flowtop

Noun

 * 1)  The solid crust on a moving lava flow.
 * 2) * 2007 September 21, Jaeger et al., "Athabasca Valles, Mars: A Lava-Draped Channel System" in Science 317(5845), page 1710 :
 * they formed as gas vented from fixed sources at depth, ripping continuous openings through the moving flowtop.