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Lava pillars
are common within collapsed sheet flow terrain. Lava pillars are hollow
inside forming a pipe-like channel between the bottom and the top of a
lava flow. They sometimes coalesce to form walls or can be attached to
other pillars by natural bridges. Lava pillars originate as gaps between
lava lobes as a lava flow initially advances. Water that is trapped beneath
the flow is heated and channeled upward through these gaps. This cold
water promotes rapid growth of the lava crust around the gaps.
Then during
flow inflation the hollow pillars continue to act as escape-routes for
seawater trapped beneath the lava flow, and so the gaps between lobes
grow upwards into pipe-like pillars. The height of the lava pillars is
a measure of the maximum thickness the lava flow attained. After lava
inflation ends and the eruption wanes, the molten interior of the sheet
flow typically subsides leaving a series of "bathtub rings"
along the sides of the pillars. These rings are formed as the crust on
the subsiding lava repeatedly adheres to and then breaks off from the
pillar's sides.
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