The formation of circular littoral cones from tube-fed pāhoehoe: Mauna Loa,Hawai'i |
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| Authors: | Zinzuni Jurado-Chichay Scott K Rowland George P L Walker |
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| Institution: | (1) Hawai'i Center for Volcanology, University of Hawai'i at M noa, 2525 Correa. Road, 96822 Honolulu, Hawai'i, USA;(2) Department of Geology and Geophysics, University of Hawai'i at Mnoa, 2525 Correa. Road, 96822 Honolulu, Hawai'i, USA;(3) Hawai'i Center for Volcanology, University of Hawai'i at Manoa, 2525 Correa Road, 96822 Honolulu, Hawai'i, USA;(4) Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Manoa, 2525 Correa Road, 96822 Honolulu, Hawai'i, USA |
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| Abstract: | Pyroclastic cones along the southwest coast of Mauna Loa volcano, Hawai'i, have a common structure: (a) an early formed circular outer rim 200–400 m in diameter composed mostly of scoria and lapilli, and (b) one or more later-formed inner rims composed almost exclusively of dense spatter. The spatter activity locally fed short lava flows that ponded within the outer rims. Based on various lines of evidence, these cones are littoral in origin: relationships between the cones and associated flows; the degassed nature of the pyroclasts; and (although not unequivocal) the position of the cones relative to known eruptive vent locations on Mauna Loa. Additional support for the littoral interpretation comes from their similarity to (smaller) littoral cones that have been observed forming during the ongoing Kilauea eruption. The structure of these Mauna Loa cones, however, contrasts with that of  standard Hawaiian littoral cones in that there is (or once was) a complete circle of pyroclastic deposits. Furthermore, they are large even though associated with tubefed p hoehoe flows instead of 'a'. The following origin is proposed: An initial flow of tube-fed phoehoe into the ocean built a lava delta with a base of hyaloclastite. Collapse of an inland portion of the active tube into the underlying wet hyaloclastites or a water-filled void allowed sufficient mixing of water and liquid lava to generate strong explosions. These explosions broke through the top of the flow and built up the outer scoria/lapilli rims on the solid carapace of the lava delta. Eventually, the supply of water diminished, the explosions declined in intensity to spattering, and the initial rim was filled with spatter and lava. |
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| Keywords: | Littoral cones Mauna Loa Volumetric flow rate Lava tubes Lava-water interaction Lava delta Bench collapse |
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