An open lava tube. This collapsed feature often occurs when the outer edges of a flow are quickly cooled by contact with seawater. The insulated lava inside remains viscous and eventually recedes, creating a void.
Lava Tube
Image courtesy of NOAA Office of Ocean Exploration and Research. Download larger version (jpg, 1.8 MB).

An open lava tube. This collapsed feature often occurs when the outer edges of a flow are quickly cooled by contact with seawater. The insulated lava inside remains viscous and eventually recedes, creating a void.

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