Apparently ‘dry’ littoral rootless cones in Hawaiʻi formed by sustained, “confined” mixing of lava and sea water

Abstract

Hydrovolcanic rootless cones form through the explosive interaction between lava and external water at locations remote from primary vents. Littoral rootless cones such as those on the coast of Hawaiʻi form at the ocean entries of basaltic lava flows and most consist dominantly of thinly bedded ash and lapilli. In contrast, the littoral rootless cones at ʻAuʻau Point and South Puʻu Kī Cone on the southwestern flank of Mauna Loa, Hawaiʻi consist of alternating layers of clastogenic lavas and thermally oxidised, tack- to moderately welded lapilli and coarse bombs with little to no ash. The pyroclastic layers have characteristics traditionally thought to be indicative of ‘dry’ explosive magmatic activity at primary vents, including the high degree of sorting in fall deposits, the paucity of ash and quench textures, extensive thermal oxidation, a high degree of welding, dominantly fluidal pyroclastic morphologies and association with clastogenic lavas. We propose that littoral rootless activity at both cones was driven by the “confined mixing” of tube-fed lava and sea water. In this configuration, the mingling of water and lava is complicated, resulting in relatively inefficient Molten Fuel Coolant Interactions (MFCIs). Cycles of more vigorous (littoral lava fountains) and less vigorous (lava bubble bursts) activity produced the alternating layers of clastogenic lava and welded bombs and lapilli. These cycles probably reflect the local depletion and replenishment of sea water injected into the lava tube. Similar apparently ‘dry’ facies found in the upper sequences of Icelandic rootless cones formed when the water content of the underlying wet sediment was depleted and MFCIs were relatively inefficient. This study suggests that both “confined mixing” of tube-fed lava and external water, and lava–water interactions with unfavourable lava–water mass ratios, such as during the late stages of formation of Icelandic rootless cones, can produce seemingly ‘dry’ deposits that nevertheless are fundamentally hydrovolcanic.