Photograph by Jack Lockwood, U.S. Geological Survey, June 14, 1982. Aa lava on pahoehoe, Hilina Pali, Kilauea. Photo by Steve Mattox, 1988. Pahoehoe lava, Kilauea, Hawaii. Photograph by J.D. Griggs, U.S. Geological Survey, June 15, 1989.
Pahoehoe lava is characterized by a smooth, billowy, or ropy surface.
A ropy surface develops when a thin skin of cooler lava at the surface of the flow is pushed into folds by the faster moving, fluid lava just below the surface. Pahoehoe flows can evolve into lava tubes. One way that tubes form is by the crusting over of channelized lava flows. As the crust on a flow becomes thicker, it insulates the lava in the interior of the flow. The lava drains down slope and feeds the advancing front or flows into the ocean.