THE FORMATION OF CIRCULAR LITTORAL CONES FROM TUBE-FED PAHOEHOE - MAUNA-LOA, HAWAII

Pyroclastic cones along the southwest coast of Mauna Loa volcano, Hawa i'i, have a common structure: (a) an early formed circular outer rim 2 00-400 m in diameter composed mostly of scoria and lapilli, and (b) on e or more later-formed inner rims composed almost exclusively of dense spatter. The spatter activity locally fed short lava flows that ponde d within the outer rims. Based on various lines of evidence, these con es are littoral in origin: relationships between the cones and associa ted flows; the degassed nature of the pyroclasts; and (although not un equivocal) the position of the cones relative to known eruptive vent l ocations on Mauna Loa. Additional support for the littoral interpretat ion comes from their similarity to (smaller) littoral cones that have been observed forming during the ongoing Kilauea eruption. The structu re of these Mauna Loa cones, however, contrasts with that of ''standar d'' Hawaiian littoral cones in that there is (or once was) a complete circle of pyroclastic deposits. Furthermore, they are large even thoug h associated with tube-fed pahoehoe flows instead of 'a'(a) over bar. The following origin is proposed: An initial now of tube-fed pahoehoe into the ocean built a lava delta with a base of hyaloclastite. Collap se of an inland portion of the active tube into the underlying wet hya loclastites 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 wa ter diminished, the explosions declined in intensity to spattering, an d the initial rim was filled with spatter and lava.