Subaerial lava pillars: Evidence for non-explosive water-lava interactions in Iceland
Christle, Kenneth W.
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Submarine lava pillars are hollow pipes of solidified basaltic lava that are interpreted to form in lobate lava flows on the deep (>2000 m water depth) seafloor as heated water rises between adjacent advancing lava lobes. As long as the flow of water within the pillar interior continues, it is inferred that the pillar conduit will remain open. If the water ceases to flow through the conduit, the pillar may be filled in with lava. Models suggest that submarine pillars remain standing while the molten interior of the lava flow drains out. Features that resemble submarine lava pillars have been observed in the subaerial environment in Iceland near Lake Mývatn and in the Skælingur region as part of the Younger Laxá and Laki fissure lava flows respectively. The subaerial pillars have similar morphologies to their submarine counterparts: they are cylinders of basalt with central conduits that display different amounts of infilling. I propose that these subaerial pillars formed in lobate lava flows that advanced over wet substrates (or possibly into shallow ponded water) and were left standing after the body of the lava flow drained away. There are differences between submarine and subaerial pillars. These include pillar interiors that, in submarine pillars, are lined with glassy lava drips and, in subaerial pillars, are filled with small (<10 cm across), bulbous lava protrusions. Additionally, submarine pillars have horizontal glassy shelves covering the external walls. These shelves are interpreted to have formed by lava quenching during drainout. In contrast, shelves on subaerial pillars are less common and with larger spaces between them. Along with horizontal shelves, subaerial pillars display curled segments of lava crust that have the appearance of peeling away from the vertical pillar wall. I infer that these features form on the exterior walls of subaerial pillars during drainout. Based on field observations of the textures on subaerial pillar walls, I infer that, during lava drainout, superheated gasses are trapped beneath a solidified lava flow roof that keeps the outside of the pillar wall hot while the molten lava drains out. This is similar to thermal conditions in a partially filled lava tube.