cocos Plate. It is inferred that water and/or water-saturated sediment was trapped in extremely thin, fluid and rapidly advancing lava flows by various processes. Bubble formation might also occur during small-scale magma-fountaining driven by magmatic volatile exsolution and extreme vent constriction or during collapse of pillows and rapid drainage of the magma, but we found no deposits clearly resulting from these processes.
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