Lava pillars are commonly observed in volcanic terrains at mid-ocean ridges and are unique to the submarine volcanic environment; they are hollow pipes that extend from the base of a lava flow to remnants of its collapsed upper crust. Pillars can be as tall as 5 m and have outer diameters typically between 0.5 and 2 m. Previous models proposed that they form in thick, fluid, fast-moving sheet flows. We present a new model for pillar formation based on sea-floor observations, and on results from laboratory and numerical models, in which lava pillars form within initially thin (<0.3 m), slow-moving lobate lava flows as gaps between neighboring lobes. Water escaping from beneath the flow acts to locally cool and thicken the crust at the gaps. Pillars attain their full height during subsequent lava-flow inflation. As the flow thickens, pillars that do not continue to channel water are melted and resorbed into the molten flow interior. Pillars are typically preserved at the margins of ponded lava flows where the flow interior is relatively stagnant and less efficient at remelting pillars, and where seawater is more accessible for circulation through the flow.

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